Circuit symbols, charge and current
Before you can work with circuits, you need to read and draw them. This page covers the symbols, what current actually is, the equation that links charge and current, and where meters go.
Part 1 of 3: Learn it
In short
- Current tells you how much charge flows past a point each second. Its unit is the ampere (A).
- Q = It: charge in coulombs equals current in amps times time in seconds.
- Ammeters go in series; voltmeters go in parallel across a component.
Where this is in your specification
Spec points: AQA 4.2.1.1 and 4.2.1.2 (Combined Trilogy 6.2.1.1 and 6.2.1.2), Edexcel Topic 10, OCR P3.1
| Board | Topic: Electric circuits |
|---|---|
| AQA 8463 | 4.2.1 and 4.2.2 (8463); 6.2.1 and 6.2.2 (8464) |
| Edexcel 1PH0 | Topic 10 |
| OCR J249 | P3.1 and P3.2 |
| Eduqas C420 | 7.1, 7.2 |
| Cambridge IGCSE 0625 | 4.2, 4.3 |
| National 5 C857 75 | Electricity: electrical charge carriers, Ohm's law, practical circuits |
Circuit symbols
| Component | What it does |
|---|---|
| Cell and battery | a cell pushes charge round; a battery is two or more cells |
| Switch (open or closed) | breaks or completes the circuit |
| Resistor and variable resistor | limits the current; a variable one can be adjusted |
| Lamp | lights up when current flows |
| Fuse | melts and breaks the circuit if the current is too high |
| Diode and LED | let current through in one direction only; an LED also emits light |
| LDR | resistance falls as light gets brighter |
| Thermistor | resistance falls as temperature rises |
| Ammeter and voltmeter | measure current and potential difference |
Charge and current
Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). A current of 1 A means 1 coulomb of charge passes a point every second.
For current to flow, there must be a closed circuit and a source of potential difference such as a cell.
Current in a series loop
In a single loop the current is the same at every point, because charge is not used up as it goes round. Placing ammeters before and after a lamp gives the same reading.
Using meters
- An ammeter is placed in series, in the loop, so the current flows through it.
- A voltmeter is placed in parallel, connected across the component it is measuring.
- Convert milliamps to amps by dividing by 1000: 500 mA = 0.5 A.
A current of 2 A flows for 10 s. How much charge passes?
Show the answer
Q = 2 × 10 = 20 C.
Part 2 of 3: See it worked
Worked examples
Example 1
A current of 0.25 A flows for 4 minutes. How much charge flows?
- t = 4 × 60 = 240 s
- Q = I × t = 0.25 × 240
Answer: 60 C.
Example 2
90 C of charge passes through a lamp in 30 s. What is the current?
- I = Q ÷ t
- I = 90 ÷ 30
Answer: 3 A.
Example 3
How long does it take for 120 C to flow with a current of 500 mA?
- I = 500 mA = 0.5 A
- t = Q ÷ I = 120 ÷ 0.5
Answer: 240 s (4 minutes).
Common mistakes
- Using minutes instead of seconds in Q = It.
- Using milliamps without converting to amps.
- Thinking current gets used up by components, so there is less after a lamp.
- Drawing a voltmeter in series or an ammeter in parallel.
How is an ammeter connected in a circuit?
Show the answer
In series, so the current flows through it.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
A current of 2 A flows for 10 s. How much charge passes?
Q = 2 × 10 = 20 C.
How is an ammeter connected in a circuit?
In series, so the current flows through it.
What happens to the resistance of an LDR when the light gets brighter?
It decreases.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Ces fiches sont en anglais car elles suivent les programmes d'examen britanniques.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses