Electric Circuits & Electrical Safety

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Notes

Circuit Diagrams & Circuit Components

Circuit symbols represent components like cells, resistors, meters, switches, and functional devices.
**Ammeters** are connected in **series**; **voltmeters** are connected in **parallel**.
**Thermistors**: resistance **decreases** as temperature increases.
**LDRs**: resistance **decreases** as light intensity increases.
**Diodes** allow current in **one direction only** (arrow direction); LEDs also emit light.
**Fuses** protect against current surges; they melt and break the circuit if current is too high.

Current in Series Circuits

A **series circuit** has a single complete loop.
Current is **the same** at every point in a series circuit.
Current depends on: voltage of the power source and total resistance of components.
Increasing voltage **increases** current; increasing number of components **decreases** current (due to higher resistance).

Current in Parallel Circuits

A **parallel circuit** has multiple loops (branches).
Current **splits** at junctions; total current entering a junction equals total current leaving (charge conservation).
Current in each branch depends on the resistance of that branch; equal only if resistances are equal.
Lighting circuits are wired in parallel so each bulb has the same brightness and bulbs can be switched independently.

EMF & Potential Difference

Cells in series: total e.m.f. = **sum** of individual e.m.f.s.
In a series circuit, sum of p.d.s across components = total e.m.f. of the power supply.
In a parallel circuit, p.d. across each branch = e.m.f. of the power supply (same for all branches).

Combined Resistance

Resistors in series: total resistance = **sum** of individual resistances.
Resistors in parallel: combined resistance is **less** than the smallest individual resistance.
For two resistors in parallel: 1/R = 1/R₁ + 1/R₂.
Two equal resistors in parallel give half the resistance of one.

Potential Dividers (Extended Tier)

A potential divider uses two resistors in series to split the e.m.f. into smaller voltages.
The p.d. across each resistor is proportional to its resistance: R₁/R₂ = V₁/V₂.
If one resistor is variable, the output voltage can be adjusted.
The sum of the p.d.s across the two resistors equals the total e.m.f. (V₁ + V₂ = V_in).

Electrical Safety

Common hazards: damaged insulation, overheating cables, damp conditions, overloading sockets.
Mains circuit wires: **live** (brown), **neutral** (blue), **earth** (green/yellow).
The **earth wire** provides a low-resistance path to ground; if the live wire touches the metal case, a large current flows and blows the fuse.
**Double insulation** (two layers of insulation, non-metallic case) means no earth wire is needed.
**Fuses** (3 A, 5 A, 13 A) melt and break the circuit if current exceeds their rating; choose the smallest rating above the normal current.
**Circuit breakers** (trip switches) automatically switch off when current is too high; they can be reset.

A simple series circuit with a cell, switch, lamp, and ammeter. Current is the same at all points.

Potential divider circuit: two resistors in series; the voltmeter measures the p.d. across R2.

Practice questions

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1.Which circuit symbol represents a fuse?
Easy
- AA resistor symbol with a zigzag line
- BA rectangle with a line through it
- CA circle with a cross inside
- DA triangle pointing to a line
2.What is the purpose of a fuse in a mains electrical circuit?
Easy
- ATo reduce the current flowing through the appliance
- BTo increase the resistance of the circuit so the appliance works safely
- CTo break the circuit if the current becomes too large
- DTo prevent the voltage from becoming too high
3.Which of the following is the correct circuit symbol for a thermistor?
Easy
- AA resistor with a diagonal arrow through it
- BA resistor with a curved line through it
- CA circle with a cross inside
- DA rectangle with a line through it
4.In a series circuit with a battery and two resistors, the current is 0.5 A. If a third resistor is added in series, what happens to the current?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
5.Two resistors of 4 Ω and 6 Ω are connected in parallel. What is the total resistance?
Medium
- A10 Ω
- B2.4 Ω
- C5 Ω
- D24 Ω
6.In a potential divider circuit, resistor R1 = 10 kΩ and R2 = 20 kΩ are in series across a 9 V supply. What is the voltage across R2?
Hard
- A3 V
- B6 V
- C9 V
- D4.5 V
7.What colour is the insulation on the earth wire in a UK mains plug?
Easy
- ABrown
- BBlue
- CGreen and yellow stripes
- DRed
8.A double-insulated appliance does not require an earth wire because:
Medium
- AIt has a metal case that is earthed
- BIt has two layers of insulation around live parts
- CIt uses a lower voltage
- DIt has a built-in fuse

Slides

Circuit Symbols & Components

Circuit diagrams use standard symbols to represent components.
Power supplies: cells, batteries, generators.
Resistors: fixed, variable, thermistors, LDRs.
Meters: ammeters (series), voltmeters (parallel).
Switches, lamps, motors, heaters, bells, fuses, diodes.

Which circuit symbol is used for a thermistor?

A rectangle with a line through it
A circle with a cross inside
A rectangle with a diagonal line and a small 't'
A triangle pointing to a line

Current in Series Circuits

A series circuit has a single complete loop.
Current is the same at every point in a series circuit.
Current depends on voltage of the power source and total resistance.
Increasing voltage increases current; increasing resistance decreases current.

In a series circuit, the current is the ____ at every point. Increasing the number of components ____ the total resistance, so current ____.

Current in Parallel Circuits

A parallel circuit has multiple loops (branches).
Current splits at junctions: total current into a junction equals total current out.
Current in each branch depends on the resistance of that branch.
Lighting circuits are wired in parallel so each bulb has the same brightness and works independently.

In a parallel circuit, the current from the source is 10 A. One branch has 6 A. What is the current in the other branch?

4 A
6 A
10 A
16 A

EMF & Potential Difference in Series

Cells in series: total e.m.f. = sum of individual e.m.f.s.
In a series circuit, the sum of p.d.s across components equals the total e.m.f.
Components share the e.m.f. of the power supply.

Three 1.5 V cells in series give a total e.m.f. of ____ V. In a series circuit, the sum of potential differences across components equals the ____ of the power supply.

Potential Difference in Parallel Circuits

In a parallel circuit, the p.d. across each branch is the same as the e.m.f. of the power source.
If a branch has multiple components, the p.d. is shared among them.

A 12 V battery is connected to two resistors in parallel. What is the p.d. across each resistor?

6 V
12 V
24 V
Depends on resistance

Combined Resistance

Resistors in series: total resistance = sum of individual resistances.
Resistors in parallel: combined resistance is less than the smallest individual resistance.
For two equal resistors in parallel, total resistance halves.

Two 4 Ω resistors are connected in parallel. What is the total resistance?

2 Ω
4 Ω
8 Ω
16 Ω

Potential Dividers

A potential divider uses two resistors in series to split the e.m.f.
The p.d. across each resistor is proportional to its resistance: R₁/R₂ = V₁/V₂.
If one resistor is variable, the output voltage can be changed.

In a potential divider, the ratio of resistances equals the ratio of ____ across them. If R₁ = 10 Ω and R₂ = 20 Ω, and V₂ = 6 V, then V₁ = ____ V.

Electrical Hazards

Mains electricity can be dangerous: voltages as low as 50 V can cause electrocution.
Common hazards: damaged insulation, overheating cables, damp conditions, overloading.
Overheating can melt insulation and cause fires.

Which of these are electrical hazards?

Damaged insulation
Using a fuse
Damp conditions near electricity
Overloading sockets

Mains Circuits: Live, Neutral, Earth

Live wire (brown): carries alternating current from mains to the circuit; most dangerous.
Neutral wire (blue): completes the circuit; lower voltage.
Earth wire (green/yellow): safety wire; provides low-resistance path to earth if the appliance becomes live.

Which wire in a mains circuit is the most dangerous?

Live wire
Neutral wire
Earth wire
All equally dangerous

Fuses & Circuit Breakers

Fuses protect appliances from excess current: a thin wire melts and breaks the circuit.
Fuse ratings: 3 A, 5 A, 13 A. Choose the smallest rating above the normal current.
Circuit breakers (trip switches) can be reset; they protect circuits in the consumer unit.

An appliance uses 3.1 A. Which fuse is most suitable?

3 A
5 A
13 A
No fuse needed

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