Electrical charge carriers: current as the flow of charge, the relationship between charge, current and time, the role of electrons as charge carriers, and how current divides in series and parallel circuits.

What this key area is asking

The SQA wants you to describe electric current as the flow of charge, use the relationship $Q = It$, identify electrons as the charge carriers in a metal, and state how current behaves in series and parallel circuits.

Current and charge carriers

Electrons carry a negative charge, so they actually flow from the negative terminal of a supply to the positive terminal. By an old convention, "conventional current" is drawn from positive to negative, the opposite way. At National 5 you should know that the real carriers in a metal are electrons.

The charge, current and time relationship

One coulomb is the charge that passes when a current of one ampere flows for one second. The most common error is leaving the time in minutes, so always convert to seconds first. You can rearrange the relationship to find any of the three quantities: $I = \frac{Q}{t}$ or $t = \frac{Q}{I}$.

Current in series and parallel

This follows from the conservation of charge: charge is never used up, so whatever flows into a junction must flow out of it. A bulb in a series string carries the full current; bulbs in parallel branches share the supply current between them. Adding more parallel branches lets more total current flow from the supply.

Conductors and insulators

A conductor (such as copper, gold or any metal) has free electrons, so it carries current easily and has a low resistance. An insulator (such as plastic, glass or rubber) has no free charge carriers, so it does not carry current and has a very high resistance. This is why wires are made of metal but covered in plastic for safety.

When a voltage is applied across a conductor, the free electrons all feel a push and drift slowly in one direction, even though they are moving randomly at high speed all the time. It is this slow overall drift that makes up the current. A larger voltage gives the electrons a bigger push, so more charge passes each second and the current is larger. Cutting the circuit (opening a switch) leaves a gap that the electrons cannot cross, so the current stops everywhere at once.

The size of a current tells you how much charge passes each second: a current of $3 \text{ A}$ means $3 \text{ C}$ of charge passes every second. This is why current and charge are so closely linked through $Q = It$, and why an ammeter (which reads current) and the charge that has flowed are two ways of describing the same flow of electrons.

Try this

Q1. Name the charge carriers in a metal wire. [1 mark]

• Cue. Free electrons.

Q2. A current of $2.0 \text{ A}$ flows for $30 \text{ s}$. Calculate the charge that passes. [2 marks]

• Cue. $Q = It = 2.0 \times 30 = 60 \text{ C}$.

Q3. State how the current behaves at every point in a series circuit. [1 mark]

• Cue. It is the same at every point.