Episode 105: Sources of electrical energy

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

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)

Demonstrations: 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 it does lose potential energy. In the power station, water loses gravitational potential energy by moving from higher GPE to lower GPE. In an electric circuit the charge ‘falls’ from high electrical potential energy to lower electrical potential energy.

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 potential energy per kilogram of water. In a similar way, the higher the EMF across a source of electrical energy the greater the change in potential energy per coulomb of charge moving between its terminals.

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 = ΔW / ΔQ

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

Introduce the terminology of electromotive force (voltage across a source of electrical energy) and potential difference (voltage across a component that uses electrical energy). 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 energy supplied equals the total energy used 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 supplied per coulomb leads to the idea that more charge must pass through the cell to increase the energy delivered to the circuit.

ΔW = V ΔQ

Student questions: Practice with the ideas
Episode 105-3: Measuring potential difference (Word, 40 KB)

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Episode 105: Sources of electrical energy (Word, 66 KB)