Electrical energy and electrical power
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Energy Transfer in Circuits
- Electric circuits transfer energy from a source (an electrical cell or mains supply) to the circuit components, and then into the surroundings
- source → components → surroundings (as heat, light, sound or kinetic energy)
- Examples:
- Lamp: electrical energy → light + heat
- Motor: electrical energy → kinetic energy + heat
- Heater: electrical energy → heat
- Loudspeaker: electrical energy → sound + heat
Every appliance transfers electrical energy into a useful output plus some wasted heat
Electrical Power
- Power is the rate at which energy is transferred, or work is done
- measured in watts (W); 1 W = 1 J/s; 1 kilowatt (kW) = 1000 W
- Formula:
- P = IV
- Where:
- P = power (W)
- I = current (A)
- V = potential difference (V)
Worked Example: Calculating Power
Museli has a lamp that operates at 12 V and draws a current of 0.5 A, and wants to find its power.
- Step 1: write down the equation. P = IV
- Step 2: substitute the values. P = 0.5 × 12
- Step 3: calculate the answer. P = 6 W
Electrical Energy
- Formula:
- E = IVt
- Where:
- E = energy (J)
- I = current (A)
- V = p.d. (V)
- t = time (s)
- Since P = IV, this can also be written as:
- E = Pt
Worked Example: Calculating Energy
Muchindu switches on a 60 W lamp for 5 minutes, and wants to find how much energy it uses.
- Step 1: convert the time to seconds. t = 5 × 60 = 300 s
- Step 2: calculate the energy. E = Pt = 60 × 300 = 18000 J = 18 kJ
The Kilowatt-hour and Electricity Costs
- Electricity companies measure energy in kilowatt-hours (kWh), also called "units" of electricity
- 1 kWh is the energy used by a 1 kW appliance operating for 1 hour
- 1 kWh = 1000 W × 3600 s = 3,600,000 J = 3.6 MJ
- Formula:
- Energy (kWh) = Power (kW) × Time (h)
- Cost of electricity:
- Cost = Energy (kWh) × Price per kWh
1 kWh is the energy a 1 kW appliance uses in 1 hour - the "unit" your electricity meter counts
Worked Example: Electricity Bill Calculation
Muumbe uses a 2 kW heater for 3 hours, and wants to find the cost if electricity is priced at 0.15 per kWh.
- Step 1: calculate the energy used. Energy = 2 × 3 = 6 kWh
- Step 2: calculate the cost. Cost = 6 × 0.15 = 0.90
- Total cost = 0.90
Interactive revision notes, videos and practice questions load below.