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Current, voltage and resistance Edexcel International GCSE Physics revision

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In plain words

Current is the flow of charge round a circuit, like water flowing through pipes. Voltage (potential difference) is the push that drives it. Resistance is how hard the circuit makes it for the current to flow. More push gives more current; more resistance gives less.

Three things to know

  1. Current is the rate of flow of charge: charge = current × time (Q = It). It is measured in amperes with an ammeter in series.
  2. Potential difference (voltage) is the energy transferred per unit charge, measured in volts with a voltmeter in parallel.
  3. Resistance = potential difference ÷ current (R = V/I), in ohms. A longer or thinner wire has more resistance.

Worked example

A 12 V battery is connected across a 4 Ω resistor. Find the current.

  1. I = V ÷ R.
  2. = 12 ÷ 4 = 3 A.

Tips and tricks

  • Ammeters go in series (in the line). Voltmeters go in parallel (across the component).
  • The triangle: V on top, I and R underneath. Cover the one you want and read off what to do.
6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Current, voltage and resistance: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. How should an ammeter be connected to measure the current in a lamp?
    • in series with the lamp (the answer)
    • in parallel with the lamp
    • across the battery
    • it does not matter

    The same current must pass through the ammeter and the lamp.

  2. A charge of 20 C passes a point in 4 s. What is the current?
    • 0.2 A
    • 5 A (the answer)
    • 24 A
    • 80 A

    I = Q ÷ t = 20 ÷ 4.

  3. The p.d. across a 10 Ω resistor is 5 V. What is the current?
    • 0.5 A (the answer)
    • 2 A
    • 15 A
    • 50 A

    I = V ÷ R = 5 ÷ 10.

  4. Which change increases the resistance of a metal wire?
    • making it shorter
    • making it thicker
    • making it longer (the answer)
    • cooling it

    A longer wire gives the electrons more collisions on the way.

  5. What is the unit of resistance?
    • volt
    • ampere
    • watt
    • ohm (the answer)

    Resistance is measured in ohms (Ω).

  6. A current of 2 A flows through a resistor when the p.d. across it is 12 V. What is its resistance?
    • 0.17 Ω
    • 6 Ω (the answer)
    • 14 Ω
    • 24 Ω

    R = V ÷ I = 12 ÷ 2.

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