PhysicsChapter 7: Current Electricity
Energy and Cost
Introduction
Electrical energy used by a device depends on power and time. E = Pt If power is measured in watts and time in seconds, energy is obtained in joules.
Definition
Kilowatt-hour
For household electricity, energy is commonly measured in **kilowatt-hours (kWh). 1 kWh = 3.6 × 10⁶ J A kilowatt-hour is a unit of energy**, not power.
Cost
Electricity cost can be calculated using: **Cost = energy used in kWh × price per kWh**
Worked Example
A 2 kW heater operates for 3 h. Energy: E = Pt `=2 kW × 3 h` **=6 kWh If electricity costs 50 currency units per kWh: Cost = 6 × 50 = 300**
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Review the key facts, formulas and checks before you move on.Energy and Cost
Sticky Notes · Quick Revision
E=Pt
- E=Pt
HOUSEHOLD UNIT
- kWh
1 kWh
- 3.6 MJ
COST
- kWh × tariff