Electrical Circuit
Electricity and Magnetism

Student starting points - number of cells

Teaching Guidance for 14-16 Supporting Physics Teaching

Diagnostic question about adding a cell

The following question, relating to the effect of increasing the number of cells in a circuit, was set to a group of students after their 11–14 teaching about electric circuits.

The bulb in circuit A is glowing normally.

How is the bulb in circuit B glowing?

  1. It's dim.
  2. It's glowing normally.
  3. It's glowing brightly.
  4. It's off.

Explain your answer.

Correct answer:

The bulb is bright.


With two cells there is a larger current everywhere in the circuit and a bigger potential difference is created across the bulb. The power output of the bulb (which depends on the potential difference and on the current) is therefore larger.

Students' answers and implications

Student 1:

c. Bright.

This is because instead of getting 100 % energy it is getting 200 % to make light up double more than normal.

Student 2:

c. Bright.

Because there is more energy and it only has to power one bulb so the bulb will be brighter than with one battery.

Student 3:

c. Bright.

There is more charges getting to it because there are charges coming out of each battery.

Student 4:

c. Bright.

The more batteries the more charges the more energy but more bulbs it would be dim or off.

Student 5:

c. Bright.

Because there are two batteries there is double the amount of energy flowing to the bulb.

Student answers – right lines or wrong track? All of the students make a correct prediction of bulb brightness.

Students 1, 2 and 5 all refer to there being more energy for the bulb. Students 3 and 4 suggest that there are more charges getting to it. This is correct in the sense that with two cells the charge moves round the circuit at a greater rate. It is clear, however, that student 3 is thinking in terms of the extra cell supplying more charges. This is not correct, unfortunately.

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