# 02Electrical working in loops

Ee02TA of the Electricity and Energy topic
• ## 01 Measurements to determine power: series connectionsEe02TAnugget01 Activity

### Making measurements and using them

What the activity is for

Making measurements gives a real connection to the quantities involved. The trail from these measurements to the final quoted value is a trail of evidence that should be laid out in such a way that others can see that the case is believable. Inevitably this shouldn't involve too many short cuts.

What to prepare

• some cells and batteries, 1.5–6 V
• some electrical wires
• a well-designed ammeter
• a well-designed voltmeter
• a collection of lamps and resistors to match the batteries and meters
• printed copies of the support sheet

Support sheet

What happens during this activity

Introduce a number of circuits containing only series connections, where the goal is to find the power dissipated in each element of the circuit and therefore the power dissipated in the whole circuit. Students need to choose where to put the ammeter and voltmeter, and then transfer their choice and readings to the calculations templates.

As with writing templates, the idea here is to encourage good practice and that students should eventually lay out their calculations by themselves. The templates are there to act as prototypes of clear communication.

• ## 02 Measurements to determine power: parallel connectionsEe02TAnugget02 Activity

### Making measurements and using them

What the activity is for

Making measurements gives a real connection to the quantities involved. The trail from these measurements to the final quoted value is a trail of evidence that should be laid out in such a way that others can see that the case is believable. Inevitably this shouldn't involve too many short cuts.

What to prepare

• some cells and batteries, 1.5–6 V
• some electrical wires
• a well-designed ammeter
• a well-designed voltmeter
• a collection of lamps and resistors to match the batteries and meters
• printed copies of the support sheet

Support sheet

What happens during this activity

Introduce a number of circuits containing only parallel connections, where the goal is to find the power dissipated in each element of the circuit and therefore the power dissipated in the whole circuit. Students need to choose where to put the ammeter and voltmeter, and then transfer their choice and readings to the calculations templates. As with writing templates, the idea here is to encourage good practice and that students should eventually lay out their calculations by themselves. The templates are there to act as prototypes of clear communication.

• ## 03 A topic mapEe02TAnugget03 Activity

### Making topic maps to reinforce and explore understanding

What the activity is for

The ideas in electric circuits are particularly densely connected. It is therefore a particularly rich area for topic mapping, where the quantity of (correct) interconnections is a guide to the student's understanding.

What to prepare

• a set of ideas to cut out and link

What happens during this activity

Pencil and paper provide the most flexible medium for this activity, but having a number of ideas ready cut out facilitates rethinking arrangements, particularly if there are several students working on the same map. Once an arrangement has been agreed then it is worth copying the ideas to paper and drawing in the links. It's the discussion and resulting clarification that are important, rather than the finished artefact, although there is much value in producing a reasonable display copy so that the similarities and differences between the different maps can be appreciated – so leading to more discussion.

It's a good idea to mix in one or two physical things, such as cells, lamps or resistors, with the physical quantities, such as potential difference and resistance. An alternative is to mix in some electrical units, such as ampere and ohm.

Here are some suggested lists:

• Power; energy; time; potential difference; lamp; current.
• Resistance; series connections; parallel connections; current; potential difference; lamp; brightness.
• Power; watt; current; ampere; potential difference; volt; energy; joule; time; second.
• Cell; battery; lamp; current; potential difference; resistance.

You'll want to adapt these in the light of what best challenges your class.

• ## 04 An earthing demonstrationEe02TAnugget04 Activity

### Making the loops in earthing circuits explicit

What the activity is for

Modelling the earthing circuit is much easier if you can show it in action. A large scale board-mounted model is not hard to make. This then provides tangible resources against which you can explain the workings of the separate loops in the circuit.

What to prepare

• 12 V power supply
• demonstration earthing board, as shown

What happens during this activity

Introduce the appliance, and the connections made to it: live, neutral and earth. Having a real plug to hand, as shown here, will help. A metal-walled container, such as the sawn kettle shown in the diagram, is ideal. Show the appliance working normally at first, without the white flying wire linking the live or neutral connections and the earth, which should usefully be as obvious as the model shows (a large copper strip is good).

Bridge the fuse so that it's replaced by a standard laboratory wire.

On making the link between the live connection and the lamp, the lamp representing the heart will glow if the voltage and lamp are well matched. This is not good for the person. Trace out the complete loops involved (remembering not to start at the power supply!), showing the extra loop that the man provides. This draws more current from the source. Break the fault link between the element and the casing by unplugging the white flying wire.

Introduce the fuse as a oneâ€“time currentâ€“limiting switch and remove the bridging link. Now adding the fault link will add in the extra loop and so cause more current in the fuse. The fuse melts and this sacrificial action prevents the man's heart from glowing.

The Physics narrative provides a much fuller explanation of the action of the earth circuit and fuse. Here we only sketch a possible series of actions and words to explain this to students.

• ## 05 Power and brightnessEe02TAnugget05 Activity

### Getting ready to measure brightness

What the activity is for

This demonstration allows you to compare:

• The brightness of two bulbs commonly used in the home by measuring this in lux at a set distance from each bulb.
• The relative effectiveness of the bulbs in shifting energy from the original chemical store (assuming fossil fuel power station) to the light pathway. Do this by dividing the lux reading at a set distance from the bulb by the power rating of the bulb to show the brightness per watt.

What to prepare

• 2 retort stands
• 3 bosses and clamps
• 1 metre ruler
• 1 digital light meter
• 2 mains energy meters
• 1 thick black card big enough to shield the lamp from the light meter
• 2 electrical lamp housings
• 1 incandescent bulb (most commonly used 60 watt) attached to one lamp
• 1 CFL (compact fluorescent lamp, most commonly used 20 watt) attached to other lamp (look for one which has energy saving bulb written on the bulb or on the packaging box)
• 1 original CFL packaging box

What happens during this activity

This demonstration should be set up in a darkened corner of a room away from direct sunlight, with lights off and normal blinds pulled down if it's a sunny day. Black-out blinds could be employed, if available. The lamps should each be plugged in to a mains meter. Bear in mind that throughout this activity the CFL bulb will need to be on for 1 minute before taking a reading because the bulb does not reach maximum brightness immediately.

### Comparing bulbs

The following contains sample data. Your own values are likely to differ and should be used instead. Show the class the two bulbs (in their housings) being used and ask which of the students have each kind in their homes.

Teacher: Why do you think that over the last 10 years or so families have been encouraged to switch to these newer CFL bulbs?

Lydia: Well, I think it is because they last longer and because they use less energy.

Teacher: These are good points. You'll notice on the side of the CFL bulb that the words energy saving bulb is printed. What do you think this means?

(This will be printed on either the bulb or the packaging box.)

Alex: This means that they use less energy.

Teacher: What do you mean by use less energy? Doesn't this really depend on how long you leave the bulb on for?

Alex: Yes it does. I mean if you leave both on for the same time, then the CFL bulb will have used less energy.

Teacher: So what quantity are you really talking about?

Lydia: Power. The CFL bulb shifts energy from one store to another at a lower rate.

Teacher: Good. We can see this from the mains meters.

Show the class that the meter reads approximately 60 watt (57 watt, sample data) for the incandescent bulb and much less for the incandescent bulb (16 watt, sample data).

Lydia: Energy is always conserved. I think what is really meant is that since the CFL bulb needs energy at a lower rate, then less fuel is burned in a power station. What is really being saved is the fuel!

Teacher: That's much better. Now, if we're to be persuaded to use the CFL bulbs we need to know that they will give a sufficient amount of light. We'll compare their brightness using a light meter.

### Brightness and lux

Show the class the meter and demonstrate how placing it near the bulb will give a reading in lux. There's no need to discuss what this unit represents; just accept that it's a useful way to compare the luminous brightness.

Teacher: We can compare the brightness if we compare the reading in lux on the meter. We do have a problem, however.

Alex: Even without the bulb, the light meter will give a reading. We need to take this away from the reading when the meter is put near the bulb.

Place the light meter at the same distance from each bulb (0.5 metre) ensuring that the meter is aligned with the centre of the bulb. Check the distance with the metre rule. Take a background light meter reading with the bulb shielded from the meter using the black card, then take the reading with the black card removed. The reading from the lamp will be the difference between the two.

It's worth writing these readings on the board: Incandescent bulb produces 408 lux at 0.5 metre; CFL bulb produces 192 lux at 0.5 metre .

Teacher: So what do our results show?

Lydia: That the incandescent bulb is better.

Teacher: Better, but how is it better?

Lydia: The incandescent bulb is giving out more light.

Teacher: That's true, so should we therefore use these bulbs instead of CFL bulbs? Do you think that the difference in brightness as read by the light meter would make a difference to how bright our rooms would be at home?

John: Well in my house we have both types of bulbs and all of our rooms are bright. I don't think it makes much of a difference really!

Teacher: Don't forget that the incandescent bulb is also shifting more energy per second than the CFL bulb.

### Luminous output and electrical input

Draw the students' attention again to the mains meters.

Teacher: Wouldn't we expect then that the incandescent would give more light since the meter reads a higher power?

Lydia: I suppose so.

Teacher: I wonder if there is a way in which we could compare the brightness, or the amount of light given out by the bulbs, which also takes into account the amount of energy being shifted, or power?

Alex: I don't know!

Teacher: Let me help you. The incandescent bulb shifts energy at a rate of 56 watt and the bulb gave a reading of 408 lux at 0.5 metre away. We could imagine that, for each 1 watt, 7.3 lux of light is measured (408 lux / 56 watt). So the incandescent bulb gives 7.3 lux of light for each watt or joule second -1. Now, what is the calculation for the CFL bulb?

Alex: For the CFL bulb the power read from the meter was 16 watt and 192 lux was recorded at 0.5 metre. So for each 1 watt the light meter recorded 12.0 lux

.

Focus attention on these values, perhaps by writing them down: the incandescent bulb produces 7.3 lux watt -1 ; the CFL produces 12.0 lux watt -1.

Teacher: Now which bulb would we consider to be better?

Lydia: It depends what you mean by better! If we mean which is better at switching power to the lighting pathway then the CFL bulb is better.

Teacher: How many times better?

Lydia: 1.6 times better!

This activity might lead to a discussion about whether or not we're convinced that using a CFL bulb is better. They run at a lower power and are better at radiating. This is surely a strong argument in their favour.

• ## 06 Talking and thinking circuit calculationsEe02TAnugget06 Activity

### Exemplifying a calculation to give students good practice to emulate

What the activity is for

This is an interactive teacher demonstration with a difference. It involves working not with apparatus, but with numbers in carrying out circuit calculations. The idea here is to make explicit to students the steps, or underlying strategy, involved in making calculations. All too often this systematic guidance is missed out and students struggle to make sense of relatively simple questions.

What to prepare

• simple circuits to demonstrate, connecting the calculations to these circuits

Here we suggest:

circuit 1: 12 volt supply and two resistors in series, 40 ohm and 80 ohm

circuit 2: 12 volt supply and two resistors in parallel, 48 ohm and 60 ohm

What happens during this activity

Set out below are the approaches to carrying out calculations for different kinds of circuit. These may be used as and when appropriate in a lesson sequence. It's a good idea to have apparatus to hand so that the steps in the calculation can be followed with the equipment.

Circuit 1: Two resistors in series

Question: What is the potential difference across the 80 ohm resistor?

In answering a question such as this, it is a good idea to talk through the overall approach with your class, making clear all of the key points:

With two resistors in series, the total voltage (12 volt) of the supply is shared between them.

Since the resistors are not of equal value, the voltage is shared with a greater potential difference across the bigger resistor. More energy is shifted as the current passes through the bigger resistor.

The current through both resistors is the same.

If we know the value of the electrical current then the potential difference across either resistor can be calculated using the equation: V = R × I.

### Talking through the calculation

Teacher: So, how do we calculate the electric current in this circuit?

Sarah: From the voltage and resistance?

Teacher: Which resistance?

Will: The total resistance.

Teacher: Exactly! We can find the current in this loop from the voltage and total resistance using our old friend I = VR, so I = 12 volt40 ohm + 80 ohm, which simplifies to I = 12 volt120 ohm, for which you can work out that the current is 0.1 A.

Teacher: So the current in the loop is 0.1 ampere.

Teacher: We can now calculate the potential difference across the 80 ohm resistor: V = R × I, so the potential difference is 80 Ω  × 0.1 A.

The potential difference across the 80 ohm resistor is equal to 8 volt.

Teacher: So what is the potential difference across the 40 ohm resistor?

Sam: Is it 4 volt?

Teacher: Why do you think that?

Sam: Because 8 from 12 volt leaves 4 volt.

Teacher: Yes! That's right. And why does that make good sense looking at the values of resistance?

The final point here is that there is twice the potential difference across the 80 ohm resistor compared with the 40 ohm resistor. In other words, the potential difference is shared in proportion to the resistance values.

Teacher: When you become expert at these calculations, like me(!), you'll be able just to glance at values such as these and give the answer straight away. Of course, the numbers aren't always as easy as this.

### Discussing the calculations

Circuit 2: two resistors in parallel

Question: What is the current in the supply?

In answering a question such as this, it's a good idea to talk through the overall approach with your class, making clear all of the key points:

• With two resistors in parallel the current in the supply is the current in both loops.
• The total current in the supply is therefore the sum of the currents in each resistor.
• In a parallel circuit, each resistor has the full battery potential difference across it.
The current in each of the resistors can therefore be calculated using I = VR for each loop in turn, so I1 = VR1 and I2 = VR2.

Teacher: So, how do we calculate the current in the 48 ohm resistor?

John: From the voltage and resistance?

Teacher: And what is the voltage?

Will: 12 volt.

Teacher: Exactly! We can find the current in the 48 ohm resistor using the connection between I, V and R.

Using I1 = VR1, in this case 12 volt48 ohm, so the current works out at 0.25 A.

In the same way, the current through the 60 ohm resistor is calculated, Using I2 = VR2, in this case 12 volt60 ohm, so the current works out at 0.20 A.

The total current in the supply must therefore be the sum of these values: 0.25 ampere and 0.2 ampere. The current in the supply is 0.45 ampere.

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