Power: An energy transfer of 1 joule per second is equal to a power of 1 watt.

The idea

Power measures how quickly energy is transferred or work is done. One watt is defined as a transfer of one joule each second, so 1 W = 1 J/s. The equation is P = E ÷ t or P = W ÷ t, where P is power in watts (W), E or W is energy or work in joules (J) and t is time in seconds (s). For example, a lamp transferring 300 J in 20 s has power 300 ÷ 20 = 15 W. A 60 W bulb transfers 60 J every second. Rearranging gives E = P t, so a 2 kW heater running for 30 s transfers 2000 × 30 = 60 000 J. Convert kilowatts to watts by multiplying by 1000, and minutes to seconds by multiplying by 60, before substituting. Higher-tier students may also combine P = W ÷ t with W = F s to obtain P = F v.

What earns the marks

• States that one watt equals one joule transferred per second, linking power to the rate of energy transfer.
• Applies P = E ÷ t or P = W ÷ t, using energy in joules and time in seconds to obtain power in watts.
• Rearranges the equation to find energy transferred, E = P t, or time taken, t = E ÷ P.
• Converts kilowatts to watts and minutes to seconds correctly before calculating.
• Interprets a power rating, such as 60 W, as 60 J transferred each second.
• Combines P = W ÷ t with W = F s to derive P = F v for a constant speed.

Common mistakes and how to put them right

• Treating watts as a unit of energy rather than power.

  Put it right by stating that watts measure joules per second.

• Substituting time in minutes without converting to seconds, giving a power 60 times too small.

  Put it right by multiplying minutes by 60 first.

• Using P = E t instead of P = E ÷ t.

  Put it right by checking that dividing a larger energy by a longer time gives a smaller power.

Exam tips

• Write the unit equation 1 W = 1 J/s at the start of a power calculation to keep the meaning clear.
• Convert all values to watts, joules and seconds before substituting, and carry units through the working.
• For a rearranged question, state the rearranged equation first, for example t = E ÷ P, then substitute.