What you'll learn

• What a unit is, and why physics answers need both a number and a unit.
• How to use ampere (A) for current, volt (V) for voltage, and watt (W) for power.
• How these units connect to charge, energy and time.
• How to avoid common unit mistakes in calculations.

Why units matter

In physics, a measurement is not complete unless it has a number and a unit.

For example, writing “12” on its own is not enough. It could mean 12 V, 12 A, 12 W, 12 N, or something else entirely.

A physical quantity is something that can be measured, such as current, voltage, power, energy, time or force.

In this part of Edexcel IGCSE Physics 4PH1 section 6.1, you need to use three electrical units correctly:

• ampere (A) for current
• volt (V) for voltage or potential difference
• watt (W) for power

This diagram links the three units to the ideas behind them.

Ampere (A): the unit of current

Electric current is the rate of flow of electric charge.

The relationship between charge, current and time is:

charge = current × time

$$Q=I\times t$$

where:

• $Q$ is charge in coulombs, C
• $I$ is current in amperes, A
• $t$ is time in seconds, s

You can rearrange it to find current:

$$I=\frac{Q}{t}$$

So an ampere is really a “coulomb per second” unit:

$$1A=1C/s$$

In electromagnetism, current is especially important because a current in a wire or coil can produce a magnetic field.

Volt (V): the unit of voltage

Voltage is also called potential difference. It is linked to the energy transferred by each coulomb of charge.

The useful relationship is:

energy transferred = charge × voltage

$$E=Q\times V$$

where:

• $E$ is energy transferred in joules, J
• $Q$ is charge in coulombs, C
• $V$ is voltage in volts, V

You can rearrange it to find voltage:

$$V=\frac{E}{Q}$$

So a volt can be thought of as a joule per coulomb:

$$1V=1J/C$$

A higher voltage means each coulomb of charge transfers more energy as it moves through a component.

Watt (W): the unit of power

Power is the rate at which energy is transferred.

The general relationship is:

power = energy transferred ÷ time taken

$$P=\frac{E}{t}$$

where:

• $P$ is power in watts, W
• $E$ is energy transferred in joules, J
• $t$ is time in seconds, s

So:

$$1W=1J/s$$

For electrical devices, another very useful relationship is:

power = current × voltage

$$P=I\times V$$

This is the same as writing $P=V\times I$ because multiplication can be done in either order.

Keeping the symbols separate

Physics uses symbols for both quantities and units, and they are not always the same thing.

For example:

• Current has quantity symbol $I$ and unit symbol A.
• Voltage has quantity symbol $V$ and unit symbol V.
• Power has quantity symbol $P$ and unit symbol W.

The letter V can therefore mean two things depending on where it appears. In the equation $P=I\times V$, $V$ means the quantity voltage. In the answer 12 V, V means the unit volt.

Unit style in exam answers

Use unit symbols carefully:

• Write A, V and W as capital letters.
• Do not pluralise symbols: write 5 W, not 5 Ws.
• Put a space between the number and the unit: write 12 V, not 12V.
• Use the correct unit for the quantity being asked for.

Sometimes power is given in kilowatts, kW.

$$1kW=1000W$$

So 2.5 kW means 2500 W. If an equation uses joules and seconds, convert kW into W first.