PHYSICS TUTORIAL NOTES

 

ELECTRICAL ENERGY IN THE HOME

 

SYLLABUS TOPIC 8.3

 

 

 

These notes are meant as a guide only and are designed to focus your thoughts on the dot points mentioned in the syllabus. They give a very brief overview of the topic and should be used in conjunction with your class notes, your textbooks and your research from other areas such as the library and the Internet.

 

 

 

Notes compiled by:

CARESA EDUCATION SERVICES

 

 

 

 


 

“1. Society has become increasingly dependent on electricity over the last 200 years.” (syllabus)

 

“Students learn to; discuss how the main sources of domestic energy have changed over time.” (syllabus)

 

DOMESTIC POWER

 

Since fire was discovered around 1½  million years ago wood remained the major source of  fuel until the industrial revolution in the 18th century. Since then fossil fuels in the form of coal and then oil and natural gas have mostly replaced wood as the major source of fuel. These original power sources were augmented to a certain extent by the domestication of animals and by mills powered by running water or wind. Wind has also been used to power boats for the last few thousand years.

 

            While the major source of power in the early days of the industrial revolution was steam engines powered by burning wood or coal this began to change when Faraday in England and Henry in America discovered electromagnetic induction which enabled the continuous generation of electricity and the advent of electric motors. This led to the building of power stations that convert fossil fuels to electricity and this is the major source of power in industrialised countries.

           

            Early humans were hunter-gatherers and led a nomadic lifestyle. As they began to grow crops they built permanent dwellings and remained in the one area. Eventually cottage industries grew up where such items as textiles or furniture were produced. The industrial revolution produced factories where large amounts of energy were used and the demand for human labour led to the growth of towns and cities as the population moved to live near the factories. As electricity became the major power source, the factories became more efficient and were able to produce a wide range of affordable goods including household electrical goods. This led to a huge demand for domestic electricity.

 

            Power stations use large alternators to produce electricity in the form of alternating current. The voltage is raised to very high levels by means of transformers and then carried through wires to the location where it is needed. Here other transformers lower the voltage to the level at which it is used; 240 volts for household consumption in Australia. The high voltage minimises power loss due to heating as the electrical energy is transferred over long distances. Power stations are built close to the coalfields and this is usually a long distance from where the majority of the electricity is consumed. Electricity is then supplied to cities, towns and remote locations through high-voltage cables.

 

            Where the cost of installing transmission lines is too expensive, remote locations produce their own electricity. This can be done by portable generators that run on petrol, oil or gas or by using methods such as solar cells or wind generators.

 

            While nuclear fission power stations are capable of supplying large amounts of electricity they pose a hazard to the environment with their radioactive waste and the possibility of a meltdown. Controlled nuclear fusion does not have these dangers and as soon as technology allows us to build fusion power stations it appears to be the energy source of the future.

 

“Students learn to; assess some of the impacts of changes in, and increased access to, sources of energy for a community.” (syllabus)

 

The availability of sources of energy enables the functioning of the modern societies of today. The most significant impact would most likely be transportation. Raw materials are collected from all over the world and transported to factories where they are converted into goods to be sold all over the world. Without affordable sources of energy we would revert to the local cottage industries of days gone by.

Overseas travel is now very common due to plentiful supplies of energy. Contrast the speed and comfort of today’s plane with that of a sailing ship of a couple of hundred years ago. The distances that a large portion of the population commutes to work or school are much greater than would be possible on horseback. Again the use of energy for transportation has made this possible.

The centralisation of industry has led to the growth of large cities which further rely on adequate sources of energy to run their factories, transport their people and to power their homes and offices.

When we think of electrical energy in the home it is amazing to think of the number and variety of electrical appliances that we use. While some are readily replaceable (e.g. an electric stove can be replaced with a gas stove) others, such as the washing machine, are not readily powered by alternate sources of energy. Most people can make a list of around 20 different appliances in their home that rely on a readily available supply of electrical energy.

 

“Students learn to; discuss some of the ways in which electricity can be provided in remote locations.” (syllabus)

 

            Electricity can be provided in remote locations by using portable generators that are powered by petrol or diesel fuel. Wind powered generators can also be used but these are unreliable as the wind is not always blowing when electricity is required. A battery storage system for the electricity can help overcome this problem. A bank of solar cells can also provide electricity and again a battery storage system will make the electricity available at night or on overcast days. In days gone by the pedal wireless was used in remote locations of Australia to contact the flying doctor. This used a pedal powered generator to provide a small amount of electricity to power a radio transmitter that could be used in emergencies.

 

“Students; identify data sources, gather, process and analyse secondary information about the differing views of Volta and Galvani about animal and chemical electricity and discuss whether their different views contributed to increased understanding of electricity” (syllabus)

 

 

 

 

 

Identify data sources:

Websites:

Learning Materials Production Centre: http://www.lmpc.edu.au

Bob Emery notes:

Other Notes

‘Timesavers” by Greg Marsden

Open Training and Educational Network

 

Gather, process and analyse secondary information:

            Luigi Galvani was a lecturer in medicine at the University of Bologna in Italy. On 26th January 1781 he was performing a frog dissection. The dissected frog was stretched out and one of his assistants touched the frog’s leg with a scalpel at the same time that an electrostatic machine discharged nearby. The frog’s leg twitched. Further experiments by Galvani showed that a dead frog’s leg could be made to twitch by electrical stimulation. Galvani also showed that the same twitching could be produced by touching the leg with a copper hook hung from an iron nail. This led him to conclude that the electricity that caused the frog’s legs to twitch was coming from within the frog and he described it as “animal electricity”. He was not aware that the copper and iron was making a cell that produced the electricity.

 

            Alessandro Volta was Professor of Experimental Physics at the University of Pavia in Italy. In 1792 he read about Galvani’s work and repeated his experiments. He concluded that the electricity was produced by the “bimetallic arc”, i.e. the two metals used to touch the frog’s leg rather than from the frog itself.

The debate continued until the death of Galvani in 1798. In 1799 Volta presented his work on the voltaic pile, the first electric battery, to the Royal Society.

 

Discuss whether their different views…….

The different views led Volta to experiment with the production of electricity from different metals. The increased understanding of electricity that resulted from these experiments led to his subsequent discovery of the voltaic pile. The debate with Galvani encouraged Volta to experiment in this area. Without this controversy Volta would no doubt have devoted his time to other areas of research and the discovery of the battery or electrochemical cell would have been delayed.

 

 

“2. One of the main advantages of electricity is that it can be moved with comparative ease from one place to another through electric circuits” (syllabus)

 

“Students learn to; describe the behaviour of electrostatic charges and the properties of the fields associated with them.” (syllabus)

 

A charged particle has a region of influence around it in which another charged particle will experience a force. This is known as an electric field. There are two types of electric charge: positive and negative. Like charges (two positive charges or two negative charges) will repel each other while unlike charges (a positive and a negative charge) will attract each other.

 

 

 

“Students learn to; define the unit of the electric charge as the coulomb.” (syllabus)

 

The electric unit of charge is the coulomb.

One coulomb is defined as one ampere second i.e. the amount of charge that passes a point when a current of one ampere flows for one second.

q = It   Where q = charge (coulombs) I = current (amps) and t = time (seconds)

 

“Students learn to; define the electric field as a field of force with a field strength equal to the force per unit charge at that point:  E = F / q” (syllabus)

 

An electric field is a region where an electric charge will experience a force.

The electric field strength at a point is equal to the force experienced by a unit positive charge (i.e. a charge of +1coulomb) placed at that point.

Note that the force gives both the magnitude and direction of the electric field.

E = F / q where E = electric field strength, F = force and q = charge.

 

“Students learn to; define electric current as the rate at which charge flows (coulombs/second or amperes) under the influence of an electric field.” (syllabus)

 

Electric current is the rate of flow of charge.

I = q / t                        Where q = charge (coulombs) I = current (amps) and t = time (seconds)

Charges need something to move them along the conductor and this is an electric field.

 

The fundamental unit of electricity is the ampere and the coulomb is defined in terms of the ampere. The ampere has an interesting definition that is not on the syllabus:

The ampere is the unit of current intensity which when maintained in two rectilinear (straight) conductors of infinite length and negligible cross-section separated by one metre in vacuo (vacuum) produces between the conductors a force of 2.0 x 10-7 S.I. units of force (Newtons) per metre of length.

 

“Students learn to; identify that current can either be direct with the net flow of charge carriers moving in one direction or alternating with the charge carriers moving backwards and forwards periodically.” (syllabus)

 

Direct Current, DC, involves the net movement of charge carriers (electrons) in one direction along a conductor. The valency electrons of the conductor are in constant random motion but in the presence of an electric field the electrons experience a net movement in the opposite direction to the electric field.

Note: electrons move in the opposite direction to the direction of the electric field.

 

Alternating Current, AC, involves the oscillation of electrons due to an electric field that is constantly changing direction. Most generators produce alternating current by means of a coil rotating in a magnetic field.

 

 

 

 

 

 

“Students learn to; describe electric potential difference (voltage) between two points as the change in potential energy per unit charge moving from one point to the other (joules/coulomb or volts)” (syllabus)

 

The potential difference between two points is equal to the work done, i.e. the change in potential energy, in moving a unit positive charge (i.e. a charge of +1 coulomb) from one point to the other. The potential difference is the work per unit charge and is expressed as joules per coulomb. One joule per coulomb is known as one volt.

V = W / q where V = potential difference (volts), W = work or energy (joules) and q = charge (coulombs)

 

“Students learn to; discuss how potential difference changes at different points around a DC circuit.” (syllabus)

 

A DC circuit has a source of potential difference such as a battery or power pack, and one or more components with a resistance connected to the terminals.  The potential difference between points on the circuit is proportional to the resistance between those two points.

 

“Students learn to; identify the difference between conductors and insulators.” (syllabus)

 

A substance that allows charge carriers (electrons) to readily flow along it is known as a conductor. A substance that does not allow charges to readily flow along it is an insulator.

No substance is a perfect conductor and no substance is a perfect insulator.

 

“Students learn to; define resistance as the ratio of voltage to current for a particular conductor: R = V / I” (syllabus)

 

Resistance is anything that hinders the flow of charge along a conductor and is measured in ohms (symbol  W). For a constant potential difference a small resistance will produce a large current and a large resistance will produce a small current. This can be expresses quantitatively as            R = V / I

where R = resistance (ohms W), V = potential difference (volts V) and

I = current (amps A)

 

“Students learn to; describe qualitatively how each of the following affects the movement of electricity through a conductor.

-          length

-     cross sectional area

-          temperature

-          material”        (syllabus)

 

The resistance of a conductor is directly proportional to its length. The longer the wire the higher the resistance.

 

The resistance of a conductor is inversely proportional to its area of cross section. A thin wire has a large resistance while a thick wire has a low resistance.

 

The resistance of a conductor increases with temperature. (When you get to semiconductors in year 12 you will find the opposite to be true of semiconductors) The resistance of a light globe will increase as it gets hotter.

 

The resistance of a conductor depends on the material of which it is made. While all metals are conductors there is a difference in their conductivity i.e. how well they conduct. Similarly there is a difference in the efficiency of materials as insulators. There is a continuum of resistance ranging from very low for conductors, medium for semiconductors and very high for insulators.

 

“Students; present diagrammatic information to describe the electric field strength and direction

-          between charged parallel plates

-          about and between a positive and negative point charge” (syllabus)

 

Electric fields can be shown diagramatically as lines of force. These show the direction of the force on a positive charge and therefore the arrows point from positive to negative.

Lines of force between two charged parallel plates.

 

Lines of force around a small positively charged particle.

 

           

Lines of force around a small particle with a high positive charge.

Note that a strong electric field has more lines of force than a weak one.

 

 

                       

Lines of force around a small negatively charged particle.

 

                       

Lines of force between a positive and a negative point charge.

 

 

“Students; solve problems and analyse information using: E = F / q” (syllabus)

 

Q.1. A polystyrene ball carries a charge of 1.0mC and is subjected to an electrostatic force of 2.0 x 10-4 N north. What is the strength of the electric field?

 

A.1. E = F / q  E = 2.0 x 10-4 / 1.0 x 10-6        = 2.0 x 102 NC-1 north.

 

Q.2. An alpha particle (charge = +3.2 x 10-19C) enters an electric field of magnitude 3.0 x 10-6 NC-1 vertically down. What force does it experience?

 

A.2. E = F / q              F = qE             F = 3.2 x 10-19 x 3.0 x 10-6     

= 9.6 x 10-25 N vertically down.

 

Q.3. An electron (charge=  –1.6 x 10-19C) enters the electric field of Q.2. What force does it experience?

 

A.3. E = F / q              F = qE             F = 1.6 x 10-19 x 3.0 x 10-6     

= 4.8 x 10-25 N vertically up.

 

Q.4.A charged oil drop of mass 0.1 mg is held so that is stationary between two charged plates.

(i)                 What is the force due to gravity acting on the oil drop?

(ii)               What is the force due to the electric field acting on the oil drop?

(iii)             If the oil drop has a charge of –6.4 x 10-19 C, what is the magnitude of the electric field strength between the plates?

(iv)             What is the direction of the electric field?

 

 

A.4.     (i) F = mg = 10-7 x 9.8             = 9.8 x 10-7 N down.

            (ii) F = mg = 10-7 x 9.8            = 9.8 x 10-7 N up

            (iii) E = F / q    = 9.8 x 10-7 / 6.4 x 10-19 = 1.53 x 1012 NC-1

            (iv) Down

 

“Students; plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuit” (syllabus)

 

Plan: Look at what you are trying to show and then work out a way to show it. From the syllabus you know you have to show the relationship between voltage across and current in a DC circuit. You would have to set up a DC circuit, measure the voltage and current and determine the relationship between them. As you plan, you think about what equipment is required to carry out your investigation.

Choose Equipment: Make an accurate list of all equipment needed to carry out your planned investigation.

Perform a first-hand investigation: This is best done in the laboratory. An example of a typical experiment is shown below.

 

Mary and John performed an experiment to show the relationship between voltage across and current in a DC circuit. They set up the circuit below containing a power pack, voltmeter, ammeter, rheostat, switch and a resistor labelled 2.0W.

                       

Circuit Diagram

 

Mary set the power pack to setting “D”. Mary then held the switch closed while John adjusted the rheostat until the reading on the voltmeter was 1.0 volt. Mary then read the ammeter and then opened the switch. Mary immediately recorded the result in a table in her science workbook.

Again Mary held the switch closed while John adjusted the rheostat until the reading on the voltmeter was 2.0 volts. Mary then read the ammeter, opened the switch and immediately recorded her results. The procedure was repeated for readings on the voltmeter of 3.0, 4.0 and 5.0 volts.

Mary then held the switch closed while John adjusted the rheostat until the voltmeter again read 1.0 volt and Mary recorded the ammeter reading. This was repeated for 2.0, 3.0, 4.0 and 5.0 volts. John and Mary then swapped jobs and took two more sets of readings so that 4 current readings were made for each voltage setting. A copy of the table of Mary and John’s results is shown below.

 

 

 

 

 

 

Table of Results: Potential Difference and Current

Potential Difference (Volts)

Current (Amps)

1st Reading

2nd Reading

3rd Reading

4th Reading

Average

1.0

0.5

0.4

0.5

0.6

0.5

2.0

1.1

0.9

1.0

1.1

1.025

3.0

1.6

1.4

1.4

1.5

1.475

4.0

2.0

2.1

2.0

1.9

2.0

5.0

2.4

2.4

2.5

2.6

2.475

 

Mary and John then drew a graph of potential difference (vertical axis) against current (horizontal axis).

Try it yourself, using Mary and John’s results, and compare your graph to the one shown below.

 

                       

Graph of Potential Difference vs Current

           

See if you can answer the following questions about Mary and John’s investigation.

 

Q.1. What is the independent variable in the above experiment?

Q.2. What is the dependent variable in the above experiment?

Q.3. Determine the gradient of the above graph.

Q.4. Why is the switch only closed when readings are being taken and left open at other times.

Q.5. What safety precautions should be taken by John and Mary in their experiment?

Q.6. Suggest a conclusion for Mary and John’s experiment.

 

A.1. Potential Difference (Voltage) is the independent variable.

A.2. Current is the dependent variable.

A.3. The gradient of the graph is 2.0 volts/amp.

A.4. The switch is closed for the minimum possible time so that the components of the circuit do not heat up.

A.5. They should only use low voltages and currents, they should ensure that the bench is dry and they should be careful not to touch any hot wires.

A.6. The voltage is directly proportional to the current.

 

“Students; solve problems and analyse information applying: R = V / I” (syllabus)

 

Q.1. A torch globe operates on a potential difference of 3.0 volts and has a current of 0.1Amp flowing through it. What is its resistance?

 

A.1. R = V / I              = 3.0 / 0.1        = 300 ohms.

 

Q.2. A hair drier operates at a potential difference of 240 volts and has a resistance of 960 ohms. How much current flows through it?

 

A.2. R = V / I              I = V / R = 240 / 960 = 0.25 Amps.

 

Q.3. What is the potential difference between the ends of a 500 W resistor when a current of 3 A flows through it?

A.3. R = V / I             V = IR            = 3 x 500         = 1500 V.

 

“Students; plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the variations in potential difference between different points around a DC circuit” (syllabus)

 

Plan: Look at what you are trying to show and then work out a way to show it. From the syllabus you know you have to show how potential difference varies between different points in a DC circuit. You would have to set up a DC circuit and measure the voltage between different points. As you plan, you think about what equipment is required to carry out your investigation.

Choose Equipment: Make an accurate list of all equipment needed to carry out your planned investigation.

Perform a first-hand investigation: This is best done in the laboratory. An example of a typical experiment is shown below.

 

Sarah and Bob carried out an experiment to investigate how the potential difference varied between different points in a DC circuit. They connected a power pack, an ammeter, a switch, 3 resistors of 1W, 2W and 5W respectively and 3 voltmeters into a circuit as shown below.

                       

                                    R1 = 1 ohm      R2 = 2 ohms    R3 = 5 ohms

 

Sarah closed the switch and read the ammeter while Bob read the three voltmeters and recorded the results. Sarah opened the switch as soon as the readings had been recorded. Sarah and Bob repeated the process twice so that they had three sets of readings. A table showing their results is shown below.

 

 

Ammeter (Amps)

V1 Volts

V2 Volts

V3 Volts

Reading 1

1.5

1.5

3.0

7.4

Reading 2

1.5

1.5

2.9

7.3

Reading 3

1.5

1.5

3.0

7.4

 

Q.1. Calculate the values (i) V1/R1,      (ii) V2/R2         (iii)V3/R3

A.1. (i) 1.5       (ii) 1.5              (iii) 1.47

 

Sarah and Bob concluded that the ratio V/R was constant and was equal to the current.

They also concluded that the voltage drop around the circuit was proportional to resistance.

                                   

“Students; gather and process secondary information to identify materials that are commonly used as conductors to provide household electricity” (syllabus)

 

Websites:

Learning Materials Production Centre: http://www.lmpc.edu.au

Bob Emery notes:

Radio Electronics School: http://www.radioelectronicschool.com/reading/reading3.pdf

 

Other Notes

‘Timesavers” by Greg Marsden

Open Training and Educational Network

 

Gather and process secondary information

By far the most common conductor used to provide household electricity is copper. It is relatively cheap and a better conductor than all metals other than silver. Consequently copper is used in most household wiring. Silver, instead of copper, is occasionally used in some high quality electronic equipment due to its higher conductivity but it is not used widely due to its high price. Aluminium is not as good a conductor as silver or copper but it is used in the wires for overhead power line distribution because of its light weight. The light weight allows the supporting structures to be placed further apart and this reduces the overall cost.

 

 

 

 

 

 

 

 

 

 

“3. Series and parallel circuits serve different purposes in households” (syllabus)

 

“Students learn to; identify the difference between series and parallel circuits” (syllabus)

 

Resistors are connected in series when there are no branches in the circuit. Any current that flows through the first resistor must also flow through the second and subsequent resistors since there are no branches in the circuit to allow it to take alternate paths. Note that current is not used up (Current is a flow of electrons so if it was used up, where would the electrons go to?) so the current through all parts of an unbranched, i.e. series circuit, is the same.

                                   

                                    Resistors in Series

Note that in the second diagram the resistors lie parallel to each other but since there are no branches in the circuit they are connected in series.

 

Resistors are connected in parallel when there are branches in the circuit so that the current can flow around the circuit via different pathways.

                                               

                                                Resistors in Parallel

 

“Students learn to; compare parallel and series circuits in terms of voltage across components and current through them” (syllabus)

 

 In a series circuit the current flowing through all points is the same. Since the current through all resistors is the same, if we apply Ohm’s Law, V = IR, we find that the potential difference is directly proportional to resistance.

In parallel circuits the potential difference across parallel components is the same Remember that it is the potential difference between two points on a circuit so this is the same regardless of which way the current travels between them. Since the voltage is the same, by applying Ohm’s Law, I = V/R, we find that the current is inversely proportional to resistance.

 

“Students learn to; identify uses of ammeters and voltmeters” (syllabus)

 Ammeters are used to measure the current through a point in a circuit.

Voltmeters are used to measure the potential difference between two points in a circuit.

 

“Students learn to; explain why ammeters and voltmeters are connected differently in a circuit” (syllabus)

 

Ammeters are connected in series with the component that we want to measure the current through. Any current flowing through the ammeter also flows through the component that is in series with it. Ammeters have a very low internal resistance so that there is very little voltage drop across the ammeter and nearly all the voltage drops across the resistor. Voltage drop is proportional to resistance.

                                               

Voltmeters are connected in parallel with the component that we want to measure the voltage across. Voltmeters have a very high internal resistance so that when the current splits up, nearly all of it will go through the component and very little through the voltmeter. Remember that the current is inversely proportional to resistance.

                                               

 

“Students learn to; explain why there are different circuits for lighting, heating and other appliances in a house” (syllabus)

 

All households have at least two circuits, one for lights and another for power points. If a house has a large number of lights or power points it may have two or more circuits for each of these functions. In addition, many households have separate circuits for such things as electric stoves, hot water systems or air conditioners. These circuits carry different amounts of current and may have wires of different thickness. The lighting circuit is normally designed to carry around 8 amps while the power circuits carry around twice this amount. All the lights or power points on one circuit are connected in parallel.

 

 

“Students; plan, choose equipment or resources for and perform first-hand investigations to gather data and use available evidence to compare measurements of current and voltage in series and parallel circuits in computer simulations or hands-on equipment” (syllabus)

 

Plan: Decide whether you are going to use a computer simulation. If you are going to use a computer simulation find details of the website or make arrangements to use a suitable program. If you are going to use hands-on equipment, decide what circuit or circuits will best demonstrate relationships involving current and voltage in both series and parallel circuits.

 

Choose equipment: If you are using a computer simulation you will require a suitable program and a computer to run it.

The following equipment is suggested for the hands-on approach using the circuits below. Power pack, tapping key, 1 ohm resistor, 2 ohm resistor, 5 ohm resistor, 10 ohm resistor, 3 ammeters, 2 voltmeters, connecting leads.

 

Gather data and use available evidence: This is best done in the laboratory. An example of a typical experiment is shown below.

 

Shirley and Greg wanted to test the variations in current and voltage around (i) a series circuit and (ii) a parallel circuit.

They first set up the series circuit below using a 1 ohm resistor as R1 and a 2 ohm resistor as R2. Shirley closed the tapping key and Greg adjusted the settings on the power pack until a current of approximately 3 amps flowed through the ammeters. Shirley then held the tapping key closed while Greg read and recorded the three ammeters and the two voltmeters. Shirley then released the tapping key. Greg then held the tapping key closed while Shirley read and recorded the three ammeters and the two voltmeters. Their results are shown in the table below.

 

 

                       

                                                Series Circuit

 

They next set up the parallel circuit below using a 5 ohm resistor as R1 and a 10 ohm resistor as R2. Shirley closed the tapping key and Greg adjusted the settings on the power pack until a current of approximately 3 amps flowed through the ammeter A3. Shirley then held the tapping key closed while Greg read and recorded the three ammeters and the two voltmeters. Shirley then released the tapping key. Greg then held the tapping key closed while Shirley read and recorded the three ammeters and the two voltmeters. Their results are shown in the table below.

 

 

                                               

                                                      Parallel Circuit

 

    Table of results: Series circuit

                          Powerpack Setting “E”

Meter

Reading

V1 (Greg)

3.0 Volts

V1 (Shirley)

3.1 Volts

V1 (average)

3.05 Volts

V2 (Greg)

6.1 Volts

V2 (Shirley)

6.1 Volts

V2 (average)

6.1 Volts

A1 (Greg)

3.15 Amps

A1 (Shirley)

3.1 Amps

A1 (average)

3.15 Amps

A2 (Greg)

3.0 Amps

A2 (Shirley)

3.1 Amps

A2 (average)

3.05 Amps

A3 (Greg)

3.1 Amps

A3 (Shirley)

3.05 Amps

A3 (average)

3.1 Amps

 

                Table of results: Parallel circuit

                        Powerpack Setting “E”

Meter

Reading

V1 (Greg)

9.15 Volts

V1 (Shirley)

9.1 Volts

V1 (average)

9.15 Volts

V2 (Greg)

9.15 Volts

V2 (Shirley)

9.15 Volts

V2 (average)

9.15 Volts

A1 (Greg)

1.8 Amps

A1 (Shirley)

1.85 Amps

A1 (average)

1.85Amps

A2 (Greg)

0.9 Amps

A2 (Shirley)

0.9 Amps

A2 (average)

0.9 Amps

A3 (Greg)

2.75 Amps

A3 (Shirley)

2.7 Amps

A3 (average)

2.75 Amps

 

Shirley and Greg noted the following about their results.

In the series circuit, the readings on all three ammeters was the same, which indicates that the current through all parts of a series (unbranched) circuit is the same.

In the series circuit, the potential difference across the 2 ohm resistor was twice that across the 1 ohm resistor which suggests that the potential difference is proportional to resistance.

In the parallel circuit the voltage across both resistors was the same, and this suggests that the potential difference across parallel components of a circuit is the same.

In the parallel circuit the current through the 5 ohm resistor was twice that through the 10 ohm resistor. This suggests that the current through parallel components of a circuit is inversely proportional to resistance.

 

“Students; plan, choose equipment or resources and perform a first-hand investigation to construct simple model household circuits using electrical components” (syllabus)

 

Plan: Decide what type of circuit you are going to construct. Draw a circuit diagram. Remember that household electrical circuits are connected in parallel.

 

Choose equipment: Choose typical components for the circuit such as a battery or power pack, switches, leads, light globes on stands, motors or devices such as a fan that contains a motor, heating coils (immersed in water) or resistors to represent any load in the circuit.

 

Perform a first-hand investigation: Hopefully you will plan your own circuit but a couple of suggestions are shown below. These circuits demonstrate how to connect components in parallel. Close the switches one at a time and both together to demonstrate that each component in a parallel circuit operates independently of other components connected in parallel with it. A variation to the circuit would be to include a short length of fine iron wire that would get red hot and melt as the current was increased and so demonstrate the operation of a fuse.

                                               

                                                  Circuit 1. Lighting

 

                                               

                                                       Circuit 2. Power

“4. The amount of power is related to the rate at which energy is transformed” (syllabus)

 

“Students learn to; explain that power is the rate at which energy is transformed from one form to another” (syllabus)

 

Power is the rate of doing work.

Power = work/time

Since work and energy are the same type of quantity and energy can not be created nor destroyed then power is the rate at which energy is transformed from one type to another e.g. From electrical to heat energy in a toaster.

 

“Students learn to; identify the relationship between power, potential difference and current” (syllabus)

 

Power is equal to the product of the potential difference and the current. P = VI

where P = power (watts W), V = potential difference (volts V) and I = current (amps A)

 

“Students learn to; identify that the total amount of energy used depends on the length of time the current is flowing and can be calculated using: Energy = VIt” (syllabus)

 

Power is the rate at which energy is transformed. Power = energy/time

It follows then that Energy = power x time

 Since P = VI then Energy = VIt

 

“Students learn to; explain why the kilowatt-hour is used to measure electrical energy consumption rather than the joule” (syllabus)

 

A watt is a joule per second. Electrical appliances consume many watts so over a period of time they would use a huge number of joules. Consequently a joule is too small a quantity for convenient measurement of the amount of electrical energy used. The kilowatt-hour has been adopted as the unit to measure electrical energy consumption. The kilowatt-hour is the amount of electrical energy used by a one kilowatt appliance operating for one hour i.e. 1000 joules per second for 1 hour.

1 kilowatt hour = 3 600 000 joules.

 

“Students; perform a first-hand investigation, gather information and use available evidence to demonstrate the relationship between current, voltage and power for a model 6V to 12 V electric heating coil” (syllabus)

 

This is best done in the laboratory. A description of a typical experiment is shown below.

 

 

 

 

 

 

 

Rebecca and Ben carried out an experiment to examine the relationship between current, voltage and power for a model electric heating coil. They took about a metre of resistance wire and wound it into a coil around a pencil. They left straight lengths of about 10 cm of wire at each end. They used alligator clips to connect the coil into a circuit with a power pack, rheostat, voltmeter and ammeter as shown in the circuit below. They then used a measuring cylinder to measure 100 mls of water into a styrofoam cup. They immersed the coil in the water and measured the temperature with a thermometer.

                                   

They set the power pack at setting “E”, turned on the power and adjusted the rheostat until the current flowing through the circuit was exactly 3.0 amps. They read the voltmeter and noted that the potential difference across the coil was 2.6 volts. They kept the current constant at 3.0 amps by making minor adjustments to the setting on the rheostat. They measured and recorded the temperature of the water and the potential difference across the coil every minute for ten minutes. A table of their results is shown below.

 

Time (minutes)

Potential Difference (Volts)

Temperature (oC)

0

2.6

18.1

1

2.6

18.9

2

2.7

19.8

3

2.7

20.7

4

2.7

21.7

5

2.8

22.6

6

2.8

23.4

7

2.8

24.2

8

2.8

25.0

9

2.8

25.7

10

2.8

26.4

            Table Showing Temperature and Voltage at 1- Minute Intervals

 

Q.1.     Given that it takes 4.2 joules of energy to raise the temperature of 1 gram of water by 1oC and that 1 gram of water occupies 1 millilitre, calculate the amount of energy absorbed by the water.

 

A.1.     Temperature rise = 26.4 – 18.1 = 8.3oC

            4.2 joules per gram = 420 joules per 100 grams per oC

            = 420 x 8.3 joules total energy = 3486 joules

 

 

 

Q.2.     At what rate in joules per second was energy being absorbed by the water?

 

A.2.     3486 joules per 10 minutes or 600 seconds = 3486/600 joules/second = 5.81 joules per second.

 

Q.3.     What is another name for the units “joules per second”?

 

A.3.     Watts

 

Q.4.     What was the average potential difference (2 d.p.) across the coil over the 10 minute period.

 

A.4.     Average = (2.8 x 6 + 2.7 x 3 + 2.6 x 2)/11     = 2.74 volts

 

Q.5.     Use the formula P = IV to calculate the average power dissipated by the coil.

 

A.5.     Average voltage = 2.74V        P = IV = 3 x 2.74 = 8.22W

 

Q.6.     Why do you think that the power dissipated by the coil was greater than the heat absorbed by the water?

 

A.6.     No allowance was made for the heat absorbed by the styrofoam cup and by the metal in the resistance wire. Heat was also lost to the surrounding air.

 

Q.7.     Suggest a reason why the voltage rose as the experiment progressed.

 

A.7.     The resistance of the coil rose as it heated up.

I = V/R  Since I is constant then as R rises so does V.

 

Q.8.     What is the independent variable in this experiment?

 

A.8.     Time

 

Q.9.     What is the dependent variable in this experiment?

 

A.9.     Temperature

 

Q.10.   Suggest a way that the accuracy of the experiment could be improved.

 

A.10.   Place a lid on the styrofoam cup and wrap it in insulating material to minimise heat loss.

            Determine the heat absorbed by the coil and include this in the calculations.

 

 

 

 

 

 

 

 

 

 

 

“Students; solve problems and analyse information using: P = VI and Energy = VIt” (syllabus)

 

Q.1.     (a) What power is consumed by a light globe operating with a potential difference of 12 volts across it if a current of 5 amps flows through it?

            (b) How much energy would it use in 5 minutes?

 

A.1.     (a) P = VI = 12 x 5 = 60 watts.

            (b) Energy = VIt = 12 x 5 x 300 = 18 000 joules. (Note that t is in seconds)

 

Q.2.     A heater dissipates 1000 watts and operates from a 250 volt supply.

(a)    How much current passes through it?

(b)   How much energy would it use in 10 minutes?

 

A.2.     (a) P = VI        I = P/V                        = 1000/250      = 4 amps.

            (b) Energy = VIt = Pt = 1000 x 600 = 600 000 joules = 600 kJ.

 

 

 

“5. Electric currents also produce magnetic fields and these fields are used in different devices in the home” (syllabus)

 

“Students learn to; describe the behaviour of the magnetic poles of bar magnets when they are brought close together” (syllabus)

 

When two bar magnets are brought close together the magnetic fields around their poles will produce a force between the two magnets. If two like poles (north pole and north pole OR south pole and south pole) are brought close together they will repel each other while if two unlike poles (a north pole and a south pole) are brought close together they will attract each other.

Like poles repel: Unlike poles attract.

 

“Students learn to; define the direction of a magnetic field at a point as the direction of force on a very small north magnetic pole when placed at that point” (syllabus)

 

The direction of a magnetic field at a point is the direction of force on a very small north magnetic pole when placed at that point. However magnetic poles don’t come as monopoles (one pole only) but come as pairs of north and south poles. If you place a suspended magnet at the point then it will line up in the direction of the magnetic field with the north pole of the magnet pointing in the direction of the magnetic field. The direction of the magnetic field is the direction that the north pole points. The practical method of determining the direction of a magnetic field at a point is to place a compass at the point and the direction of the magnetic field is the direction that the compass points.

 

 

 

 

 

 

“Students learn to; describe the magnetic field around pairs of magnetic poles” (syllabus)

 

The magnetic field around pairs of magnetic poles is best described by describing the lines of magnetic flux. Lines of flux are directed from the north pole to the south pole and never cross. In a strong magnetic field they are close together while in a weak magnetic field they are well apart.

The simplest example of a magnetic field around a pair of magnetic poles is shown by the bar magnet.

                       

 

 

 

Two examples of the magnetic field around pairs of magnets are shown below           

 

 

 

 

 

 

 

 

 

“Students learn to; describe the production of a magnetic field by an electric current in a straight current-carrying conductor and describe how the right hand grip rule can determine the direction of current and field lines” (syllabus)

 

Moving electric charges produce a magnetic field. When these charges are moving through a conductor the lines of magnetic flux form concentric circles around the conductor. The direction of the magnetic field can be found by applying the right hand grip rule. Grip the wire with the thumb pointing in the direction of the flow of current. (i.e. conventional current, which is the direction of positive charge flow). The fingers now wrap around the wire in the direction of the magnetic field.

                                               

The existence and direction of the magnetic field can be verified by holding a compass at various positions around a straight, current-carrying conductor. The existence of the magnetic field can also be verified by passing the conductor through a sheet of cardboard that is perpendicular to the conductor and sprinkling iron filings onto the cardboard, tapping gently and noting the pattern formed by the iron filings.

 

“Students learn to; compare the nature and generation of magnetic fields by solenoids and a bar magnet” (syllabus)

 

A solenoid is a coil of insulated wire of many turns. It is long compared to its diameter. A current through the solenoid will produce a magnetic field and the direction of the field can be determined by the solenoid rule. Grip the solenoid with the fingers wrapped around the solenoid pointing in the direction of the current. The thumb will point to the north pole of the solenoid. The lines of magnetic flux are continuous and the direction of the magnetic field is from north to south outside the solenoid but from south to north inside the solenoid.

 

 

 

 

Magnetic Field Around a Solenoid

 

Since magnetic fields are due to moving electric charges it seems puzzling why a piece of iron but not other metals can become magnetised. In most metals the atoms have a random alignment so that the electrons orbit the atom in random directions and the minute magnetic fields due to each atom cancel out. In iron, cobalt and nickel atoms tend to group together in domains where the atoms are aligned so that their electrons spin in the same direction.  In an unmagnetised piece of metal, the domains are randomly orientated so they have no overall magnetic effect. In a magnetised piece of iron the domains are aligned so that the electrons spin in the same direction and produce a magnetic field. Consequently, when it is in a magnetic field, an iron bar can be magnetised to produce a north pole at one end and a south pole at the other. The lines of flux travel from the north pole to the south pole. Cobalt and nickel can also be magnetised but not as readily as iron.

 

                       

           

Magnetic Field around a Single Bar Magnet

 

 

 

 

 

 

 

“Students; plan, choose equipment or resources for and perform a first-hand investigation to build an electromagnet” (syllabus)

 

Plan: You have to build an electromagnet. Find out what you need and how it is assembled.

Choose Equipment: Make an accurate list of all equipment needed to build your electromagnet.

Perform a first-hand investigation: You can do this in the laboratory but you can also make an electromagnet at home.

 

 

A possible approach to making an electromagnet is described below.

 

A soft iron core is the most suitable starting point for an electromagnet since this will produce a temporary magnet that loses its magnetic properties when the current is switched off. However most commercially available iron products such as bolts or nails are hard steel and retain their magnetic properties after the current has been switched off i.e. they become permanent magnets. If no soft iron core is available then iron bolts and nails can be softened to a certain extent by annealing them, i.e. heating them to red heat and allowing them to cool slowly.

 

Procedure: Heat three long iron nails to red heat with a bunsen burner and allow them to cool slowly. When cool, bind them together with sticky tape. Wind about two metres of thin, insulated wire around the nails and connect the ends of the wire to the terminals of a 6-volt lantern battery (This is a large battery and can deliver a high current.) Test the effectiveness of your electromagnet by seeing how many small nails or paperclips it can pick up. Disconnect the wire from one terminal of the battery and again see how many nails or paperclips can be picked up when no current is flowing.

 

“Students; perform a first-hand investigation to observe magnetic fields by mapping lines of force

-          around a bar magnet

-          surrounding a straight DC current-carrying conductor

-          a solenoid

-          present information using o and k to show the direction of a current and direction of a magnetic field ” (syllabus)

 

Bar Magnet: Place a bar magnet on a bench and cover it with a sheet of thin, firm cardboard. Sprinkle iron filings over the cardboard and tap the cardboard lightly a couple of times so that the iron filings align themselves with the magnetic field. Take a photo of the iron filings showing the lines of force.

 

An alternative is to use diazo paper (a light sensitive paper). Place the diazo paper over the bar magnet and sprinkle iron filings over the diazo paper. Leave the paper exposed to sunlight for three minutes (the time depends on how sunny it is). Return the iron filings to their container and develop and fix the diazo paper by exposing it to ammonia. This is done by pouring some strong ammonia solution in the bottom of a desiccator and placing the diazo paper above the ammonia in the desiccator. An example of a magnetic field recorded with diazo paper is shown below.

 

 

 

           

Magnetic Field around a Single Bar Magnet

 

Straight DC current-carrying conductor: Set up a tall retort stand with a bosshead and clamp at the top and at the bottom of the retort stand and with a large retort ring in the middle. Use a small nail to make a small hole in the centre of a sheet of thin, firm cardboard about 20 cm square. Thread a length of insulated wire through the cardboard and place the cardboard on the retort ring. Wrap the wire around the clamps at the top and bottom of the retort stand so that there is a vertical length of wire running through the cardboard. Connect the ends of the wire to a power pack. And sprinkle iron filings onto the cardboard. Turn on the power and tap the cardboard lightly a couple of times. Take a photo of the iron filings showing the lines of force.

 

Solenoid: Make a solenoid by winding a long length of insulated wire around a cardboard tube. (The cardboard tube from the centre of a toilet roll is ideal). Cut a sheet of firm cardboard so that it goes around the solenoid with a tongue to insert into the solenoid at each end. Place the solenoid between two books on a bench and place the cardboard on the books with the tongues inserted into the solenoid. Connect the wire at the two ends of the solenoid to the terminals of a power pack and switch it on. Sprinkle iron filings over the cardboard and tap gently. Take a photo of the iron filings showing the lines of force.

 

Note: In the above demonstrations, the lines of force can be mapped by placing a compass at various points in the magnetic field instead of using iron filings. The compass has the advantage of showing the direction of the magnetic field as well as being able to demonstrate weaker magnetic fields.

 

 

 

 

 

 

 

 

 

Direction of Magnetic Field:

Q.                Show the direction of the magnetic field above and below a wire carrying a current to the right in the plane of the paper.

 

A.              k          k          k          k          k

 

 

k          k          k          k          k

 

   I

                        o          o          o          o          o

 

                        o          o          o          o          o

 

Q.    A wire carries a current vertically into the plane of the page. Show the direction of the magnetic field above and below the wire.

 

A.  By the right-hand grip rule the direction is shown by the arrows below.

 

         B

o

                                  B

 

“Students; identify data sources, gather, process and analyse information to explain one application of magnetic fields in household appliances” (syllabus)

 

Applications that can be studied include electric motors, switches and relays, electric bells, magnetic audio and video tapes and magnetic computer discs.

The application studied here will be the loudspeaker used in radios, televisions, audio equipment and computers.

 

The loudspeaker consists of a powerful magnet with light cylindrical coils, known as the voice coils, mounted over the central pole piece without touching it. The output current from the audio device fluctuates as it passes through the voice coil and causes a fluctuating magnetic field in the voice coil. The interaction of the fluctuating magnetic field of the voice coil with the fixed magnetic field of the pole piece produces a changing force on the voice coil that causes it to vibrate. The rigid paper cone is attached to the voice coils so the vibration is transmitted to the paper cone. This causes the air around it to vibrate and this is detected as sound.

                                   

 

“6. Safety devices are important in household circuits” (syllabus)

 

“Students learn to; discuss the dangers of an electric shock from both a 240 volt AC mains supply and various DC voltages, from appliances, on muscles of the body” (syllabus)

 

An electric shock occurs when an electric current passes through the body. Most schools use power packs with circuit breakers that prevent currents in excess of 5 amps flowing through the circuit but a current of a fraction of an amp flowing through the human body is sufficient to cause death. Thankfully the human body is covered with skin that is a pretty good insulator but this insulation breaks down if the skin is wet. The resistance of around 0.5 MW for dry skin falls to about 100W if the skin is wet. Add this to the 100W internal resistance of the human body and you can see that a wet person has a total resistance of only around 200W.

 

While Australia has a 240 volt electricity supply, many overseas countries use a 110 volt supply. A 240 volt supply has less current running through the circuit than a 110 volt supply when dissipating the same amount of power. At first glance it would appear that this would make it safer, but remember: it is the current through the body, not the current in the circuit that is dangerous. Since the resistance of the human body would be the same in each case, a 240 volt shock running to earth would produce more than twice the current as a 110 volt shock. Consequently it is more dangerous. It is used in Australia because less power is lost as heat in the transmission wires.

 

The severity of an electric shock depends on several factors, among them being

(i)                 The amount of current passing through the body.

(ii)               The length of time that the current passes.

(iii)             The path of the current through the body.

(iv)             Whether the current is A.C. or D.C.

(v)               The general fitness and well being of the victim.

 

The muscles of the body operate in response to electrical impulses from nerve cells that cause the muscles to contract. An electric shock will interrupt these nerve impulses and can cause muscles to contract independently. For this reason a person who grasps hold of a wire carrying a current can find that they are unable to let go.

 

 

 

As a general guide, currents of 100-200mA are most dangerous. These can cause ventricular fibrillation, i.e. rapid irregular contractions of the heart muscles, a situation where the heart flutters rapidly and doesn’t pump properly. CPR cannot normally restart a fibrillating heart and death usually results. A fibrillating heart can be restarted with a defibrillator, a device designed to apply a controlled electric shock across the heart. These are available in most NSW ambulances so their rapid response to electrocution cases is vital. Currents above 200mA do not usually cause ventricular fibrillation although they can cause the heart to stop beating. These victims will normally recover with prompt CPR and efficient medical treatment.

 

The longer a current flows through the human body the more dangerous it is. For example, a current of  0.1A  flowing for a tenth of a second would normally have no ill-effects on the body but the same current flowing for ten seconds or more would probably cause death.

 

A current through the hand will cause the muscles of the hand to contract so that the person is unable to let go of the wire. This will increase the time that the current passes through the body with possible fatal consequences. A current across the chest can cause paralysis of the chest muscles so that the victim is unable to breathe. However a much more serious consequence of a current across the chest is its disruption of the heart rhythm and the onset of ventricular fibrillation. Current across the chest is most likely to occur when the current enters through one hand and leaves through the other hand or a foot. This can occur if a person experiencing a shock grabs something connected to earth such as a tap or pipe or if a person experiencing a shock is standing on something connected to earth such as a concrete floor.

A.C. is much more dangerous than D.C. as its changing polarity is much more likely to cause fibrillation. This is particularly true of the 50 Hz frequency used in Australia as this is within the range most likely to induce fibrillation. Alternating current will cause the same ill effects on the body as a direct current around five times its value. For example the “let go threshold” i.e. the current at which a person is unable to release their grip on a conductor is about 10.5A for A.C. and around 50A for D.C.

 

A fit and robust person is more likely to survive an electric shock that someone in poor health, particularly if the poor health is related to the heart. However, just as there are unmeasurable individual differences that make some people more susceptible to some diseases than others, so too some people are more affected by an electric shock than others.

Time Effect of Shock on the Human Body

Electric Current (mA)

Duration (ms)

Effect

50

10-200

200-4000

>4000

Usually no dangerous effect

Fibrillation unlikely

Up to 50% probability of fibrillation

100

10-100

100-600

600-10 000

>10 000

Usually no dangerous effect

Fibrillation unlikely

Up to 50% probability of fibrillation

More than 50% probability of fibrillation

500

10-40

40-500

>500

Fibrillation unlikely

Up to 50% probability of fibrillation

More than 50% probability of fibrillation.

Table copied from O.T.E.N. notes:

Electrical Energy in the Home Part 6: Electrical Safety

 

“Students learn to; describe the functions of circuit breakers, fuses, earthing, double insulation and other safety devices in the home” (syllabus)

 

Properly constructed circuits have safety devices that cut off the current if it exceeds a level that is deemed safe.

An excess of current can

(i)                 Cause expensive components in a circuit to burn out.

(ii)               Cause wires in inaccessable places to overheat and cause fires.

(iii)             Cause appliances to become “live” and electrocute anyone who touches them.

 

The most basic safety device in a circuit is a fuse. A fuse is just a piece of wire in a circuit and will heat up and melt if too much current flows through it. When the wire melts it breaks the circuit. While some older houses still have fuses they have been superseded in household circuits by circuit breakers and safety switches. Fuses have the disadvantage of being awkward to replace and not cutting off the electricity supply as quickly as safety switches. Fuses are still used in cars and in some appliances such as televisions.

 

Original circuit breakers worked on one of the following principles. In the first type, too much current would cause a bimetallic strip to heat and bend away from its contact points. In the second type, too much current would increase the strength of an electromagnet so that it pulled an iron bar away from its contacts.

 

The most efficient type of circuit breaker is earth leakage device known as a safety switch. This detects a difference between the current flowing into the house through the active line and the current leaving in the neutral line. If current leaks to earth the safety switch will cut off the current in a few milliseconds, which is less time than it takes to electrocute someone. Because safety switches are safer than fuses or conventional circuit breakers they are mandatory on all new buildings and on those undergoing renovations.

 

While some double insulated appliances have only two pins in the plug that is inserted into the power point, any appliance with a metal casing will still have a three-pin plug. The vertical pin at the bottom is an earth connection. Normally the earth wire does not carry any current. However, if there is a short circuit in the appliance and the outer casing becomes “live” then the earth wire will carry the excess current to earth and either blow the fuse or trip the circuit breaker.

 

You can recognise a double insulated appliance by its two-pin plug. The internal components of a double insulated device are completely enclosed in plastic and this prevents contact with any outside metal parts.

 

 

 

 

 

 

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