V=dW/dQ
Electricity and Magnetism

Episode 105: Sources of energy in circuits

Lesson for 16-19 IOP TAP

It is worth discussing energy transfer in electric circuits and linking this by analogy to other more familiar examples.

Lesson Summary

  • Demonstrations: Human and lemon batteries (10 minutes)
  • Discussion: Energy and work in an electric circuit (10 minutes)
  • Discussion: Quantitative energy transfers (10 minutes)
  • Student questions: Practice with the ideas (30 minutes)

Demonstration: Human and lemon batteries

Two fun demonstrations showing that there is nothing special about the chemical substances that are needed to make a battery. The limitation is, of course, the high internal resistance of the cells.

Episode 105-1: The human battery (Word, 24 KB)


Episode 105-2: Making electricity (Word, 27 KB)


Discussion: Energy and work in an electric circuit

Show a cell connected to a lamp. The idea to get across is that charge carriers are pushed around a circuit by the electromotive force (EMF) of the cell. The charge carriers are rather like water in a hydroelectric power station – they do work (e.g. in the lamp) just as the flowing water does work in the turbo-generators. Neither the charge nor the water is used up but the current transfers energy. In the power station, water moves from behind a raised dam to a lower level. In an electric circuit the charge falls from high electrical potential to lower electrical potential.

This can lead to the idea that a cell provides a potential difference and that charges move around the circuit from higher to lower potential (beware of signs here – negative charges fall from  −  to  +  whilst positive charges would fall the other way!). The greater the vertical drop in the hydroelectric station the greater the change in energy per kilogram of water stored gravitationally. In a similar way, the higher the EMF across a power supply the greater the change in energy per coulomb of charge moving between its terminals stored electrically.

Discussion: Quantitative energy transfers

The volt is defined as the energy transfer per coulomb of charge as charges move between two points in a circuit.

V =  Δ E Δ Q

i.e. energy change per unit charge (so that 1 V = 1 J C-1 )

Introduce the terminology of electromotive force (voltage across an power supply) and potential difference (voltage across a component in which electrical work is done). Stress that, despite its name, EMF is not a force but a voltage, measured in volts.

Kirchhoff’s second law comes later, but there is no harm in preparing the way here. They will be familiar with the concept of energy conservation and this can be applied to a single charge carrier (or more simply 1 C) as it is followed around any closed loop in a circuit. The essential idea is that the total change in energy stored (e.g. chemically in a cell) equals the total change in energy transferred by components around any loop (leading to sum of EMFs = sum of pds).

This leads on to the basic principle behind the chemical cells shown in the demonstrations. Different metals have different affinities for electrons. This pushes electrons from one to the other through the intervening electrolyte. The accumulation of charge on the cell terminals provides the push that drives charge carriers around the external circuit. Large EMF can be obtained by connecting cells in series. Larger currents can be drawn if they are connected in parallel.

Discuss the energy per coulomb from the human and lemon batteries and compare it with familiar AA cells.

The idea that EMF is energy per coulomb leads to the idea that more charge must pass through the cell to increase the energy transferred to the circuit.

Δ E = V ×  Δ Q

Student questions: Practice with the ideas

Episode 105-3: Measuring potential difference (Word, 40 KB)


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