IGCSE ELECTRICITY

MADE BY CLAUDE

What unit is electric charge measured in?

Coulombs (C)

What are the two types of electric charge?

Positive (+) and negative (−)

State the rule for like and unlike charges.

Like charges repel; unlike charges attract

When a rod is charged by friction, which particles are transferred?

Only electrons (negative charge) are transferred — positive charges (protons) do not move

If an object gains electrons by friction, what charge does it acquire?

Negative charge

Define an electric field.

A region in which an electric charge experiences a force

What is the direction of an electric field at a point?

the direction of the electric force that would act on a positive test charge placed at that location

Use opposites attract and like repels

Describe the electric field pattern around a positive point charge.

Radially outward in all directions, evenly spaced arrows pointing away from the charge

Describe the electric field between two oppositely charged parallel plates.

Uniform and parallel, directed from positive plate to negative plate (end effects ignored)

Why do metals conduct electricity but plastics do not?

Metals have free (delocalised) electrons that can move; insulators have no free electrons so charge cannot flow

Define electric current.

The charge passing a point per unit time: I = Q/t

State the equation linking current, charge and time.

I = Q/t — Current (A) = Charge (C) ÷ Time (s)

In which direction does conventional current flow?

From positive to negative (outside the cell)

In which direction do electrons actually flow?

From negative to positive — opposite to conventional current

How is an ammeter connected in a circuit?

In series — it has very low resistance so it doesn't affect the circuit

What is the difference between DC and AC?

DC (direct current) always flows in the same direction; AC (alternating current) reverses direction periodically

What is electrical conduction in metals due to?

The movement of free (delocalised) electrons through the metal lattice

Define electromotive force (EMF).

The electrical work done by a source in moving a unit charge around a complete circuit: E = W/Q

Define potential difference (PD).

The work done by a unit charge passing through a component: V = W/Q

What is the unit of both EMF and PD?

Volts (V)

How is a voltmeter connected, and why?

In parallel across a component; it has very high resistance so it draws negligible current

What is the key difference between EMF and PD?

EMF is energy given to each coulomb by the source; PD is energy transferred from each coulomb to a component

State Ohm's law / resistance equation.

R = V/I — Resistance (Ω) = Voltage (V) ÷ Current (A)

How do you determine resistance using an ammeter and voltmeter?

Connect voltmeter in parallel and ammeter in series across the component; R = V/I

Describe the I–V graph for a resistor at constant resistance.

A straight line through the origin — resistance is constant (Ohmic conductor)

Describe the I–V graph for a filament lamp and explain its shape.

A curve that flattens — as current increases, the filament heats up, increasing resistance

Describe the I–V graph for a diode.

No current in reverse; above ~0.6V forward, current rises sharply. Conducts in one direction only

How does the resistance of a wire depend on its length?

Resistance is directly proportional to length — double the length, double the resistance

How does the resistance of a wire depend on its cross-sectional area?

Resistance is inversely proportional to cross-sectional area — double the area, half the resistance

State the equation for electrical power.

P = I × V — Power (W) = Current (A) × Voltage (V). Also P = I²R or P = V²/R

State the equation for electrical energy.

E = P × t = V × I × t — Energy (J) = Power (W) × Time (s)

Define the kilowatt-hour (kWh).

The energy used by a 1 kW appliance running for 1 hour; 1 kWh = 3,600,000 J (3.6 MJ)

How do you calculate the cost of using an electrical appliance?

Cost = Power (kW) × Time (hours) × price per kWh

What happens to energy in an electric circuit?

Energy is transferred from the source (e.g. battery) to components and then to the surroundings (e.g. as heat/light)

What is true about current at every point in a series circuit?

The current is the same at every point in a series circuit

State the voltage rule for a series circuit.

The total PD equals the sum of the individual PDs: V = V₁ + V₂ + V₃

How do you calculate total resistance for resistors in series?

R_total = R₁ + R₂ + R₃ (add them up)

State the current rule for a parallel circuit.

The sum of currents entering a junction equals the sum leaving it: I = I₁ + I₂ + I₃

State the voltage rule for a parallel circuit.

The PD across each branch is the same as the total PD across the parallel arrangement

How do you calculate the combined resistance of two resistors in parallel?

R_total = (R₁ × R₂) / (R₁ + R₂) — always less than either individual resistor

State two advantages of connecting lamps in parallel in a lighting circuit.

1) Each lamp gets the full supply voltage. 2) If one lamp fails, the others remain on. 3) Each can be switched independently

Describe the action of a variable potential divider.

Two resistors in series across a supply; the output voltage is taken across one resistor. Adjusting the ratio of resistances changes V_out

State the potential divider equation.

V_out = V_in × R₂ / (R₁ + R₂)

How does an LDR behave as light intensity increases?

Its resistance decreases as light intensity increases

• Brighter Bulbs: If a bulb is in series with the LDR, the increased current will make the bulb glow more brightly.

• Louder Buzzers: A buzzer would sound louder because it is receiving more electrical energy per second.

• . The Heating Effect

  When more current flows through a wire or component, it generates more thermal energy

• In a parallel circuit, increasing light intensity only increases the current in the LDR's specific branch; the voltage across the LDR and the behavior of other parallel branches remain unchanged because they are independent

How does an NTC thermistor behave as temperature increases?

Its resistance decreases as temperature increases

State four hazards of electricity from a mains supply.

1) Damaged insulation. 2) Overheating cables. 3) Damp/wet conditions. 4) Overloading plugs, sockets or extension leads

Name the three wires in a mains circuit and their colours (UK).

Live (brown) — 230V AC; Neutral (blue) — 0V; Earth (green/yellow) — safety

Why must a switch be connected to the live wire?

So that when the switch is open, the appliance is disconnected from the high-voltage live wire and is safe to touch

Explain how a fuse works.

A thin wire that melts when current exceeds the fuse rating, breaking the circuit and protecting the wiring

Explain how a trip switch (MCB) works.

Detects excess current and automatically switches the circuit off; can be reset after the fault is fixed

Why must the outer casing of a metal appliance be earthed?

If the casing becomes live, current flows to earth, creating a large current that blows the fuse, making the appliance safe

to protect users from fatal electric shocks. If internal insulation fails and a live wire touches the metal casing, earthing provides a low-resistance path for the current to flow safely into the ground, causing the circuit breaker to trip instantly. [1, 2, 3, 4]

What is double insulation and what protection is needed?

A plastic (non-conducting) casing that prevents electric shock without needing an earth wire; a fuse alone protects the circuit

How do you choose an appropriate fuse rating?

Choose the rating just above the normal operating current of the appliance (e.g. 3A for <700W, 13A for high-power devices)