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CIE iGCSE Co-Ordinated Science P4.2.5 Electrical energy and electrical power Exam Style Questions Paper 3

CIE iGCSE Co-Ordinated Science Paper 3 - All Chapters

Question

Fig. 10.1 shows an electric kettle with a heating element.
(a) The heating element heats the water at the bottom of the kettle. Thermal energy is then transferred to all the water in the kettle.
State the name of this important method of energy transfer in liquids.
(b) The water in the kettle is heated. The water boils and changes into steam.
Water is a liquid and steam is a gas.
Fig. 10.2 shows diagrams of the arrangements of particles in a gas, liquid and solid.
Identify the diagrams for a gas and for a liquid.
(c) The kettle is connected to a 240 V supply. Power is supplied to the kettle at 3000 W.
(i) Calculate the current in the kettle.
(ii) The kettle is used for 10 minutes. Show that the energy used is 0.50 kWh.
(iii) 1 kWh costs $0.50. Calculate the cost of the energy used in (ii).
(d) Fig. 10.3 shows an electrical hazard for a person using the kettle.
State the electrical hazard shown and explain why it is dangerous.

Most-appropriate topic codes (Cambridge IGCSE Coordinated Sciences 0654):

• Topic P2.3.2 — Convection (Part (a))
• Topic P2.1.1 — States of matter (Part (b))
• Topic P4.2.5 — Electrical energy and electrical power (Part (c)(i), (c)(ii), (c)(iii))
• Topic P4.4 — Electrical safety (Part (d))

▶️ Answer/Explanation

(a) convection
Convection is the transfer of thermal energy in fluids (liquids and gases) by the movement of the fluid itself. Heated water rises and cooler water sinks, creating convection currents.

(b) (gas is diagram) C and (liquid is diagram) B
In a gas (C), particles are far apart and move randomly. In a liquid (B), particles are close together but can slide past each other.

(c)(i) P = IV (in any form) OR (current =) 3000/240; 12.5 (A)
Using P = IV, I = P/V = 3000/240 = 12.5 A.

(c)(ii) energy = power × time (in any form); conversion of minutes to hours; conversion of W to kW; (= 0.5 kWh)
Energy = 3000 W × (10/60) h = 3.0 kW × 0.167 h = 0.5 kWh.

(c)(iii) ($) 0.25
Cost = 0.5 kWh × $0.50/kWh = $0.25.

(d) insulation damaged / exposed (live) wire; electric shock
Damaged insulation exposes live wires. If a person touches the exposed wire, they may receive an electric shock, which can be fatal as current passes through the body.

Question

(a) Complete Table 12.1 by drawing the circuit symbol for each electrical component.
(b) Fig. 12.1 shows a house with solar panels on the roof.
The solar panels contain many solar cells.
State one advantage and one disadvantage of generating electricity using solar cells.
(c) A washing machine in the house uses electricity generated from the solar cells.
Fig. 12.2 shows the washing machine connected to a 230V supply.
(i) The washing machine uses 2500W of power when switched on.
Calculate the current in the washing machine.
(ii) The water in the washing machine is heated by a heating element.
A current of 3.0A passes through the heating element when the voltage across the heating element is 230V.
Calculate the resistance of the heating element.
(iii) Inside the washing machine, some of the water evaporates.
During evaporation water changes state from liquid to gas.
Describe the process of evaporation in terms of particles.

Topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic P4.3.1 — Circuit diagrams and circuit components
• Topic P1.6.3 — Energy resources
• Topic P4.2.4 — Resistance
• Topic P4.2.5 — Electrical energy and electrical power
• Topic P2.2.2 — Melting, boiling and evaporation

▶️ Answer/Explanation

(a) Motor: circle with ‘M’ inside. Variable resistor: rectangle (or zigzag) with a diagonal arrow across it.
The motor symbol indicates conversion of electrical energy to kinetic energy.
The arrow on the variable resistor symbol shows that its resistance can be adjusted, making it useful for controlling current in circuits such as dimmer switches or volume controls.

(b) Advantage: renewable / produces no greenhouse gases. Disadvantage: does not work at night / only works in sufficient sunlight.
Solar cells convert light energy directly into electricity with no fuel costs and zero emissions during operation.
However, their output depends entirely on sunlight availability, making them unreliable without battery storage, and initial installation costs can be high.

(c)(i) Current in the washing machine:
\(I = \dfrac{P}{V} = \dfrac{2500}{230} \approx \textbf{11\,A}\)
Using \(P = IV\) and rearranging for current, dividing the power (2500 W) by the supply voltage (230 V) gives approximately 10.87 A, which rounds to 11 A to 2 significant figures.

(c)(ii) Resistance of the heating element:
\(R = \dfrac{V}{I} = \dfrac{230}{3.0} \approx 77\,\Omega\)
Applying Ohm’s law \(V = IR\) and rearranging gives resistance as voltage divided by current.
The high resistance of the element is what causes electrical energy to be converted efficiently into heat.

(c)(iii) The most energetic particles near the liquid surface have enough kinetic energy to overcome intermolecular attractions and escape into the gas phase.
Because it is the fastest-moving particles that escape, the average kinetic energy of the remaining liquid falls, causing the liquid to cool.
Evaporation can occur at any temperature, unlike boiling which requires the entire liquid to reach a fixed boiling point.

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