Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equation: V = IR
Notes
Circuit basics
- Learn the symbols for: cell, battery, switch (open and closed), lamp, fuse, resistor, variable resistor, diode, LED, thermistor, LDR, ammeter and voltmeter.
- An ammeter goes in series; a voltmeter goes in parallel across the component.
- For current to flow there must be a closed circuit with a source of potential difference. The current is the same at every point in a single closed loop.
Series and parallel
| Series | Parallel | |
|---|---|---|
| Current | Same through each component | Total = sum of the branch currents |
| Potential difference | Supply p.d. is shared | Same across each branch |
| Resistance | Rtotal = R1 + R2 | Total is less than the smallest resistor |
Mains electricity
UK mains is a.c. at 50 Hz and about 230 V. Most appliances use three-core cable:
- Live (brown): carries the alternating p.d. from the supply.
- Neutral (blue): completes the circuit; at or close to 0 V.
- Earth (green and yellow): a safety wire at 0 V; only carries current if there is a fault.
Touching the live wire is dangerous because your body is at 0 V: the large p.d. between the live wire and you drives a current through your body, causing an electric shock. A live wire can be dangerous even when a switch is open.
Questions
Q1 (F) How should an ammeter and a voltmeter be connected in a circuit?
Show answer
Ammeter in series; voltmeter in parallel across the component.
Q2 (F) Two lamps are in series with a 6.0 V battery. One lamp has 2.0 V across it. What is the p.d. across the other?
Show answer
The supply p.d. is shared in series: 6.0 − 2.0 = 4.0 V
Q3 (F) Two parallel branches carry 0.20 A and 0.30 A. What is the current from the supply?
Show answer
0.20 + 0.30 = 0.50 A
Q4 (F) What colour is the earth wire, and what is it for?
Show answer
Green and yellow. It is a safety wire that stops the metal casing of an appliance becoming live if there is a fault.
Q5 (F/H) A 4.0 Ω and a 6.0 Ω resistor are in series with a 5.0 V supply. Calculate the current.
Show answer
Total resistance = 4.0 + 6.0 = 10 Ω
F: V = I R
I: 5.0 = I × 10
A: I = 0.50 A
Q6 (F/H) Explain why adding a resistor in parallel decreases the total resistance of a circuit.
Show answer
It gives the current another path to flow through. The p.d. across each branch is the same, so the total current increases – and a bigger current for the same p.d. means a smaller total resistance.
Q7 (F/H) Explain why touching a live wire can give you an electric shock.
Show answer
The live wire is at about 230 V and your body is at 0 V. This large potential difference drives a current through your body to earth.
Q8 (H) A thermistor is in series with a lamp. Explain what happens to the lamp as the room warms up.
Show answer
As the temperature rises, the thermistor’s resistance decreases. The total resistance of the circuit falls, so the current increases and the lamp gets brighter.
