Recall and use the equation: charge (coulomb, C) = current (ampere, A) ×…

The idea

Charge is the quantity of electricity that flows past a point, measured in coulombs (C). Current is the rate of flow of charge, measured in amperes (A), where one ampere is one coulomb per second. Time is measured in seconds (s). The equation charge = current × time links the three: a current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged, current = charge ÷ time and time = charge ÷ current. Always convert minutes to seconds (multiply by 60) and milliamperes to amperes (divide by 1000) before substituting, then state the unit with the answer.

What earns the marks

• Selects the equation charge = current × time and substitutes values with time in seconds and current in amperes.
• Converts a time given in minutes to seconds by multiplying by 60 before calculating.
• Converts a current given in milliamperes to amperes by dividing by 1000 before calculating.
• Rearranges the equation to current = charge ÷ time or time = charge ÷ current when the unknown is not charge.
• States the answer with the correct unit, coulomb (C) for charge, ampere (A) for current or second (s) for time.

Common mistakes and how to put them right

• Substitutes time in minutes directly into the equation.

  Put it right: Convert minutes to seconds by multiplying by 60 first.

• Uses milliamperes as if they were amperes.

  Put it right: Divide milliamperes by 1000 to obtain amperes.

• Adds current and time instead of multiplying them.

  Put it right: Apply the product charge = current × time.

• Omits the unit or gives the wrong unit with the numerical answer.

  Put it right: Attach C, A or s to match the quantity found.

Exam tips

• Write the equation down first, then rearrange it before substituting numbers so the arithmetic is clear.
• Check every unit against the equation: seconds for time, amperes for current, coulombs for charge.
• Estimate the answer roughly to catch a slipped decimal point before finalising your choice.