Circuit symbols, charge and current
Before you can work with circuits, you need to read and draw them.
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.
Practise this topicCircuit symbols, charge and current, with exam-style questions
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