Lesson focus: Electric currentThe rate of flow of electric charge, measured in amperes (amps, A). is the rate of charge flow around a complete circuitA complete, closed loop of conductors that current can flow around., while energy is transferred from a source to components.
Build and interpret complete circuits, measure current correctly and distinguish charge flow from energy transfer.
Build on earlier learning
Charging upStatic discharge involved brief charge movement. This lesson develops sustained charge flow in a closed circuit.
Practical
Building circuits
- Build a series circuit with a cell, switch, bulb and ammeter.
- Move the ammeter to different points: the current is the same everywhere in a series loop.
- Use the model on this page to build circuits and read the meters.
Record: Ammeter readings at each position.
Circuit builder
Choose how the bulbs are connected, add bulbs and change the cell. The meters read like real ones.
Build the circuit
Each bulb has a resistance of 6 Ω (illustrative). Brighter bulbs transfer more energy each second.
You will be successful when you can…
- Recognise a complete circuit.
- Define current as charge flow rate.
- Connect an ammeter in series.
- Separate current from energy transfer.
Words that carry the science
- current
- Rate of flow of electric charge, measured in amperes.
- ampere
- The unit of current; one ampere is one coulomb per second.
- circuit
- A complete conducting path through which charge can flow.
- ammeter
- A meter connected in series to measure current through a point.
- conventional current
- The defined current direction from positive to negative outside a source.
- charge carrier
- A moving charged particle, such as an electron in a metal.
A current requires a complete conducting loop
01A current requires a complete conducting loop
A cell establishes an electric field through a complete circuitA complete, closed loop of conductors that current can flow around.. In metal wires, existing conduction electrons respond throughout the circuit; the source does not need to send one particular electron all the way to the lamp before it lights.
Opening a switch breaks the conducting path, so sustained currentThe rate of flow of electric charge, measured in amperes (amps, A). stops. Insulators lack mobile charge carrierA charged particle that moves to carry current, such as an electron in a wire. under ordinary conditions.
02Current measures charge per second
Current is defined by I = Q ÷ t. A current of 1 A means 1 C of charge passes a point each second. In a single unbranched loop, the same current passes every point because charge does not steadily accumulate in one component.
conventional currentThe direction we agree current flows: from the positive (+) end of a battery round to the negative (−) end. points from positive to negative around the external circuit. Electron drift in a metal is in the opposite direction. Either description must be labelled consistently.
03Meters and energy accounts have different jobs
An ammeterA meter that measures electric current, connected in series in a circuit. is connected in series so the same charge flow passes through it. It is designed to have very low resistance; placing it directly across a cell can cause excessive current.
Charge circulates and is conserved. Components transfer energy from the circuit to thermal, light, sound or kinetic stores. Saying that a lamp “uses current” confuses the conserved charge flow with energy transfer.
Calculate charge flow
A current of 0.30 A flows for 40 s.
- Use I = Q ÷ t.
- Rearrange to Q = I × t.
- Substitute Q = 0.30 × 40.
- Calculate Q = 12 C.
How to make the reasoning visible: Twelve coulombs of charge pass each point in the unbranched circuit.
Circuit-building
Diagnose each circuit from its connection and meter evidence.
| Circuit | Connection | Reading | Diagnosis |
|---|---|---|---|
| A | cell–switch–lamp loop closed | 0.24 A | complete conducting loop |
| B | switch open | 0.00 A | path broken |
| C | ammeter in series | 0.24 A | correct current measurement |
| D | ammeter across cell | very large | unsafe low-resistance path |
| E | plastic gap in loop | 0.00 A | insulating break |
Why are readings before and after the lamp equal in circuit A?
Why is circuit D unsafe?
Check your understanding
Answer from memory, then check against the suggested answer.
What does current measure?
How is an ammeter connected?
What is one ampere?
A motor current is 2.5 A for 8 s. How much charge passes?
Why can a lamp light almost immediately although individual electron drift is slow?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Two ammeters are placed either side of a lamp in a single-loop (series) circuit. Both show the same current.
Explain why both ammeters show the same current.
(3)Explain how the lamp transfers energy if the current is unchanged.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., keep the charge account and energy account separate.