Electrical energy and electrical power
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Energy Transfer in Circuits Electric circuits transfer energy from a source (battery, cell, mains supply) to circuit components and then into the surroundings . Energy Flow Source (chemical/electrical energy) → Components (electrical energy) → Surroundings (heat, light, sound, kinetic energy) Lamp: Electrical energy → light + heat Motor: Electrical energy → kinetic energy + heat Heater: Electrical energy → heat Loudspeaker: Electrical energy → sound + heat Electrical Power Power The rate at which energy is transferred or work is done.
Measured in watts (W). P = IV Where: P = power (W), I = current (A), V = potential difference (V) Units 1 watt (W) = 1 joule per second (J/s) 1 kilowatt (kW) = 1000 W Example: Calculating Power Question: A lamp operates at 12 V and draws a current of 0.5 A.
Calculate its power. Answer P = IV = 0.5 × 12 = 6 W Electrical Energy E = IVt Where: E = energy (J), I = current (A), V = p.d. (V), t = time (s) This can also be written as: E = Pt (since P = IV) Example: Calculating Energy Question: A 60 W lamp is switched on for 5 minutes.
How much energy does it use? Answer t = 5 × 60 = 300 s E = Pt = 60 × 300 = 18000 J = 18 kJ The Kilowatt-Hour (kWh) Electricity companies measure energy in kilowatt-hours (kWh) , also called "units" of electricity.
Kilowatt-hour (kWh) The energy used by a 1 kW appliance operating for 1 hour. Energy (kWh) = Power (kW) × Time (h) Conversion 1 kWh = 1000 W × 3600 s = 3,600,000 J = 3.6 MJ Calculating Electricity Costs Cost = Energy (kWh) × Price per kWh Example: Electricity Bill Calculation Question: A 2 kW heater is used for 3 hours.
If electricity costs 0.15 per kWh, what is the cost? Answer Energy = 2 × 3 = 6 kWh Cost = 6 × 0.15 = \0.90
Interactive revision notes, videos and practice questions load below.