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Energy transfer

is the rate of transfer of energy between .

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 is done and energy transferred. The energy transferred can be calculated using the equation:

energy transferred (work done) (J) = charge (C) 脳 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?

Learn more on power in this podcast