Physics · OCR H556 · Topic 4.1

Charge and current

Charge, mean drift velocity.

  • 2 sub-topics
  • Taught in Year 12
  • 24 practice questions
  • 2 animated lessons
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What the specification says

Every sub-topic OCR can examine in this topic, with its reference number from the official specification. The Y12 and Y13 tags show when each sub-topic is usually taught.

  1. 4.1.1ChargeY12
  2. 4.1.2Mean drift velocityY12

Try 2 questions

Easy and hard questions from this topic, each with a worked solutions and the mark for every step. All 24 questions in this topic, free, no account needed.

Electric circuits

Easy

Electric current · Year 12 · 3 marks

A current of 0.25 A flows in a wire for 30 s.
  1. (a)
    Calculate the charge that flows.
    (1)
  2. (b)
    Calculate the number of electrons that pass a point in the wire.
    (2)

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Part (a)1 mark

  1. Q=7.50 CQ = 7.50\ \mathrm{C}

Part (b)2 marks

  1. 7.501.60×10−19\frac{7.50}{1.60 \times 10^{-19}}
  2. N=4.69×1019N = 4.69 \times 10^{19}

Total3

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  • A
  • B
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Electric circuits

Hard

Drift velocity · Year 12 · 6 marks

A copper wire of diameter 0.5 mm carries a current of 1.5 A. Copper has 8.5 × 10²⁸ free electrons per m³.
  1. (a)
    Calculate the mean drift velocity of the electrons.
    (3)
  2. (b)
    The wire is joined to a thinner wire of the same metal. Explain what happens to the drift velocity.
    (2)
  3. (c)
    Explain why semiconductors have much higher resistivity than metals.
    (1)

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Part (a)3 marks

  1. AA=1.96×10−7 m2{}= 1.96 \times 10^{-7}\ \mathrm{m^2}
  2. v=InAqv = \frac{I}{nAq}
  3. vv=5.62×10−4 m s−1{}= 5.62 \times 10^{-4}\ \mathrm{m\,s^{-1}}

Part (b)2 marks

  1. The current is the same in both (series).
  2. A is smaller, so v is larger (v ∝ 1/A).

Part (c)1 mark

  1. They have far fewer charge carriers per unit volume (n is much smaller).

Total6

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  • A
  • B
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