Energy transfer
powerThe energy transferred each second, measured in watts (W). Power = work done Ă· time taken. is the rate of transfer of energy between energy storeThe different ways in which energy can be stored, including chemical, kinetic, gravitational potential, elastic potential and thermal stores..
Energy transferred
The energy transferred can be calculated using the equation:
\(power = \frac{energy~transferred}{time}\)
This is when:
- power is measured in watts (W)
- energy is measured in joules (J)
- time is measured in seconds (s)
Example
When a lamp is switched on for 60 s, 3,000 J of energy are transferred. Calculate the power of the lamp.
\(power = \frac{energy~transferred}{time}\)
\(= 3,000 \div 60\)
\(= 50~W\)
The energy transferred can be calculated using the same equation but with different units:
- energy is measured in kilowatt-hours (kWh)
- power is measured in kilowatts (kW)
- time is measured in hours (h)
This is normally used when considering domestic electricity usage.
Example
A 500 W television set is switched on for 4 hours. Calculate the energy transferred.
\(500~W = \frac{500}{1,000} = 0.5~kW\)
\(power = \frac{energy~transferred}{time}\)
Rearrange the equation:
\(energy~transferred = power \times time\)
\(= 0.5 \times 4\)
\(= 2~kWh\)
Energy, voltage and charge
When a charge moves through a potential difference, electrical workEnergy transferred by a force. Work done = force Ă— distance moved in the direction of the force. is done and energy transferred. The energy transferred can be calculated using the equation:
energy transferred (work done) (J) = charge (C) Ă— potential difference (V)
\(E = V \times Q\)
This is when:
- energy (E) is measured in joules (J)
- potential difference (V) is measured in volts (V)
- charge (Q) is measured in coulombs (C)
One volt is the potential difference when one coulomb of charge transfers one joule of energy.
Example
How much energy is transferred when 3 C of charge moves through a potential difference of 6 V?
\(E = V \times Q\)
\(E = 6 \times 3\)
\(E = 18~J\)
Question
What is the potential difference between two points if 2 C of charge shifts 4 J?
\(E = V \times Q\)
\(V = \frac{E}{Q}\)
\(V = \frac{4}{2}\)
\(V = 2~V\)
Learn more on power in this podcast
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