Define current, voltage (potential difference) and resistance, state their units, and explain how they relate in a simple circuit

What this dot point is asking

SEAB wants you to define the three core electrical quantities - current, voltage (potential difference) and resistance - to state their units, and to explain how they describe what is happening inside a circuit. The central insight is that current is a flow of charge, voltage is the push that drives that flow and the energy it carries, and resistance is the opposition to the flow. Everything else in Electronics is built on these three ideas.

The answer

Current is the rate of flow of charge

Electric current is the rate at which electric charge flows past a point in a circuit:

where $Q$ is charge in coulombs ($\text{C}$) and $t$ is time in seconds. The unit of current is the ampere ($\text{A}$): one ampere is a flow of one coulomb per second. In a metal wire the moving charges are free electrons.

Conventional current

By long-standing convention, current is drawn flowing from the positive terminal of the supply, round the external circuit, to the negative terminal. This conventional current is opposite to the direction the electrons actually drift, but every circuit rule in the syllabus is stated in terms of conventional current, so you always work with it.

Voltage is energy per unit charge

Potential difference, usually called voltage, across a component is the electrical energy transferred per unit charge as charge passes through it:

The unit is the volt ($\text{V}$): one volt is one joule per coulomb. A cell provides an electromotive force that pushes charge round the circuit; as the charge passes through components, it transfers that energy, so the voltage across each component tells you how much energy each coulomb gives up there.

Resistance is opposition to current

Resistance is the opposition a component offers to the flow of charge. The unit is the ohm ($\Omega$). For a given voltage, a larger resistance allows less current. Resistance arises because moving charges collide with the atoms of the material, transferring energy to them as heat. The formal link between the three quantities is Ohm's law, covered in its own dot point.

Putting the three together

A useful picture is water in a pipe: voltage is the pressure pushing the water, current is the rate at which water flows, and resistance is how narrow the pipe is. A bigger push (voltage) gives more flow (current); a narrower pipe (resistance) gives less flow. This is why the three quantities are always discussed together.

Examples in context

Example 1. A car indicator bulb. When a $12\ \text{V}$ supply drives $0.5\ \text{A}$ through an indicator bulb, each coulomb of charge transfers $12\ \text{J}$ of energy as it passes through the filament, heating it until it glows. The current is the same in the wire and the bulb because they are in one series loop, and the resistance of the cold filament sets how much current flows at switch-on.

Example 2. A potentiometer volume control. Turning the volume knob on a radio changes a variable resistance. Increasing the resistance reduces the current to the speaker for the same supply voltage, which lowers the energy delivered each second and so reduces the loudness. This shows the three quantities working together in a real control.

Try this

• Cue. Define resistance and state its unit. Resistance is the opposition of a component to the flow of electric current; its unit is the ohm ($\Omega$). A higher resistance allows less current for the same voltage.

• Cue. A wire carries a current of $2.0\ \text{A}$ for $5.0\ \text{s}$. Find the charge that passes. Use $Q = It = 2.0 \times 5.0 = 10\ \text{C}$.

• Cue. Explain why current is the same at every point in a simple series circuit. Charge is not created or destroyed and has only one path, so the rate of flow of charge is identical everywhere in the loop.