What do voltages measure?
Teaching Guidance for 11-14
A puzzling quantity
Thinking about the learning
Thinking about voltage either in terms of size of push or in terms of a second factor (as well as the current) that sets the power switched by an element in a circuit (see Physics Narrative) is likely to make intuitive good sense to pupils at this stage in the teaching. Batteries of greater voltage provide a bigger push, which leads to more power dissipated in the circuits that they're a part of.
Thinking about the teaching
Why are voltage measurements important? What kind of information do they provide? What do voltage measurements tell us that is different from measurements of current?
These are important questions to be addressed in teaching. The short answer to all of them is that voltage measurements provide an
energy picture of the electric circuit. Whilst current measurements tell us about the flow of charge (coulomb per second), voltage measurements provide information about power being dissipated by that charge (watt per ampere) in different parts of the circuit.
We do think that it's important not to try and go too far with younger children. Voltage is a hard idea.
We do think the use of the rope loop can help here. This is because pupils can act out what is happening in the circuit with the rope. As the rope passes through their hands, so their hands warm up, and this neatly models the shifting of energy to a thermal store by electrical working. The power dissipated is a function of two factors: the flow of the rope (current) and how hard you grip (voltage).
The more firmly the pupils grasp the rope, the warmer their hands become for each metre of rope that passes through their hands. The voltage is a similar signal, albeit electrical. It functions in a similar way to the firmness of the pupils' grasp: the larger the voltage, the more power for each ampere. (More precisely, the frictional force impeding the flow of rope is an exact analogue of the voltage.)
Because what pupils do with their hands enables them to predict where the energy will end up being shifted in the rope loop, we call this a teaching model. It's a model because it has a predictive power. The structure of this model is the same as the electric circuit model – that's why it's such a powerful heuristic tool.
Different treatments of the voltage/current and energy/power story
It is possible to introduce the idea of voltage at different levels of difficulty that can be used with pupils of different age and ability. Going into the idea in any detail is probably best done through thinking about power. This is the approach taken in the SPT: Electricity and energy topic.
At the phenomenological level, the battery voltage sets the power for each ampere. A larger battery results in brighter bulbs, because:
- There is more current in each bulb.
- There is more voltage across each bulb.
The accumulation of all this electrical working is that energy is shifted to and from stores.
The voltage compares the change of energy in different stores, predicting or measuring the comparative quantities of energy shifted by different parts of the circuit. But of course these changes accumulate over time – the longer the time for which the circuit runs, the greater the energy change. The power remains constant, so long as the circuit is functioning.
At a more advanced level, the battery voltage is taken as a measure of the number of joules of energy shifted for each passing coulomb of charge. The voltage across a bulb or some other circuit resistance is taken as a measure of the number of joules of energy shifted by each coulomb of charge as that much charge passes through that component.