Electrical charge and current: charge flow, Q, in coulombs, C

The idea

Charge flow, Q, is the total quantity of electrical charge that passes a point in a circuit. It is measured in coulombs, symbol C. One coulomb passes when a current of one ampere flows for one second, so Q = I t. Charge is carried by electrons in metal wires; each electron carries 1.6 × 10⁻¹⁹ C, so 1 C corresponds to about 6.25 × 10¹⁸ electrons. For example, a current of 0.2 A for 50 s transfers 10 C. Charge is not used up around a series circuit: the same charge passes each point, though energy is transferred to components. In parallel branches, charge splits between branches and recombines. Always give Q in coulombs and avoid confusing charge with current or energy.

What earns the marks

• Identify Q as charge flow and state that its unit is the coulomb, symbol C.
• Define one coulomb as the charge passing a point when one ampere flows for one second.
• Use Q = I t to calculate charge flow, ensuring current is in amperes and time in seconds.
• Recognise that charge is conserved around a circuit and is not used up by components.
• Link charge to the number of electrons using the charge on one electron, 1.6 × 10⁻¹⁹ C.
• Distinguish charge in coulombs from current in amperes and energy in joules.

Common mistakes and how to put them right

• Writing the unit as C s⁻¹ or A; correct by using C for charge, since C s⁻¹ is the unit of current.

• Treating charge as used up by a lamp.

  Put it right by stating charge is conserved and energy is transferred instead.

• Mixing up Q and I in the equation.

  Put it right by checking that Q is the quantity in coulombs and I is the rate in amperes.

Exam tips

• Underline the quantity asked for, then choose the matching unit before calculating.
• Show the equation and substitution so the examiner can credit correct physics even if arithmetic slips.
• Use standard form for very small charges such as the charge on an electron, 1.6 × 10⁻¹⁹ C.