Circuit Builder

Three quantities in a loop

A circuit is just a loop that lets charge flow round, and almost every circuit question comes down to three quantities and how they share out. CURRENT is the flow of charge, measured in amperes. POTENTIAL DIFFERENCE is the push that drives that flow, measured in volts. RESISTANCE is whatever opposes the flow, measured in ohms. ⭐⭐ These three are tied by a single equation: potential difference equals current times resistance. Rearrange it and you can find any one of the three from the other two, so it is the workhorse of the whole topic. ⭐ A bigger push, more potential difference, drives a bigger current. More resistance opposes the flow, so for the same push the current is smaller. That is the whole behaviour of a simple component in one sentence. ⭐⭐ What changes from circuit to circuit is how the quantities SHARE OUT. In a SERIES loop there is only one path, so the current is the same at every point, and the potential difference is shared between the components. In PARALLEL there are separate branches, so each branch gets the full potential difference, and the current splits between the branches. This module is about the circuit itself. Mains electricity, the plug wires and the national grid belong to a different module. ⚠️ Carry one question the whole way through: is this a series loop or parallel branches, and what does the equation give me?

Current, push, and opposition

Each column is one of the three quantities: what it is, its unit, and how a series or parallel circuit treats it.

Tap the two true of a series loop

Tap the TWO statements that are TRUE of a simple SERIES circuit.

• The current is the same at every point around the loop.
• The potential difference is shared between the components.
• The current splits between separate branches.
• Every component gets the full potential difference of the supply.

The current in a series loop

In a simple series circuit with two bulbs, what is true of the current?

• It is the same at every point in the loop
• It splits in half between the two bulbs
• It is zero everywhere
• It grows larger after each bulb

The language of circuits

Six terms this topic turns on. The first three are the quantities; the rest are the circuit types and a unit.

Match each to what it means

Two true of a parallel circuit

Select the TWO statements that are TRUE of a PARALLEL circuit.

• Each branch gets the full potential difference of the supply
• The current from the supply splits between the branches
• The current is the same in every branch and the supply
• The potential difference is shared out between the branches

Answering a circuit question

Circuit questions reward knowing the sharing rules and using the equation cleanly. ⭐⭐ DECIDE SERIES OR PARALLEL FIRST. In series, the current is the same everywhere and the potential difference is shared between components. In parallel, each branch gets the full potential difference and the current splits. Almost every answer starts by naming which one you have. ⭐ USE THE EQUATION BOTH WAYS. Potential difference equals current times resistance. To find a resistance, divide the potential difference by the current; to find a current, divide the potential difference by the resistance. Rearranging it is where most marks are won or lost. ⭐⭐ REMEMBER WHAT MORE RESISTANCE DOES. For the same push, more resistance means a smaller current. So adding resistors in series increases the total resistance and cuts the current, while adding branches in parallel gives the current more paths and increases the total current. MEASURE IN THE RIGHT PLACE. An ammeter goes in series, in the line of the current it measures. A voltmeter goes across a component, connected in parallel with it, to read the potential difference. ⚠️ A habit that saves marks. Whenever you use the equation, write it out and keep the units in amperes, volts and ohms, so a rearrangement does not slip. Keep the one question to hand: is this a series loop or parallel branches, and what does the equation give me?

Order how to measure a resistance

Put the steps of measuring the resistance of a component into a sensible order.

• Build a circuit with the component and a power supply
• Put an ammeter in series to read the current
• Connect a voltmeter across the component to read the potential difference
• Read the current and the potential difference
• Divide the potential difference by the current to get the resistance

Work out the potential difference

A current of 3 amperes flows through a resistor of 4 ohms. Using potential difference equals current times resistance, what is the potential difference, in volts?

The circuit run

Five quick decisions about circuits. Three lives.

A crowd through the corridors

Leave electricity aside for a screen. Picture a crowd leaving a stadium along the corridors, and think about what decides how fast people get out. ⭐ The number of people passing a point each minute is the flow. Two things set it: how hard the crowd behind is pushing to get out, and how much the corridor holds them back. A keener, more pressed crowd flows faster; a narrow doorway slows everyone down. ⭐⭐ Now suppose there is only ONE corridor from the hall to the exit. Then the same number of people per minute must pass every point along it, because there is nowhere else for them to go. Squeeze in a narrow doorway and the whole corridor slows to match it. But suppose the hall opens onto SEVERAL corridors side by side. Now the crowd splits, some down each, so each corridor carries only a share of the people, and adding another corridor lets more people leave in total. ⚠️ Hold that picture. The flow of people is the flow of charge, the push of the crowd is the push that drives it, a narrow doorway is what opposes it, one corridor behaves like a single loop where the flow is the same throughout, and several side by side behave like branches that share the flow out.

Finish each circuit line

The flow of electric charge around a circuit, measured in amperes, is the _____. The push that drives the current, measured in volts, is the potential _____. What opposes the current, measured in ohms, is the _____. A circuit that is a single loop, with the same current at every point, is a _____ circuit. A circuit with separate branches, each getting the full potential difference, is a _____ circuit.

Three circuits on the bench

Three situations arise while building circuits. For each, choose the response that the sharing rules and the equation support.

• Two identical bulbs are wired in one series loop, and a student expects the current to be twice as big in the first bulb as the second.
• A component has 6 volts across it and a current of 2 amperes through it, and you need its resistance.
• A room has two lamps wired in parallel, and someone worries that switching one off will dim the other.

Compare series and parallel

Explain the difference between a series and a parallel circuit, using current and potential difference, and explain how you would use the equation to find the resistance of a component. Say why house lighting is wired in parallel.

• State the three quantities: current, potential difference and resistance
• Explain how current and potential difference share out in a series loop
• Explain how they share out in parallel branches
• Explain how to find a resistance from a current and a potential difference
• Explain why parallel wiring lets one lamp be switched without the others