# 03Power and voltage

El03TA of the Electric circuits topic
• ## 01 VoltageEl03TAnugget01 Activity

### Part 1: introducing the idea of voltage

What the activity is for

First of all, attention is focused on introducing what is meant by voltage. It is important that pupils have an understanding of what it is that they are measuring when they measure voltages in different parts of a circuit.

What to prepare

• 12 volt direct current power supply
• 12 volt, 24 watt bulb in holder

What happens during this activity

It is a good idea to start with a demonstration of what happens in a simple circuit when the voltage of the supply is changed. You might start with the output of the power supply at about 6 volt and then switch it up to about 12 volt. You might also make links here to the practical work carried out by the pupils earlier in which they changed the number of batteries in a circuit.

Teacher: OK, so we start with the power supply set down here at around 6 volt. What do you notice about the brightness of the bulb?

Bill: It's dim.

Teacher: Yes, that's right. Now supposing I turn up the power supply here, increase the voltage on this scale. What do you think will happen? Predictions?

Jill: It'll get brighter.

Teacher: Well, let's give it a go [teacher turns up voltage]. Brighter yes. Where have you seen this kind of thing before?

Jill: If you add batteries it makes the bulb brighter.

Teacher: That's right! So if I add batteries or turn up the voltage, what happens with the bulb?

Bill: You get more energy from the bulb.

Teacher: Excellent! In fact the voltage is a measure of how much energy is provided by the battery for the charged particles in the circuit. If I increase the battery voltage more energy is shifted by the bulb. The bulb gets brighter.

### Part 2: using a voltmeter

What the activity is for

The purpose of this activity is to introduce the voltmeter as an instrument that is used to measure voltages, and to develop further what those measurements actually mean.

What to prepare

• 12 volt direct current power supply
• 12 volt, 24 watt bulb in holder
• demonstration voltmeter

What happens during this activity

Following on from the previous demonstration and discussion, introduce the voltmeter as the instrument to be used in making measurements of the energy provided by the battery and shifted by the bulb.

Demonstrate how to connect the voltmeter by talking through the following kind of sequence with the pupils:

Step 1: make the complete circuit to light the bulb.

Step 2: connect the voltmeter, the right way around, across the power supply and take the reading.

Teacher: First of all let's make a measurement of the energy provided by the power supply. To do this we must connect the meter across the supply and we need to connect it the right way around, with the positive terminal of the voltmeter to the positive side of the supply.

Step 3: connect the voltmeter, the right way around, across the bulb and take the reading.

When connecting the voltmeter to the circuit emphasise the differences between using an ammeter to measure electric currents and using a voltmeter to measure voltages. The key point is that whereas the circuit must be broken to insert an ammeter, the voltmeter is connected across the relevant component in the complete circuit. The ammeter is connected in series; the voltmeter is connected in parallel.

The voltmeter reading across the power supply and the reading across the bulb will be the same. This is a fundamental point which needs to be talked through with the class:

Teacher: So the reading across the battery is 10.5 volt. The voltage value, 10.5 volt, is a measure of how much energy is shifted by the battery each time a coulomb of charge passes through the battery. A bigger voltage means more energy per coulomb of charge. What was the reading across the bulb?

Jill: Same.

Teacher: That's right, 10.5 volt. Why must it be the same?

Bill: Is it because the energy given by the battery equals the energy given out in the bulb?

From here the ideas might be taken further:

Teacher: When we say that the voltage across the battery is 10.5 volt, this actually means that the battery shifts 10.5 joule of energy per coulomb of charge. The voltage is a measure of the number of joule of energy per coulomb of charge. With the bulb, the voltmeter reading is also 10.5 volt. This means that 10.5 joule of energy are shifted by each coulomb of charge as it passes through the bulb.

An energy hill diagram for the circuit might also be introduced as a focal point for further discussion and to support pupil understanding.

• ## 02 Predicting and measuring voltagesEl03TAnugget02 Activity

### Making predictions

What the activity is for

Having demonstrated what is involved in measuring voltages, the pupils are now given the opportunity to make some measurements for themselves. The approach taken is to encourage the pupils to think about and talk through the voltage ideas first, before making the actual voltage measurements. To this end, the pupils are asked to make predictions of voltage values before they make each measurement.

What to prepare

• batteries, bulbs, voltmeters, connecting leads
• printed copies of the support sheet: Predicting and measuring voltages

Support sheet

What happens during this activity

You might introduce this activity in the following kind of way:

Teacher: OK, we have the idea that the voltmeter measures how much energy is shifted by the charged particles as they move around the circuit. So, in this first circuit you measure the battery voltage and this gives the number of joules of energy which are shifted by each coulomb of charge. If you then measure the voltage across the bulb what would you expect to get?

Teacher: First of all talk it through with your partner. Think about what we've been saying about energy and voltage.

Teacher: OK, people are suggesting that the voltage should be the same in both places. As Anita says the energy put in at the battery comes out at the bulb.

Teacher: Well, collect the equipment and use a voltmeter to measure the voltage values for yourselves. There are three circuits to investigate. Go to it!

For this activity, the pupils should ideally work in pairs. Each pair should have one voltmeter, which is placed in the different positions in the circuits as shown on the support sheet.

As the pairs of pupils complete their measurements, it is a good idea to collect the voltage values on the board or on a large sheet of poster paper, so that the pattern of findings becomes apparent to all.

Be sure to give the pupils an opportunity to talk through these new ideas during the lesson as they review the voltage values for the whole class.

• ## 03 Checking an initial understandingEl03TAnugget03 Activity

### Diagnostic questions on voltage

What the activity is for

The diagnostic questions can be used to check the pupils' understanding of key ideas introduced in this episode.

What to prepare

• printed copies of these three questions

Support sheet

What happens during this activity

The questions might be used for homework or as the basis for discussion in class.

Four batteries: designed to probe pupils' understanding of battery voltage.

1. Battery d
2. It has the largest voltage, so it pushes the biggest current round.

Battery voltage: probes understanding of the relationship between the battery voltage and the voltage across the external circuit.

1. V1 is 3 volt
2. This must be the reading on the voltmeter because the voltage across the battery equals the voltage across the single resistor. The energy per coulomb supplied to the circuit at the battery equals the energy per coulomb shifted to the surroundings at the resistor.

Batteries: probes pupils' understanding of current and voltage.

1. You cannot buy a 1.5 ampere battery because batteries are specified in terms of voltage. The current in a circuit depends on both the battery voltage and the circuit resistance. It does not make sense to talk of a 1.5 ampere battery. The battery can supply a full range of currents.
• ## 04 Bulb and buzzer in seriesEl03TAnugget04 Activity

### Combining components of unequal resistance

What the activity is for

This activity provides the opportunity for pupils to talk and think about a circuit in which there are two components (bulb and buzzer, for example) of unequal resistance. The pupils are directed to set up the series circuit and to measure the voltage across each component and relate that to the power dissipated by each component. They must then come up with an explanation for their findings.

What to prepare

• batteries, bulbs, buzzers, voltmeters, connecting leads
• printed copies of the support sheet: Bulb and buzzer

Support sheet

What happens during this activity

You might introduce this activity in the following way:

Teacher: OK, so what happened with the circuit with the two bulbs in series. What were the voltage readings?

Zac: The voltage drop across the bulbs was the same.

Teacher: Good! Now in this circuit we have two components in series again, but this time it's a bulb and a buzzer. The plan is for you to set up the circuit, measure the voltage across the different parts and then figure out what's going on.

• ## 05 Testing pupils on series circuitsEl03TAnugget05 Activity

### Diagnostic questions on series circuits

What the activity is for

The diagnostic questions can be used to check the pupils' understanding of key ideas introduced in this episode.

What to prepare

• printed copies of these diagnostic questions

Support sheet

What happens during this activity

The questions might be used for homework or as the basis for discussion in class.

Resistors in series: designed to probe pupils' understanding of how voltage is shared between two identical resistors in series.

(a) V1 reads 3 volt (b) The battery voltage is shared, half across each resistor. Different batteries: probes pupils' understanding of the relationship between the battery voltage and the total voltage across external resistors.
1. V1 is 6 volt; V2 is 3 volt

Different resistors: probes pupils' understanding of how voltage is shared between unequal resistors.

1. V1 is 4 volt; V2 is 5 volt; V3 is 4 volt

Bigger resistance: probes pupils' understanding of how voltage is shared between unequal resistors.

1. V1 reads between 6 volt and 3 volt
2. The battery voltage is shared, with the larger share across the larger resistor.
• ## 06 Predicting and measuring voltages in a parallel circuitEl03TAnugget06 Activity

### Voltage in parallel circuits

What the activity is for

The approach taken is to encourage the pupils to make predictions of voltage values before they make each measurement.

What to prepare

• batteries, bulbs, voltmeters, connecting leads
• printed copies of the support sheet: Predicting and measuring voltages in a parallel circuit

Support sheet

What happens during this activity

For this activity, the pupils should ideally work in pairs and each pair has one voltmeter that is placed in the different positions in the circuits as shown on the worksheet.

As the pairs of pupils complete their measurements, it is a good idea to collect the voltage values on the board or on a large sheet of poster paper, so that the pattern of findings becomes apparent to all.

Give the pupils a chance to talk through these new ideas, as they review the voltage values for the whole class.

• ## 07 Bulb and buzzer in parallelEl03TAnugget07 Activity

### Combining components in parallel

What the activity is for

This activity provides the opportunity for pupils to talk and think about a circuit in which there are two components (bulb and buzzer) of unequal resistance. The pupils are directed to set up the parallel circuit and to measure the voltage across each component and relate that to what's happening in the circuit. They must then come up with an explanation for their findings.

What to prepare

• batteries, bulbs, buzzers, voltmeters, connecting leads
• printed copies of the support sheet: Bulb and buzzer in parallel

Support sheet

What happens during this activity

This activity follows on from bulbs and buzzers in series. You might introduce this activity in the following kind of way:

Teacher: OK, so what happened with the circuit with the two bulbs in parallel. What were the voltage readings?

Aarn: The voltage drop across both bulbs was the same as the battery voltage.

Teacher: Good! Now in this circuit we have two components in parallel again, but this time it's a bulb and a buzzer. The plan is for you to set up the circuit, measure the voltage across the different parts and then figure out what's going on.

• ## 08 Testing pupils on parallel circuitsEl03TAnugget08 Activity

### Diagnostic questions on parallel circuits

What the activity is for

The diagnostic questions can be used to check the pupils' understanding of key ideas introduced in this episode.

What to prepare

• printed copies of the diagnostic questions

Support sheet

What happens during this activity

The questions might be used for homework or as the basis for discussion in class.

Identical resistors: designed to probe pupils' understanding of how voltage is dropped across identical resistors in a simple parallel circuit.
1. V1 is 8 volt
2. V2 is 8 volt
Power supply: designed to probe pupils' understanding of how voltage is dropped across identical resistors in a simple parallel circuit.
1. Voltage across bulb B1 is 4 volt
2. Voltage across bulb B1 is 4 volt; B2 is 4 volt
• ## 09 Electrical powerEl03TAnugget09 Activity

### Introducing the idea of voltage

What the activity is for

The aim of this interactive demonstration is to introduce the idea of electrical power by examining a range of electrical appliances. In addition, for those pupils who are able to follow the line of argument, the link is made from basic ideas of current and voltage to that of power.

What to prepare

• collection of electrical appliances
• joule and watt meter

Some joulemeters are designed to work at low voltages only (for example up to 16 volt), whilst others operate at the mains supply voltage (around 240 volt). You will need to select electrical appliances to demonstrate according to the type of joule and watt meter.

What happens during this activity

Use this part of the activity to get pupils talking about a range of familiar electrical appliances.

These might include:

• A 40 watt bulb in a lamp holder
• A 100 watt bulb in a lamp holder
• A CD player
• Hair tongs
• A convector heater

Ask the pupils how the two lamps differ from one another and move the discussion on to considering how you would know which kind of bulb to buy in a supermarket:

Teacher: OK, so we have these two bulbs, one dim, one bright. How would you know which kind to get in a supermarket?

: Is it called the watts? Like a 100 watt?

Teacher: That's exactly right. In fact this here is a 100 watt bulb. What do you think the dim one is? How many watts?

Move towards the idea that this value in watts is the electrical power of the device and that the power measures the amount of energy shifted per second (see narrative and teaching and learning issues).

### Different appliances: using a wattmeter

Look at each of the appliances in turn and ask the pupils to predict their power output. If you have access to a wattmeter, it is interesting at this point to connect it up to the different appliances.

Wattmeters measure the power dissipated by a device as time goes by. Most have some kind of flashing display, on which each flash indicates 100 joule of energy shifted. The faster the flashing, the greater the rate at which energy is shifted, the bigger the power output. With some meters the output can be connected up to an amplifier and speaker so that you can hear the rate of flashing.

It is helpful to make the link from the rate of flashing to the model of charged particles working in the device and shifting energy as they go:

Teacher: Wow! Just look at that. Can't you just see the charged particles working hard and shifting energy as they pass through the tongs! The power output is really huge here.

For those classes which are able to cope with the ideas involved you might now return to the base ideas of current (as charged particles passing per second) and voltage (as energy shifted per coulomb) and develop the idea of power (as energy shifted per second).

• ## 10 The electrical homeEl03TAnugget10 Activity

### A survey of electrical appliances

What the activity is for

This is a homework activity in which pupils make a survey of the different electrical appliances used in their own homes. The aim is for the pupils to collect information about the use and electrical power output of each appliance and then to list the appliances in terms of relative power. This will allow the pupils to make the link between high power outputs and appliances which have a heating function.

What to prepare

• Some backup sample data
• homework sheet: The electrical home

Support sheet

What happens during this activity

The pupils follow the instructions on the homework sheet and collect information about the different electrical appliances that they have in their own homes. Having collected the information the pupils then make a table listing the appliances in order of their power output. The aim here is to demonstrate that the appliances, which have a high power output (those which shift energy quickly), all have some kind of heating function.

Direct the pupils to find power output figures by examining:

• Information on the casing of the appliance
• Information leaflets for the appliance
• Information available on the internet
• ## 11 How things have changed!El03TAnugget11 Activity

### The progression of electrical devices

What the activity is for

This is a discussion activity which focuses on the rapid expansion in the range of electrical devices in the home within the last 30–40 years.

What to prepare

What happens during this activity

The pupils might be asked, for homework, to collect some information about the electrical appliances that older neighbours or relatives had in their homes some 30–40 years ago. They can report back their findings in a class discussion which also draws upon the data shown here.

• ## 12 Check questionsEl03TAnugget12 Activity

### Diagnostic questions on power

What the activity is for

The diagnostic questions can be used to check the pupils' understanding of key ideas introduced in this episode.

What to prepare

• copies of these questions

Support sheet

What happens during this activity

The questions might be used for homework or as the basis for discussion in class.

Best word: is designed to check pupils' ability to use the correct terms in relation to a simple electric circuit.

1. An electric current
2. Electric charge
3. Voltage
4. Energy
5. Power output

Make a match: probes understanding of three fundamental electrical units.

1. 1 ampere is the same as 1 coulomb / second (B)
2. 1 volt is the same as 1 joule / coulomb (D)
3. 1 watt is the same as 1 joule / second (A)

Who do you agree with?: probes pupils' understanding of a series circuit with two bulbs of different rating.

1. Tanya: agree
2. Ben: disagree
3. Jane: agree
•