Every electrical appliance transfers energy from the electrical store:
ELECTRIC MOTOR (washing machine, fan, drill): electrical → kinetic + thermal
ELECTRIC HEATER (oven, toaster): electrical → thermal
LAMP/LED: electrical → light + thermal
SPEAKER: electrical → sound
CHARGER: electrical → chemical (battery)
All appliances dissipate some energy as thermal — no appliance is 100% efficient.
Calculating Energy Transferred
E = P × t (joules; time in SECONDS)
EXAMPLE 1:
2 kW kettle for 3 minutes:
P = 2000 W, t = 180 s
E = 2000 × 180 = 360,000 J
KILOWATT-HOUR (kWh) — energy company unit:
Energy (kWh) = Power (kW) × time (hours)
1 kWh = 3,600,000 J
EXAMPLE 2:
3 kW shower for 0.5 hours:
Energy = 3 × 0.5 = 1.5 kWh
Energy Cost and Comparison
Cost = energy (kWh) × price per kWh
EXAMPLE:
1.5 kWh at 30p per kWh = 45p
COMPARING APPLIANCES:
High power × short time vs low power × long time:
Fridge: 200 W × 24 h = 4.8 kWh/day
Kettle: 2 kW × 5 min/day ≈ 0.17 kWh/day
Fridge uses far more despite lower power — it runs continuously.
REDUCING ENERGY USE:
LED bulbs instead of filament.
Shorter usage times.
Better insulation → less heating demand.
⚠️ Common Mistake
For E = Pt in joules, time must be in SECONDS. For kWh, use kW and hours. Mixing units is the most common error. High-power appliance used briefly can use LESS energy than a low-power one running for hours.
📐 Variables
EEnergy transferred (E) is measured in joules (J)
PPower (P) is measured in watts (W)
tTime (t) is measured in seconds (s)
📐 Key Equations
E = P × t
Energy (kWh) = Power (kW) × time (hours)
📌 Key Note
E = Pt (J, t in seconds). kWh = kW × hours. 1 kWh = 3.6 MJ. Cost = kWh × price/unit. Motor → kinetic; heater → thermal; lamp → light; speaker → sound; charger → chemical.
🎯 Matching Activity — Appliance Energy Transfers
Match each appliance to its primary useful energy output. — drag the symbols on the right to match the component names on the left.
Electric motor
Drop here
Electric heater
Drop here
Lamp/LED
Drop here
Speaker
Drop here
Phone charger
Drop here
Kinetic energy — spinning drum, fan or drill
Sound — kinetic energy of vibrating air
Light — plus thermal as waste
Chemical energy — stored in rechargeable battery
Thermal energy — heating room or food
⚽ FIFA Worked Examples
Energy Calculation
A 1.5 kW toaster is used for 4 minutes. Calculate the energy transferred in joules.
F
E = P × t
I
P = 1500 W, t = 4 × 60 = 240 s
F
E = 1500 × 240
A
E = 360,000 J (360 kJ)
🎯 Test Yourself
Question 1 of 2
1. A 2 kW heater runs for 3 hours. How much energy does it use and what does it cost at 25p/kWh?
2. A 200 W fridge runs continuously. A 2000 W kettle runs 5 min/day. Which uses more energy in 24 hours?
⭐ How Well Do You Understand This Topic?
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