Episode 120: Energy transfer in electric circuit

This episode revises the idea of EMF, and introduces the idea of internal resistance and consequent energy losses.

Summary

  • Demonstration and discussion: Revising ideas about energy in circuits (20 minutes)
  • Student experiment: Leading to the idea of 'lost volts' (30 minutes)
  • Demonstration: (Optional) Drop in terminal PD of a source on load (5 minutes)

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

Episode 120-1: Energy transfer from a chemical cell (Word, 24 KB)

Battery driven torch

Use any suitable battery-driven device as a prop - e.g. toy car, MP3 player, torch - and ask the class a number of questions. (For the sake of illustration, we have assumed a supply voltage of 3.0 V from two AA cells, but this should be adapted to the particular example used.)

Some questions to ask:

  • What is the main energy transfer going on in the batteries? (Chemical to electrical.)
  • This is powered by two 1.5 V AA cells in series - what is the supply voltage? (3.0 V)
  • What is the EMF of the supply? (3.0 V - 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 replaced; why? (Steer them away from the idea that it ‘runs out of charge’ - you are looking for an answer based on energy transfer – the chemical reaction that allows the battery to do work on charge has completed – like burning fuel.)
  • What determines how quickly it runs down? (Look 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? (Use two pairs of two AA batteries in parallel - effectively more ‘chemical fuel’ but running at the same voltage so each cell provides half the previous power and current and lasts twice as long. This is unwise in practice though, if one pair of cells runs down faster than the other current can be forced backwards through the cells and they get very hot.)
  • What determines how much current is drawn from the supply? (Load resistance - the lower the load resistance the greater the current and power and the sooner the battery needs to be replaced.)

Now 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. Since the cells themselves are made of material 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 energy are the same ones that are responsible for the cell’s internal resistance (this term can be introduced quite naturally through discussion) it is convenient to separate these two roles so that they end up with an idea of energy transfers as summarised in this diagram.

Energy transfers in electric circuit

Hence we represent a ‘real’ cell as two components in a circuit diagram:

  • an ‘ideal’ cell, with no internal resistance, and
  • a small resistor, representing the internal resistance.

It can make things clearer if you draw a box or circle around these two, to indicate that they are aspects of the same component.

Energy is conserved, so the energy transferred from chemical to electrical by the EMF of the cell must ‘pay’ for the energy transferred to other forms in the circuit - this is the sum of the energy transferred in the cell itself and the energy transferred by the external load.

Clearly the energy transferred to heat in the cell is not doing useful work and so is in a sense ‘lost’. This sets up the idea of ‘lost volts’ that will be referred to later.

Student experiment: Leading to the idea of ‘lost volts’
This could be carried out as a demonstration but, if resources are available, the point comes across more strongly as a quick class practical. It can be advisable to do these experiments using rechargeable cells otherwise it can be quite expensive! However, rechargeable cells have a very low internal resistance so the experiment doesn’t work well. One could always cheat and place a rechargeable cell in a box with a series resistor.

Episode 120-2: Loading a battery (Word, 24 KB)

The idea is simple. Students add lamps in parallel to a suitable battery. Introduce the idea of terminal voltage (terminal pd) and get them to measure it. They could also measure the current drawn 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 the number of lamps in parallel is increased.

Can they explain what is happening? Draw 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 ‘push charge’ through the internal resistance of the cell. As the current I increases, the PD Ir 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 ‘lost voltage’ across the internal resistance of the battery, given by Ir.
  • The more current that is drawn from the battery the larger the ‘lost voltage’.

Class discussion after the experiment should prepare the way for the theory session which follows. It is worth pointing out that the EMF of a cell can be measured by connecting a high resistance voltmeter directly across its terminals (‘high’ here means high compared to the internal resistance of the cell).

Demonstration: (optional) Drop in terminal PD of a source on load (5 minutes)
This shows the effect of a high internal resistance and an EHT supply.

Episode 120-3: Drop in terminal pd of a source on load (Word, 26 KB)

Download this episode
Episode 120: Energy transfer in electric circuit (Word, 64 KB)