A student sets up a simple series circuit containing a battery, a switch and a lamp. When the switch is closed, charge flows around the circuit and the lamp lights up. Explain what is meant by electric current in this circuit and describe how the amount of charge passing a point in the circuit is related to both the current and the time for which the switch is closed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Electric current is the rate of flow of charge. In a circuit, charge is carried by electrons moving through the conducting wires.
Model answer (5 marks)
Electric current is the rate of flow of charge through a point in the circuit.
Charge is measured in coulombs (C) and current in amperes (A).
The amount of charge that passes a point in the circuit is directly proportional to the current: a greater current means more charge flows in a given time.
It is also directly proportional to the time the switch is closed: the longer the switch is closed, the greater the total charge that passes a point.
Mathematically, charge = current × time.
Charge is measured in coulombs (C) and current in amperes (A).
The amount of charge that passes a point in the circuit is directly proportional to the current: a greater current means more charge flows in a given time.
It is also directly proportional to the time the switch is closed: the longer the switch is closed, the greater the total charge that passes a point.
Mathematically, charge = current × time.
Examiner tips
- Use the definition of current (rate of flow of charge) first. Show the units of charge (C) and current (A). Explain the proportionality to both current and time. State the formula Q=I×t to link the concepts.
Common mistakes
- Confusing current with charge, or giving the definition of charge instead of current. Forgetting to mention the units of charge and current. Not recognising that the total charge depends on both current and time, or giving only one relationship.
Mark scheme (5 marks)
- Electric current is the rate of flow of charge (through a point in the circuit)
- Charge is measured in coulombs (C) and current is measured in amperes / amps (A)
- The greater the current, the greater the charge that passes a point (in a given time)
- The longer the switch is closed (the greater the time), the greater the total charge that passes a point
- Charge = current × time (charge is directly proportional to both current and time)
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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