# Student starting points – number of cells(Exposition)

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?

CharList

• It's dim.
• It's glowing normally.
• It's glowing brightly.
• It's off.
• EndList

The bulb is bright.

Explanation:

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.

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.