Electric Circuits & Electrical Safety

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Lesson 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.

Electrical symbols used in circuit diagrams

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).

A simple series circuit

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.

Calculation of current splitting at a junction in a parallel circuit

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).

Example calculation for potential difference in series

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.

Combined resistance of resistors in series

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).

Potential divider diagram

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.

Cross-section of a mains cable showing conducting and insulating materials

Slides

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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?
Easy
- 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?
Medium
- 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:
Easy
- 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

More questions

9.An electric kettle has a metal casing. The cable contains an earth wire connected to the plug's earth pin. Which danger does this guard against?
Medium
- AThe cable to the kettle becoming too hot
- BThe casing of the kettle becoming live
- CThe casing of the kettle becoming wet on the outside
- DThe casing of the kettle overheating
10.A fuse is placed in series with an appliance. The ammeter in the circuit reads 1.2 A. Which fuse rating should be used?
Medium
- A1.0 A
- B3.0 A
- C5.0 A
- D13.0 A
11.Which component's resistance decreases as temperature increases?
Easy
- ALDR
- BThermistor
- CDiode
- DFixed resistor
12.A security light uses an LDR to switch on when dark. In the dark, the LDR's resistance is high. What happens to the potential difference across the LDR when it gets dark?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
13.In a circuit containing a lamp connected to an ammeter and a voltmeter, which component is connected in parallel with the lamp?
Easy
- AAmmeter
- BVoltmeter
- CCell
- DSwitch
14.In the circuit diagram shown, what is the purpose of component labelled X?
Easy
- ATo measure current
- BTo protect the circuit from overcurrent
- CTo vary resistance
- DTo store charge
15.A student sets up the circuit shown. The ammeter reads 0.5 A. What is the current through the battery?
Medium
- A0.5 A
- B1.0 A
- C0.25 A
- D2.0 A
16.In the circuit shown, the potential difference across the lamp is 6 V. What is the potential difference across the resistor?
Medium
- A6 V
- B3 V
- C9 V
- D12 V
17.Two resistors, 6 Ω and 12 Ω, are connected in parallel. What is the combined resistance?
Medium
- A18 Ω
- B4 Ω
- C9 Ω
- D2 Ω
18.A potential divider consists of a 10 kΩ resistor and a 20 kΩ resistor in series across a 12 V supply. What is the voltage across the 20 kΩ resistor?
Medium
- A4 V
- B8 V
- C6 V
- D12 V
19.An electric heater is rated at 240 V, 2.4 kW. What is the most suitable fuse rating for its plug?
Medium
- A3 A
- B5 A
- C10 A
- D13 A
20.In the circuit shown, ammeter X reads 3 A, ammeter Y reads 1 A. What is the reading on ammeter Z?
Medium
- A1 A
- B2 A
- C3 A
- D4 A
21.A security light circuit uses an LDR and a fixed resistor in series. In bright light, the LDR resistance is low. What happens to the voltage across the LDR?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
22.Which of the following is a correct safety measure to prevent electric shock from a metal-cased appliance?
Medium
- AConnecting the earth wire to the metal case
- BUsing a fuse in the neutral wire
- CDouble insulation of the live wire only
- DConnecting the earth wire to the live terminal
23.What is the circuit symbol for a diode?
Easy
- AA triangle pointing to a line
- BA circle with a cross inside
- CA zigzag line
- DTwo parallel lines
24.A student connects a 3 V battery, a 6 Ω resistor, and a 3 Ω resistor in series. What is the current in the circuit?
Medium
- A0.33 A
- B0.5 A
- C1 A
- D3 A
25.Which of the following statements about current in a parallel circuit is correct?
Easy
- AThe current is the same in all branches
- BThe total current is the sum of branch currents
- CThe current splits equally at every junction
- DThe current is zero in the branch with highest resistance
26.In a series circuit containing a cell, a switch, a lamp and an ammeter, where should the ammeter be connected?
Easy
- AIn parallel with the lamp
- BIn series anywhere in the circuit
- CBetween the cell and the switch only
- DAfter the lamp only
27.A circuit contains a 12 V battery and two resistors of 4 Ω and 8 Ω connected in series. What is the current in the circuit?
Medium
- A1.0 A
- B3.0 A
- C0.5 A
- D12 A
28.A 6 V battery is connected to a 10 Ω resistor and an LDR in series. In bright sunlight, the LDR resistance is 5 Ω. What is the potential difference across the LDR?
Medium
- A2 V
- B3 V
- C4 V
- D6 V
29.Which of the following is a common electrical hazard?
Easy
- AUsing a fuse with a lower rating than required
- BConnecting an ammeter in parallel
- CDamaged insulation on a wire
- DUsing a circuit breaker instead of a fuse
30.An electric kettle has a metal casing and is connected to the mains via a three-core cable. The earth wire is connected to the metal casing. If the live wire inside the kettle becomes loose and touches the metal casing, what happens?
Hard
- AThe kettle stops working immediately
- BA large current flows through the earth wire, blowing the fuse
- CThe user receives a mild electric shock but the kettle continues to work
- DThe kettle becomes double insulated
31.A student connects a circuit with a battery, an ammeter, and a variable resistor in series. The ammeter reads 2 A when the variable resistor is set to 3 Ω. What will the ammeter read if the variable resistor is adjusted to 6 Ω, assuming the battery voltage remains constant?
Medium
- A1 A
- B2 A
- C4 A
- D0.5 A
32.In the circuit shown, three identical lamps are connected in parallel to a battery. If one lamp blows (breaks), what happens to the brightness of the other two?
Medium
- AThey become dimmer
- BThey become brighter
- CThey remain the same
- DThey go out
33.A circuit contains a 9 V battery, a 20 Ω resistor, and a thermistor in series. At room temperature, the thermistor has a resistance of 10 Ω. What is the current in the circuit?
Medium
- A0.3 A
- B0.45 A
- C0.9 A
- D0.15 A
34.Which of the following is the correct circuit symbol for a light-dependent resistor (LDR)?
Easy
- AA circle with a diagonal arrow pointing into it
- BA rectangle with a line through it
- CA zigzag line with a diagonal arrow pointing into it
- DA triangle with a line at the tip
35.What is the colour of the insulation on the neutral wire in a UK mains plug?
Easy
- ABrown
- BBlue
- CGreen and yellow stripes
- DRed
36.Which of the following is an example of an electrical hazard?
Easy
- AUsing a double-insulated appliance
- BConnecting an earth wire to a metal casing
- CDamaged insulation on a cable
- DUsing a fuse with the correct rating
37.What is the colour of the insulation on the live wire in a UK mains plug?
Easy
- ABrown
- BBlue
- CGreen and yellow stripes
- DRed
38.Which component's resistance decreases as light intensity increases?
Easy
- AThermistor
- BLight-dependent resistor (LDR)
- CFixed resistor
- DDiode
39.In a series circuit, the current is the same at any point.
Easy
True or false?
40.A voltmeter is always connected in series in a circuit.
Easy
True or false?
41.Two resistors of 4 Ω and 6 Ω are connected in series. What is the combined resistance?
Easy
- A2.4 Ω
- B10 Ω
- C24 Ω
- D0.4 Ω
42.In a parallel circuit, if one bulb breaks, the others remain lit.
Easy
True or false?
43.Which of the following are safety features in domestic electrical appliances? (Select all that apply)
Medium
- ADouble insulation
- BFuse
- CEarth wire
- DAmmeter
- EThermistor
44.Arrange the following steps in the correct order for how an earth wire and fuse protect a metal-cased appliance if the live wire touches the case.
Medium
- A surge of current flows through the earth wire
- The fuse melts and breaks the circuit
- The live wire touches the metal case
- The appliance is disconnected from the mains supply
45.Match each wire in a UK mains plug to its correct colour.
Easy
- Live wire
- Neutral wire
- Earth wire
- Brown
- Blue
- Green and yellow stripes
46.Complete the sentence:
Easy
In a UK mains plug, the neutral wire is coloured ____.
47.A circuit has a 12 V battery and two resistors of 3 Ω and 5 Ω in series. What is the current in the circuit?
Medium
48.A potential divider consists of a 10 kΩ resistor (R1) and a 20 kΩ resistor (R2) connected in series across a 9 V supply. What is the voltage across R2?
Medium
49.A circuit has a 12 V battery and two resistors of 3 Ω and 5 Ω in series. What is the current in the circuit?
Medium
- A1.5 A
- B2.5 A
- C0.5 A
- D3.5 A
50.A potential divider consists of a 10 kΩ resistor (R1) and a 20 kΩ resistor (R2) connected in series across a 9 V supply. What is the voltage across R2?
Medium
- A7 V
- B8 V
- C6 V
- D5 V

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