pearsons science yr8 chap 5.3

5.3.1 What is electrical energy?

•    Electrical energy is the energy carried by moving electric charges (electrons) in a conductor.

•    Common sources: mains electricity, batteries, solar cells converting light to electrical, generators.

•    Everyday examples: lights, computers, motors, charging devices.

•    Practice question: Identify three devices in your home that use electrical energy and state what they do (purpose).

5.3.2 Circuits and components

•    An electric circuit is a closed path through which electric current flows.

•    Key components: power source (battery, cell, mains), connecting wires, load (lamp, motor, resistor), switch.

•    Circuit diagrams use symbols to represent components.

•    Current in a circuit is the flow of charge (measured in amperes, A).

•    Practice question: Draw a simple circuit diagram with a battery, switch and lamp; label each component.

5.3.3 Energy transformations in electrical systems

•    When electrical energy flows through a device (load), it is often transformed into other forms of energy:

◦    Lamp: electrical → light + heat

◦    Motor: electrical → kinetic + heat

◦    Speaker: electrical → sound + heat

•    Efficiency: Some electrical energy is “useful” (e.g., light) and some is wasted (e.g., heat).

•    Practice question: A heater uses 2000 J of electrical energy and gives off 1800 J as heat and 200 J as light. What is the useful energy, and what is wasted?

5.3.4 Measuring electrical energy & cost

•    Electrical energy often measured in kilowatt-hours (kWh) in homes (1 kWh = energy used by a 1 kW appliance running for 1 hour).

•    Formula examples: Energy (kWh) = Power (kW) × time (h).

•    Cost calculation: cost = energy used (kWh) × rate (cost per kWh).

•    Practice question: A 1500 W (1.5 kW) heater runs for 3 hours. How many kWh does it use? If the rate is $0.30 per kWh, how much does it cost?

5.3.5 Safety and efficiency in electrical systems

•    Safety features: fuses, circuit breakers, insulation, earthing — to protect against overloads and faults.

•    Reducing wasted energy in electrical systems improves efficiency and reduces cost and environmental impact.

•    Consider energy rating for electrical appliances: higher ratings = more efficient devices (less wasted energy).

•    Practice question: Explain why replacing an incandescent bulb with an LED saves energy and cost.

Key Vocabulary

Term

Definition

Electric circuit

A complete path through which electric current flows.

Current

The rate of flow of electric charge, measured in amperes (A).

Load

The part of a circuit that uses electrical energy (lamp, motor etc).

Electrical energy

Energy carried by moving electric charges.

Kilowatt-hour (kWh)

A measure of electrical energy: 1 kW used for 1 hour.

Energy rating label

A label showing how much energy an appliance uses and how efficient it is.

Fuse / circuit breaker

Safety device that interrupts the circuit if too much current flows.

Example questions you should be able to answer

1    What is the difference between an open and closed circuit?

2    Draw a circuit diagram with two cells in series, a switch and a lamp.

3    If a fridge uses 2.4 kWh per day and electricity costs $0.28 per kWh, what is the daily cost?

4    Explain how electrical energy is transformed in a TV (including useful and wasted forms).

5    Name three safety features in household electrical systems and explain their purpose.