Electrical Circuit
Electricity and Magnetism

Student starting points - resistance

Teaching Guidance for 14-16 Supporting Physics Teaching

A diagnostic question about thin wire

The following question, about the idea of electrical resistance, was set to a group of students after their 11–14 teaching about electric circuits.

In circuit B there is a thin piece of wire. The thin wire forms part of the circuit.

  1. Explain why the current is smaller in circuit B.
  2. What will be the brightness of the bulb in circuit B?
  3. Explain your answer.

Correct answers:

  1. The current is smaller in circuit B because the thin wire introduces extra resistance and reduces the rate of flow of charge (the electric current) in all parts of the circuit.
  2. The bulb will get dimmer.
  3. The bulb will get dimmer because the current through it is reduced. Also, the potential difference of the supply is now shared between the wire and the bulb, so there is a smaller potential difference across the bulb. The brightness of the bulb is determined by the input power (P = I × V), and both current and potential difference have been reduced.

Some students' answers and implications

Student 1:

  1. Because it has to go through the bulb then go through the wire and the wire slows the charges down.
  2. It will go dim.
  3. Because the energy is slower than normal so it is dim because the wire is slowing the charges down.

Student 2:

  1. Because the power is taking more energy to get to the bulb to make it light.
  2. It will go dim.
  3. Because the energy has to get through a lot of wire to power the bulb and go through an ammeter to get to the bulb.

Student 3:

  1. Because the wire is slowing the charge down in the circuit.
  2. It will get dimmer.
  3. It will be dimmer because there is less charges getting to it.

Student 4:

  1. The current is smaller in circuit B because the charges slow down when they flow through the wire.
  2. The brightness of the bulb will be normal.
  3. The bulb is normal because if the wire would be in front of the bulb then you could change the brightness but because it is behind you cannot.

Student 5:

  1. Because the longer the wire the greater the resistance.
  2. It will glow very dimly.
  3. Because there is not as much energy flowing through because it is the long piece of wire it has to travel through.

Thinking about the teaching

Student answers – right lines or wrong track? Once again, the good news is that four out of five students give a correct bulb brightness prediction, but as before the explanations aren't so convincing.

Student 1 correctly refers to the wire slowing the charges down but it makes much less sense to state, the energy is slower than normal. Student 2 seems to be arguing from a current going around the circuit components in sequence point of view rather than a simultaneous all-at-once point of view (as modelled by the rope loop). Student 3 gives a correct version of the charge story but there is no reference to potential difference. Student 4 is correct in stating that the charges slow down in circuit B, but is clearly wrong in predicting a normal bulb brightness. It is quite clear that student 4 is using the same kind of sequential reasoning as student 2. Student 5 correctly refers to resistance and is the only one to do so; less convincing is the reference to not as much energy flowing.

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