Power: energy transferred, E, in joules, J

The idea

Energy transferred, E, is the total amount of energy moved or converted during a process, measured in joules, J. One joule equals one newton metre, so lifting a 1 N weight through 1 m transfers 1 J. In power calculations, E is the numerator: power P = E ÷ t, so a 600 J transfer in 20 s gives 30 W. Energy is conserved, so the energy transferred to a device equals the useful energy plus any energy dissipated to the surroundings, often by heating. When a device runs for a known time, E = P × t. Always convert kilojoules to joules (1 kJ = 1000 J) and megajoules to joules (1 MJ = 1 000 000 J) before substituting. Record E in J, not W or N, and check the value is sensible for the context.

What earns the marks

• State that E is energy transferred, measured in joules, J, and that 1 J = 1 N m.
• Use E = P × t to calculate energy transferred when power in watts and time in seconds are known.
• Convert kJ to J by multiplying by 1000, and MJ to J by multiplying by 1 000 000, before substitution.
• Recognise that energy transferred to a device equals useful energy plus energy dissipated, usually by heating.
• Substitute values into P = E ÷ t correctly, keeping E in joules and t in seconds.
• Give the unit J with the numerical answer and reject answers left in W or N.

Common mistakes and how to put them right

• Writing the unit as J/s or W.

  Put it right by remembering J is energy, while J/s and W are power.

• Substituting kilojoules directly into E = P × t.

  Put it right by converting kJ to J first, for example 2.4 kJ = 2400 J.

• Confusing E with power P.

  Put it right by checking whether the quantity is total energy in J or energy per second in W.

Exam tips

• Underline the quantity asked for and its unit before calculating, so you know whether to find E in J or P in W.
• Write the equation, substitute numbers with units, then give the answer with the unit J.
• Check the size of your answer: a 2 kW heater running for 30 s transfers 60 000 J, not 60 J.