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Episode 120: Energy transfer in an electric circuit

This episode revises the idea of emf, and introduces the idea of internal resistance andconsequent energy losses.

Summary

Demonstration and discussion: Revising ideas about energy in circuits. (20 minutes)

Student eperiment: !eading to t"e idea of #lost volts$. (%0 minutes)

Demonstration: (&ptional) Drop in terminal pd of a source on load (' minutes)

Demonstration and discussion:

Revising ideas about energy in circuits

Here you have the opportunity to check and reinforce students basic ideas about current, voltageand energy in electrical circuits. This could be truncated or skipped with an able group.

TAP !"#$ %nergy transfer from a chemical cell

&se any suitable battery#driven device as a prop # e.g. toy car, mp' player, torch # and ask theclass a number of questions. ()or the sake of illustration, we have assumed a supply voltage of'." * from two AA cells, but this should be adapted to the particular e+ample used.

-ome questions to ask$

hat is the main energy transfer going on in the batteries/ (0hemical to electrical.

This is powered by two .1 * AA cells in series # what is the supply voltage/ ('." *

hat is the emf of the supply/ ('." * # remind them that emf is the name given to the

potential difference across the energy supply when it is not loaded.

After a while the battery needs to be replaced2 why/ (-teer them away from the idea that

it 3runs out of charge # you are looking for an answer based on energy transfer 4 the

chemical reaction that allows the battery to do work on charge has completed 4 likeburning fuel.

hat determines how quickly it runs down/ (5ook for answers linked to power, the rate at

which energy is transferred and link this to current drawn and supply voltage.

How could the design be modified so it runs for longer before batteries need replacing/

(&se two pairs of two AA batteries in parallel # effectively more 3chemical fuel but runningat the same voltage so each cell provides half the previous power and current and laststwice as long. This is unwise in practice though, if one pair of cells runs down faster than

http://var/www/apps/conversion/tmp/scratch_5/TAP120-1-Energy-transfer.dochttp://var/www/apps/conversion/tmp/scratch_5/TAP120-1-Energy-transfer.doc

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the other current can be forced backwards through the cells and they get very hot.

hat determines how much current is drawn from the supply/ (5oad resistance # the

lower the load resistance the greater the current and power and the sooner the batteryneeds to be replaced.

6ow open up the discussion to include the energy transfer within the cells. Point out that current

flows through all parts of the circuit, including the cells. -ince the cells themselves are made ofmaterial with resistance there will be some energy lost in the cell. This will increase with current,so there is more loss at higher currents. Although the chemicals that provide the electrical energyare the same ones that are responsible for the cells internal resistance (this term can beintroduced quite naturally through discussion it is convenient to separate these two roles so thatthey end up with an idea of energy transfers as summarised in the diagram below$

Hence we represent a 3real cell as two components in a circuit diagram$

an 3ideal cell, with no internal resistance, and

a small resistor, representing the internal resistance.

7t can make things clearer if you draw a bo+ or circle around these two, to indicate that they areaspects of the same component.

%nergy is conserved, so the energy transferred from chemical to electrical by the emf of the cellmust 3pay for the energy transferred to other forms in the circuit # this is the sum of the energytransferred in the cell itself and the energy transferred by the e+ternal load.

!

%lectrical energyto heat$ 7nternalresistance r

0hemical energy to electricalenergy$ 0ell of emf E

%lectrical energy to work and heat$e+ternal load resistance R

E!!

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0learly the energy transferred to heat in the cell is not doing useful work and so is in a sense3lost. This sets up the idea of 3lost volts that will be referred to later.

Student eperiment:

!eading to t"e idea of #lost volts$

This could be carried out as a demonstration but, if resources are available, the point comesacross more strongly as a quick class practical. 7t can be advisable to do these e+periments usingrechargeable cells otherwise it can be quite e+pensive8 However, rechargeable cells have a verylow internal resistance so the e+periment doesnt work well. 9ne could always cheat and place arechargeable cell in a bo+ with a series resistor:

TAP !"#!$ 5oading a battery

The idea is simple. -tudents add lamps in parallel to a suitable battery. 7ntroduce the idea ofterminal voltage (terminal pd and get them to measure it. They could also measure the currentdrawn from the cell, although this is less important.

Ask them to record the terminal voltage of the battery with no lamps connected and then as thenumber of lamps in parallel is increased.

0an they e+plain what is happening/ ;raw out the following ideas$

As the number of parallel lamps increases, the terminal voltage drops and the current

increases.

This is because energy is used to 3push charge through the internal resistance of the cell.

As the current 7 increases, the pd 7r increases.

The terminal voltage of the cell is equal to only its emf when no current is drawn (no

lamps attached.

The difference between the battery emf and its terminal voltage is a 3lost voltage across

the internal resistance of the battery, given by 7r. The more current that is drawn from the battery the larger the 3lost voltage.

0lass discussion after the e+periment should prepare the way for the theory session whichfollows. 7t is worth pointing out that the emf of a cell can be measured by connecting a highresistance voltmeter directly across its terminals (3high here means high compared to the internalresistance of the cell.

Demonstration: (&ptional)

Drop in terminal pd of a source on load (' minutes)

This shows the effect of a high internal resistance and an %HT supply.

TAP !"#'$ ;rop in terminal pd of a source on load

'

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*+ 120,1: Energy transfer from a c"emical cell

*pparatus re-uired:

Any battery#powered device (e.g. toy car

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*+ 120, 2: !oading a battery

*pparatus re-uired:

&p to si+ > * lamps

> * battery pack

@ultimeter (voltmeter

9ptional second multimeter (ammeter

5eads (enough to connect all the lamps in parallel

+rocedure

. 0onnect the voltmeter to the battery pack and measure its terminal voltage.

!. epeat with lamp connected across the battery.

'. epeat with !, ' ...etc... lamps in parallel.

?. ecord the voltage and number of lamps.

9PT796A5 # use a second meter to measure the current drawn from the supply as , !, '... etc...lamps are added in parallel.

+ractical advice

6B -witch the circuit on only when you intend to make measurements (otherwise, with manylamps in parallel and large currents flowing, the cells will quickly run down.

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*+ 120, %: Drop in pd of a source on load

&ptional demonstration

Re-uirements

%HT power supply, "#1 k* dc with voltmeter built in

" mA dc ammeter

leads, ?mm, (6.B. no side screws in plugs

Set,up:

-witch on the power supply and set to """ * with no equipment attached.

-witch off the supply and add the " mA ammeter

-witch on and observe the voltmeter on the supply and the ammeter.

-witch off.

)rom your observations what is siCe of the internal resistance of the supply/

-uggest why it is constructed in this way.

+ractical advice

This demonstration should not be laboured. The aim is to show how real power supplies canbehave when a current is drawn. Take care when using %HT.

Eternal references

This activity is an adaptation of evised 6uffield Advanced Physics e+periment BD.

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