KS3 current, voltage and resistance: a guide for parents

Current, voltage and resistance, explained

An electric current is a flow of electric charge around a complete circuit. In a metal wire the charges that move are electrons. Current is measured in amperes (amps, symbol A) using an ammeter, which is connected in series, meaning in line with the component being measured so the current passes through it. The current is the same amount of charge passing each point every second. It is not used up: the charge that leaves the battery is the same as the charge that returns to it, and what is transferred to a bulb or motor is energy, not charge. By convention, circuit diagrams show current flowing from the positive terminal round to the negative, the opposite way to the electrons.

Pupils compare two ways of wiring components. In a series circuit there is a single loop, so the current is the same everywhere in it. Adding more bulbs makes each one dimmer, and if one bulb fails the whole circuit goes off. In a parallel circuit the current splits at a junction and rejoins later. The currents in the branches add up to the current coming from the battery, so where branches meet the currents add. If one branch is broken the others keep working, which is why the lights in a house can be switched on and off independently.

Potential difference (p.d.), measured in volts (V), tells you how much energy each unit of charge transfers between two points. In everyday terms it is how hard the cell or battery pushes the current round, or how much energy is transferred to a component. A voltmeter is connected across a component, in parallel with it. Pupils read ratings such as a 1.5 V cell or a 6 V bulb, and learn that cells joined in series add their voltages: three 1.5 V cells give 4.5 V. In a series circuit the p.d. is shared between the components, while in a parallel circuit each branch gets the full battery p.d.

Resistance, measured in ohms (symbol Ω), says how hard it is for current to flow through a component. It is defined as the ratio of potential difference to current: resistance = potential difference ÷ current, or R = V ÷ I. For the same p.d., a component with a large resistance lets only a small current flow. Pupils measure p.d. and current for a lamp or resistor and calculate its resistance, and rearrange the equation as V = I × R and I = V ÷ R. Conductors such as copper have very low resistance, while insulators such as plastic, rubber and dry air have such a high resistance that almost no current flows. The difference is huge, not slight: a short copper connecting wire has a resistance of a small fraction of an ohm, while a plastic rod of a similar size has a resistance of millions of ohms or far more, so with a 6 V battery the current through it would be far too small to measure with a school ammeter.

Teachers stress units and symbols: current is I in amps, p.d. is V in volts and resistance is R in ohms. They also stress that school practicals use cells and low-voltage supplies only. Pupils often use an analogy, such as water pumped round a closed pipe loop, to picture a circuit, but analogies have limits and a good answer uses the correct terms: current flows through a component and p.d. is across it.

Words your child will hear

Current

The flow of electric charge round a circuit, measured in amperes (A).

Potential difference (p.d.)

The energy transferred per unit of charge between two points, measured in volts (V).

Resistance

How much a component opposes current, measured in ohms (Ω). R = V ÷ I.

Series circuit

A circuit with one loop, where the same current flows through every component.

Parallel circuit

A circuit with branches. The current splits at a junction and the branch currents add up.

Ammeter and voltmeter

An ammeter measures current and goes in series. A voltmeter measures p.d. and goes across a component.

Conductor and insulator

A conductor has low resistance and lets current flow easily. An insulator has very high resistance.

What the National Curriculum says

The programme of study for Key Stage 3 says:

• Pupils should be taught about electric current, measured in amperes, in circuits, series and parallel circuits, currents add where branches meet and current as flow of charge
• Pupils should be taught about potential difference, measured in volts, battery and bulb ratings; resistance, measured in ohms, as the ratio of potential difference (p.d.) to current
• Pupils should be taught about differences in resistance between conducting and insulating components (quantitative)

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Common mistakes and how to help

How to help at home

• Build a simple circuit. With a torch bulb in a holder, a cell and some wires, make the bulb light. Then add a second bulb in series and notice it is dimmer, and try it in parallel and see the difference. Use batteries only, never mains electricity.
• Conductor or insulator? Add a gap in the circuit and bridge it with household objects such as a coin, foil, a wooden spoon, a plastic lid and a paper clip. Predict first, then sort them. Never connect a wire straight across a battery with nothing in between, because it can get hot.
• Act out a circuit. Sit family members in a ring and pass a bean bag round, one person at each point. The number passing per minute is the current. Add a slow person to show resistance, or two parallel paths to show current splitting.
• Read the labels. Look at the ratings on cells, torches and chargers, and ask what the volts and amps mean. UK mains electricity is about 230 V, which is far more dangerous than the cells used in school. Look at the labels but never open or poke anything connected to the mains.
• Quick calculations. Make up quick questions for each other. A component on a 6 V supply has a current of 2 A through it. What is its resistance? Check the answer is 3 Ω. A short daily question builds the habit of writing the equation and the units.

Related topics

Everything in Key Stage 3 science