Batteries and voltage
Physics Narrative for 11-14
Batteries come in many shapes and sizes, often of different voltage
The batteries commonly used in school science practical work are torch batteries rated at 1.5 volt. If two of these batteries are connected into a circuit one after the other (in series), the total rating is 3.0 volt. If three batteries are used we have 4.5 volt and so on. A single 9 volt battery might be used to supply a radio, whilst a 12 volt battery is used in cars.
A key specification for any battery is its voltage, and this is what we need to know when buying a battery in a shop. But what exactly does the voltage tell us?
There are two ways of thinking about voltage. These are, in fact, two ways of telling the same story and relate directly to ideas introduced in earlier episodes.
Voltage as an electrical
Firstly you can think of voltage as being a measure of the size of the force or
push provided by a battery on the charged particles in a circuit. The idea of a battery providing a push to set charged particles in motion was first introduced in episode 01. Furthermore, we saw in episode 02 that adding an extra battery to a circuit (or using a battery with a higher voltage) provides a bigger push, moving charged particles around more quickly and increasing the electric current.
Sometimes the battery voltage is referred to as the battery e.m.f., which stands for electromotive force. This term captures the idea of the battery providing a force or push on all of the charged particles in the circuit.
Thinking about voltage as a push links up closely with the rope loop model, in modelling a larger battery is effected by pulling the rope around with a greater force.
Voltage in terms of power: energy shifted in each second
Whilst it makes sense to refer to voltage as being a measure of the push of a battery, it makes little sense at all to talk about voltage as being a measure of the push of a bulb. However the impeding caused by the resistance does provide an opposing effect to the
push of the battery (again, you'd model this by gripping the rope loop).
Here we need to turn to the alternative view of voltage as a measure of the power dissipated by each current in different parts of the circuit.
More precisely, it is a measure of:
- The amount of power per unit current by the battery (energy shifted from the chemical store in a battery as charge flows through the battery).
- The amount of power per unit current to the surroundings in any circuit resistance (energy shifted to thermal store in surroundings as charge flows through resistance).
If the circuit is to conserve energy (it must!), then the voltage across the battery and across the bulb must be equal, as the power input and output must be equal.