Energy transfers in everyday appliances: time, t, in seconds, s

The idea

Time, t, is the duration for which a device operates or a charge flows, measured in seconds (s). In this section it links the charge flow through an appliance to the current: Q = I × t, so t = Q ÷ I. It also appears in energy transfer equations such as E = P × t, where t must be in seconds for E to come out in joules when P is in watts. For example, a 2 A current flowing for 30 s transfers Q = 2 × 30 = 60 C. If a question gives minutes, convert first: 5 min = 5 × 60 = 300 s. Always check the unit before substituting, and remember that t is a scalar quantity with the symbol t and the unit s.

What earns the marks

• States that time is measured in seconds (s) and uses the symbol t.
• Uses t in Q = I × t, rearranging to t = Q ÷ I when the charge flow and current are known.
• Uses t in E = P × t, ensuring time is in seconds so energy is in joules.
• Converts minutes to seconds by multiplying by 60, and hours to seconds by multiplying by 3600, before substitution.
• Interprets t as the duration of operation of an appliance or the time for which a current flows.

Common mistakes and how to put them right

• Substituting minutes directly into Q = I × t.

  Put it right by converting to seconds first.

• Confusing t with other quantities such as charge Q or power P.

  Put it right by checking the symbol and unit in the question.

• Forgetting to convert hours to seconds in energy calculations.

  Put it right by multiplying hours by 3600.

Exam tips

• Write the equation, substitute values with units, then give the unit for the answer.
• If time is given in minutes or hours, convert to seconds as the first step and show the conversion.
• Check that the final unit matches what the question asks for, for example C for charge or J for energy.