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2.1.1Electricity

Charge and Current

5 min read 5 parts Lesson 2 of 18 1 quiz

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Section 1 of 5Charge and Current

2.1.1 Charge and Current

Electric current is a flow of charge, and it is measured in amps.

Electric charge
  • A property of particles such as electrons and protons, measured in coulombs (C).
Electric current
  • The rate of flow of electric charge round a circuit, measured in amperes (A).
Electron flow
  • Electrons carry negative charge and move from the negative terminal to the positive terminal of a cell.
Conventional current
  • The direction of current taken as flowing from positive to negative, which is opposite to electron flow.
One coulomb
  • One coulomb is the charge that flows past a point when a current of one amp flows for one second.

Section 2 of 5Calculating Charge

The equation Q = I × t is rearranged to find current or time when the other values are known.

1
Identify the current in amperes and the time in seconds.
2
Convert any time in minutes into seconds by multiplying by 60.
3
Multiply the current by the time to find the charge in coulombs.
4
Rearrange Q = I × t if the question asks for current or time instead.

Section 3 of 5Measuring Current

Measuring Current
Ammeter position
  • An ammeter is placed in series with the component so that the same current flows through both.
Low resistance
  • An ideal ammeter has very low resistance, so it does not reduce the current it measures.
Reading the scale
  • The ammeter reading gives the current directly in amps, which may need converting to milliamps.
Mistake to avoid
  • Connecting an ammeter in parallel with a component can damage the meter or the circuit.

Section 4 of 5Worked Example

Question

  • A current of 0.25 A flows through a lamp for 2 minutes.
  • Calculate the charge that flows through the lamp.

  1. Convert time2 minutes = 2 × 60 = 120 s.
  2. Use Q = I × t = 0.25 A × 120 s.
  3. Q = 30 C.

Answer The charge that flows through the lamp is 30 C.

Section 5 of 5Exam Checklist & Tips

2.1.1 – Key Points for the Exam

  • Charge The quantity of electricity carried by particles, measured in coulombs.
  • Current The rate of flow of charge, equal to charge divided by time.
  • Ampere The SI unit of current, equal to one coulomb per second.
  • Ammeter A meter used to measure current, placed in series in a circuit.
  • Conventional current The direction of current from positive to negative terminal.

Common Mistakes

  • Using minutes instead of seconds in Q = It, which gives a charge that is 60 times too small.
  • Confusing the direction of conventional current with the direction of electron movement.
  • Connecting an ammeter in parallel rather than in series with the component.
  • Stating that current is a quantity of charge; current is the rate of flow of charge.
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