Energy and Power in Circuits

Electrical Energy and Power

Electrical energy is the ability of an electric current to do work. Energy is measured in Joules (J).
Power is the rate at which electrical energy is transferred or converted. Power is measured in Watts (W).
The formula linking power (P), voltage (V) and current (I) is P = V x I.
Another formula for power in terms of resistance (R) and current is P = I^2 x R. ‘I^2’ stands for current squared (I multiplied by I).
The amount of electrical energy transferred can also be calculated using the formula E = P x t, where E is energy, P is power, and t is time in seconds.

Charge is a property of particles which causes them to experience a force when close to other charged particles.
Electric current is the flow of electrical charge. It is measured in Amperes (A).
The formula that links charge, current and time is Q = I x t, where Q is charge in Coulombs (C), I is current, and t is time in seconds.

Not all energy used by an electrical device is converted into useful forms. Some is ‘wasted’ as heat or sound.
The efficiency of an electrical device is the ratio of useful power out to the total power in.
Efficiency can be improved by reducing wasted energy (for example, by using insulation or more efficient technology).

The cost of electrical energy depends on the power of the device, the amount of time it is used and the cost per unit of electricity (usually given in pence per kilowatt-hour, p/kWh).
The formula to calculate cost is Cost = Power (kW) x Time (hours) x Cost per unit (p/kWh).
Converting power in Watts to kilowatts involves dividing the power by 1000. Converting time in seconds to hours involves dividing the time by 3600.