Electric Circuits and Electrical Safety — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

Section 1 of 26Power supplies, meters and switches

Power supplies, meters and switches

Power supplies

• Power supplies cells, batteries, power supplies and generators all supply current to a circuit.

Meters

• Meters ammeters measure current and are connected in series;
  • voltmeters measure potential difference and are connected in parallel.

Switches

• Switches open and close a circuit and so decide whether current can flow.

Functional components

• Functional components motors rotate, lamps emit light, heaters transfer thermal energy and bells emit sound.

Section 2 of 26Resistors that change with conditions

Resistors that change with conditions

Fixed and variable resistors

• Fixed and variable resistors used to control current;
  • a potential divider is a resistor arrangement that controls p.d.

Thermistor

• Thermistor a non-ohmic, temperature-dependent resistor whose resistance decreases as the temperature rises.

Light-dependent resistor (LDR)

• Light-dependent resistor (LDR) a non-ohmic sensory resistor whose resistance decreases as the light intensity increases.

Section 3 of 26Electromagnetic components, fuses and diodes

Electromagnetic components, fuses and diodes

Magnetising coils, relays and transformers

• Magnetising coils, relays and transformers use electromagnetic effects.
• A relay uses a small current in one circuit to switch on a much larger current in another.
• A transformer steps potential difference up or down.

Fuse

• Fuse protects components from current surges and acts as a safety measure against fire.

Diode (Extended)

• Diode (Extended) allows current in one direction only, the way the triangle points;
  • reversed, it blocks current.
• A light-emitting diode (LED) behaves in the same way and also emits light when current flows.

Section 4 of 26Current in a series circuit

Series circuit

• Series circuit a single complete loop containing all the components.

Same current everywhere

• Same current everywhere the number of electrons per second passing any point is the same.

Factors

• Factors current rises if the source p.d. rises and falls if more components are added because the total resistance goes up.

Section 5 of 26Current in a parallel circuit

Parallel circuit

• Parallel circuit several loops, with components on separate branches.

Splitting

• Splitting the current from the source divides at a junction, so each branch current is smaller than the source current.

Lighting circuits

• Lighting circuits parallel wiring gives each bulb the same p.d. and brightness, lets other bulbs keep working if one breaks,
  • and allows each lamp to be switched separately.

Junction rule (Extended)

• Junction rule (Extended) current into a junction equals current out of it, because charge and electrons are conserved.
• The branch currents are equal only if the branch resistances are equal.

Section 6 of 26Meters and the effect of components

Meters and the effect of components

Adding a lamp in series

• Adding a lamp in series the current falls in every part of the loop because the total resistance increases.

Adding a lamp in parallel

• Adding a lamp in parallel the other lamps are unaffected and the supply current increases.

Dark and light

• Dark and light in a circuit with an LDR, the p.d. across the LDR rises in the dark because its resistance is large.

Switches

• Switches a switch in a parallel branch controls only that branch, while a switch in the main wire controls the whole circuit.

Section 7 of 26Comparing series and parallel circuits

Feature	Series	Parallel
Current	Same everywhere	Splits at junctions
If one lamp breaks	All go out	Others stay on
Total resistance	Increases when components are added	Decreases when branches are added

Section 8 of 26More from the specification

More from the specification

Circuit Components and Current in Series

• In a series circuit, electric current flowing from the power supply is identical through every component,
• and the combined total resistance equals the sum of all individual resistor resistances.
  • (R_total = R1 + R2)
    What the symbols mean: R_T = total resistance of resistors in series (Ω) · R₁, R₂ = the separate resistances (Ω)

Electrical Safety Fuses

• A fuse is a safety device wired in series with the live wire of an appliance containing a thin wire designed to melt and break the circuit if excessive current flows, protecting wiring from overheating.

Earth Wire and Metal Casings

• For mains appliances with conductive metal chassis, an earth wire connects the exterior case to ground;
  • an internal electrical fault causes heavy current to surge through the earth wire, blowing the fuse.

Double Insulation in Modern Appliances

• Electrical appliances bearing double insulation possess non-conductive plastic outer casings,
• which means internal live components cannot be contacted, eliminating the need for an earth wire.

Earth wire protection

• Earth wire protection earth wires route fault currents from metal appliance casings safely into ground, preventing lethal electrical shocks.

Mains fuses

• Mains fuses excessive current flowing through an appliance melts the thin fuse wire, breaking the circuit instantly.

Double insulation

• Double insulation plastic casings encase internal electrical wiring completely, eliminating the necessity for a dedicated earth wire.

Mains supply

• Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

Circuit diagram

• Drawing of a circuit using standard symbols.

Circuit symbol

• Standard picture for a component.

Insulation

• Material that reduces thermal energy transfer.

Combined resistance

• Single resistance that could replace several resistors.

Section 9 of 26E.m.f. of cells in series

Rule

• Rule when cells are connected in series, their combined e.m.f. is the sum of the individual e.m.f.s.

Example

• Example three 1.5 V cells in series give 4.5 V.

Section 10 of 26Potential difference in series circuits (Extended)

Rule

• Rule the sum of the p.d.s across the components equals the e.m.f. of the supply, so the components share the e.m.f.

Equation

• E = V_X + V_Y for two components X and Y in series.

Section 11 of 26Potential difference in parallel circuits (Extended)

Potential difference in parallel circuits (Extended)

Rule

• Rule the p.d. across each branch equals the e.m.f. of the supply, so every branch has the same p.d.

Several components on one branch

• Several components on one branch they share that branch p.d. between them.

Section 12 of 26Combined resistance

Combined resistance

Series

• Series combined resistance is the sum of the resistances: R = R₁ + R₂.
  What the symbols mean: R_T = total resistance of resistors in series (Ω) · R₁, R₂ = the separate resistances (Ω)

Parallel

• Parallel combined resistance is less than the smallest individual resistor, because the current has more paths.
• Two equal resistors in parallel give half the resistance of one.

Parallel equation (Extended)

• 1/R = 1/R₁ + 1/R₂.
  What the symbols mean: R_T = total resistance of resistors in parallel (Ω), always smaller than the smallest one · R₁, R₂ = the separate resistances (Ω)

Section 13 of 26Method for two resistors in parallel

Method for two resistors in parallel

Work out 1/R₁ and 1/R₂ and add them to find 1/R.

Use the reciprocal key (x⁻¹) to turn 1/R into R.

Check that R is smaller than both R₁ and R₂.

Section 14 of 26Potential dividers (Extended)

Potential dividers (Extended)

Idea

• Idea two resistors in series share the supply p.d., so a potential divider gives a component only a fraction of the supply p.d.

Larger resistance, larger share

• Larger resistance, larger share the resistor with the greater resistance has the greater p.d.

Ratio equation

• R₁ / R₂ = V₁ / V₂ and V₁ + V₂ = e.m.f.

Variable resistor

• Variable resistor changing one resistor changes the p.d. across the other, and across any component in parallel with it.

Sensors

• Sensors with an LDR or thermistor, a change in light or temperature changes the share of p.d., which can switch on a lamp or other component.

Section 15 of 26Using potential dividers in sensor circuits

Light sensor

• Light sensor an LDR in series with a fixed resistor: in the dark the LDR has high resistance and takes a large share of the p.d.

Temperature sensor

• Temperature sensor a thermistor in series with a fixed resistor: when it gets hot its resistance falls and the p.d. across the fixed resistor rises.

Variable resistor

• Variable resistor moving the slider changes the output p.d. smoothly from zero to the full supply voltage.

Section 16 of 26More from the specification

More from the specification

Equivalent circuit calculations

• Equivalent circuit calculations applying series and parallel resistance formulas provides an algebraic means to determine total circuit load.

Series and parallel circuit rules

• Series circuits share equal current with source voltage divided among components;
  • parallel circuits share equal voltage across branches with total current dividing at junctions.

Electrical power and energy equations

• Electrical power is P = VI = I² R; electrical energy transferred over time is E = P t = V I t.

Domestic electrical safety devices

• Fuses and circuit breakers disconnect the live conductor when current exceeds rated limits;
  • earth wires provide a low-resistance path to trip fuses during electrical faults.

Current entering a junction

• Current entering a junction the sum of currents flowing into any circuit junction equals the sum of currents flowing out.

Earth connection and fuses

• Earth connection and fuses earthing connects metal casings to ground so fault currents trigger fuses before damaging the appliance.

Insulation safety

• Insulation safety double insulation prevents users touching live wires connected directly to the mains power source.

Potential difference

• Work done by a unit charge passing through a component.

Mains supply

• Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

Circuit diagram

• Drawing of a circuit using standard symbols.

Source

• Reservoir at T_H providing heat Q_H

Junction

• Place where a pipe splits or joins; flow rates add.

Insulation

• Material that reduces thermal energy transfer.

Section 17 of 26Electrical hazards

Danger level

• Danger level p.d.s as low as 50 V can cause serious risk of electrocution.

Damaged insulation

• Damaged insulation exposed metal wire can give a lethal shock, and current can flow to any metal it touches.

Overheating cables

• Overheating cables excess current heats a wire, which may melt the insulation or start a fire.

Damp conditions

• Damp conditions water conducts, so moisture near a live wire can cause a short circuit and fire, or a path through a person to earth.

Overloading

• Overloading too many appliances on plugs, extension leads or sockets draws excess current and can melt insulation or cause a fire.

Section 18 of 26The three mains wires

Wire	Colour	Purpose
Live	Brown	Carries the alternating current from the mains to the appliance; the most dangerous wire.
Neutral	Blue	Completes the circuit; at a much lower p.d. so it is far less dangerous.
Earth	Green and yellow stripes	Safety wire that stops the appliance becoming live.

1. Other countries use different plugs, such as two-pin plugs, but the earth wire is still present.

Section 19 of 26Double insulation

Meaning

• Meaning the wires have a first layer of insulation and a non-metallic case acts as the second layer.

Result

• Result there is no metal case that could become live, so no earth wire is needed or it cannot touch the casing.

Section 20 of 26Earthing

Why needed

• Why needed if the live wire touches a metal case, the case becomes live and a person touching it could be electrocuted.

How it works

• How it works the earth wire provides a low resistance path to earth, which causes a surge of current in the live wire and fuse.
• The fuse melts and breaks the circuit, cutting off the supply and making the appliance safe.

Section 21 of 26Fuses

Position

• Position in the plug, to protect an individual appliance and its cable.

Construction

• Construction a glass cylinder containing a thin metal wire that melts when the current is too large.

Ratings

• 3 A, 5 A and 13 A; choose the value just above the normal working current.
• For a 3.1 A appliance, 3 A would blow at once, 13 A would let an extra 10 A pass, so 5 A is correct.

Symbol

• Symbol a rectangle with a wire through it, not to be confused with a resistor.

Section 22 of 26Trip switches (circuit breakers)

Position

• Position in the consumer unit (fuse box), where current enters the house;
  • each trip switch controls one circuit.

Action

• Action when the current is too high the switch trips to the off position and stops the current.

Advantages

• Advantages protection from surges and faults, and they can be reset once the fault is fixed.

Section 23 of 26Why the earth wire is not enough on its own

Fuse needed

• Fuse needed the earth wire only works safely because the large current it causes melts the fuse.

Low resistance

• Low resistance the earth wire has very low resistance, so the fault current is large enough to blow the fuse quickly.

Section 24 of 26More from the specification

More from the specification

Protective electrical grounding

• Protective electrical grounding connecting metal appliance casings to earth wire conductors provides a reliable means to direct fault currents safely away.

Potential difference in series and parallel circuits

• In a series circuit, total potential difference from the power source is shared across components,
• whereas in a parallel circuit, full source voltage is applied across each parallel branch.

Current rules at circuit junctions

• According to Kirchhoff's first law, total electric current entering any junction equals the sum of currents leaving that junction, ensuring charge is conserved in parallel branches.

Combined resistance of connected components

• Resistors connected in series have an equivalent resistance equal to their sum,
  • (R_total = R1 + R2)
• whereas in parallel the combined resistance is always lower than any individual branch resistor.
  • (1/R_total = 1/R1 + 1/R2)

Power dissipation and electrical safety

• Electrical power delivered to components depends on current and voltage (P = VI);
  • excessive current drawn through parallel branches risks overheating, requiring fuses or circuit breakers.

Potential difference in series and parallel circuits

• In a series circuit, total potential difference from the power source is shared across components,
• whereas in a parallel circuit, full source voltage is applied across each parallel branch.

Current rules at circuit junctions

• According to Kirchhoff's first law, total electric current entering any junction equals the sum of currents leaving that junction, ensuring charge is conserved in parallel branches.

Combined resistance of connected components

• Resistors connected in series have an equivalent resistance equal to their sum,
  • (R_total = R1 + R2)
    What the symbols mean: R_T = total resistance of resistors in series (Ω) · R₁, R₂ = the separate resistances (Ω)
• whereas in parallel the combined resistance is always lower than any individual branch resistor.
  • (1/R_total = 1/R1 + 1/R2)
    What the symbols mean: R_T = total resistance of resistors in parallel (Ω), always smaller than the smallest one · R₁, R₂ = the separate resistances (Ω)

Power dissipation and electrical safety

• Electrical power delivered to components depends on current and voltage (P = VI);
  • excessive current drawn through parallel branches risks overheating, requiring fuses or circuit breakers.

Potential difference

• Work done by a unit charge passing through a component.

Combined resistance

• Single resistance that could replace several resistors.

Potential divider

• Series resistors that share the supply p.d.

Circuit diagram

• Drawing of a circuit using standard symbols.

Components

• Perpendicular parts of a vector that add to give the vector.

Series

• Components in one loop, sharing the e.m.f. between them.

Parallel

• Components on separate branches, each with the full e.m.f.

Source

• Reservoir at T_H providing heat Q_H

Branch

• Path connecting two junctions.

Junction

• Place where a pipe splits or joins; flow rates add.

Section 25 of 26Key terms

Term	Meaning
Series circuit	A circuit with one loop where the same current flows through all components.
Parallel circuit	A circuit with multiple loops, so the current splits at junctions.
Thermistor	A resistor whose resistance changes with temperature.
LDR	A resistor whose resistance decreases as light intensity increases.
Relay	A switch that uses a small current to switch a larger current.
Fuse	A safety device that melts and breaks the circuit when the current is too high.
Diode	A component that allows current in one direction only.

Term	Meaning
E.m.f.	The work done per unit charge by the source, in volts.
Series	Components in one loop, sharing the e.m.f. between them.
Parallel	Components on separate branches, each with the full e.m.f.
Combined resistance	The single resistance that has the same effect as a group of resistors.
Potential divider	Series resistors that split a supply p.d. in proportion to their resistances.
Reciprocal	1 divided by the number; the x−1 key on a calculator.

Term	Meaning
Live wire	The wire carrying the mains p.d. to the appliance; colour brown.
Neutral wire	The wire completing the circuit; colour blue.
Earth wire	A safety wire that gives a low resistance path to earth; green and yellow.
Fuse	A thin wire that melts to break the circuit when the current is too high.
Double insulation	Two layers of insulation, so no earth wire is needed.
Trip switch	A resettable switch that opens when the current is too high.

Section 26 of 26Exam technique

1

Connecting an ammeter in parallel — ammeters are in series.

2

Thinking current is used up round a series circuit — it is the same at every point.

3

Saying branch currents are always equal — they are equal only if the branch resistances are equal.

4

Mixing up thermistor and LDR — one responds to temperature and the other to light.

1

Adding parallel resistances like series ones — use the reciprocal method.

2

Forgetting the final reciprocal step and giving 1/R as the answer.

3

Thinking branches in a parallel circuit split the e.m.f. — each branch has the full e.m.f.

4

Putting the larger p.d. across the smaller resistor in a divider — larger resistance means larger p.d.

1

Thinking the neutral wire is the dangerous one — the live wire is the most dangerous.

2

Fitting a much larger fuse than needed — it will not protect the appliance.

3

Believing double-insulated appliances need an earth wire — they do not.

4

Forgetting that water is a conductor — damp conditions are a hazard.