Volts, amps and joules

The voltmeter

As you have seen, voltage is a measure of electrical energy; its proper name is potential difference.

The voltmeter is therefore connected in parallel with the bulb as you see in the circuit diagram (Figure 1), it measures the difference in the energy of the electricity between points A and B, that is before and after it has passed through the bulb.

At A the electricity has a lot of energy but at B most of this energy has been changed into heat and light in the bulb.

Current and voltage

It is often easy to confuse current with voltage and so it is a good idea to remember the following:

It is therefore possible to have two wires carrying the same current but one with much more energy than the other.
The two circuits in Figure 2 explain this.

The low voltage bulb has a current of 1 A flowing through it, it runs on 5 V and gives out energy as heat and light.
However the mains bulb runs on 240 V and also has a current of 1 A flowing through it. It gives out a lot more energy and so is much brighter.

The amount of electricity passing through each bulb per second (the current) is the same, but the electric charges passing through one bulb have more energy than those passing through the other one.

The volt

Energy is measured in joules and so we need to know the connection between volts and joules.

In other words if the potential difference between two points is 12 V then every coulomb of electricity passing from one point to the other loses 12 J of energy.
If we think about the input of energy, then a 240 V supply will give each coulomb 240 J of energy while a 5 V supply only gives each coulomb 5 J.

We can write these ideas in a formula as:


These mathematical examples clearly explain the results with the two bulbs.
Another way of thinking about energy is the length of time that a certain battery will supply a certain amount of current. This is usually written in Ampere hours (Ah).

Energy capacity of a battery

If a battery is labelled 12V 48 Ah it means that it can supply 48A for 1 hour at 12V, or 24 A for 2 hours at 12V or 1 A for 48 hours at 12V and so on.

The energy stored in a 12V 48 Ah battery is = 12x48x3600 = 2.074 MJ

The energy stored in a 6V 48 Ah battery would be half that = 6x48x3600 = 1.037 MJ

A car battery will have a capacity of between 30 and 50 Ah while a AAA torch battery is usually around 700mAh.

The photograph shows two car batteries, the larger one has a capacity of 32 Ah while the smaller one is 16 Ah.

Problems

Copy and complete the following table:

10	Voltage	Charge	Current	Time	Energy
1	12V		2 A	20s
2	2V		3 A	2 min
3	6V		2 A		240 J
4	20V		100 mA	½ hour
5	100 mV	200 C		20s
6		space	2A	60 s	600 J
7		20 C		5 s	10 kJ
8	1000V	space	0.5 kA	10 s
9	2 kV	120 C		2 ms
10	space	150 C	space	7.5 s	20 kJ