Energy transfers in everyday appliances: charge flow, Q, in coulombs, C

The idea

Charge flow, Q, is the quantity of electric charge that passes a point in a circuit, measured in coulombs (C). It links current and time through Q = I × t, where I is current in amperes and t is time in seconds. For example, a current of 3 A flowing for 20 s gives Q = 3 × 20 = 60 C. Charge flow also connects to energy transfer: the energy transferred by an appliance depends on the charge flow and the potential difference across it, E = Q × V. One coulomb is the charge transferred when a current of one ampere flows for one second. When solving problems, identify I and t, convert time to seconds, then multiply; rearrange to I = Q ÷ t or t = Q ÷ I if needed.

What earns the marks

• States that charge flow is measured in coulombs (C) and uses the symbol Q.
• Uses Q = I × t to calculate charge flow from current in amperes and time in seconds.
• Rearranges Q = I × t to find current or time when the other two quantities are known.
• Uses Q in E = Q × V to calculate energy transferred when potential difference is known.
• Recognises that one coulomb is the charge transferred by a current of one ampere in one second.

Common mistakes and how to put them right

• Mixing up charge Q with current I.

  Put it right by remembering Q is in coulombs and I is in amperes.

• Using time in minutes without converting to seconds.

  Put it right by multiplying minutes by 60 before calculating Q.

• Forgetting that Q = I × t requires consistent units.

  Put it right by checking that I is in A and t is in s.

Exam tips

• Write the equation Q = I × t, substitute the values, and include the unit C in the answer.
• If the question gives time in minutes, convert to seconds and show the conversion in your working.
• Check whether the question asks for charge flow or energy transfer, and choose Q = I × t or E = Q × V accordingly.