Electric Current
Electricity and Magnetism

Student starting points - charge flow

Teaching Guidance for 14-16 Supporting Physics Teaching

Diagnostic question on ammeter readings

The following question, on electrical current, was set to a group of students after their 11–14 teaching on electric circuits:

In this circuit the ammeter at position 1 reads 0.3 ampere. Predict the value of the current at positions 2 and 3.

Current at ammeter 2 is …

Current at ammeter 3 is …

Explain why you have predicted these values.

Correct answers:

Current at ammeter 2 is 0.3 ampere

Current at ammeter 3 is 0.3 ampere


The current is the same in every element round this simple series circuit. This is because the rate of flow of charge (the number of coulombs of charge passing each point every second) is the same for every point in the circuit.

Some students' answers and implications

Student 1

Ammeter 2: 0.6

Ammeter 3: 0.3

Because the energy in the charges go past the ammeter with energy in the charges then when it gets to number 2 the charges are empty so it is low.

Student 2

Ammeter 2: 0.3 ampere

Ammeter 3: 0.3 ampere

Because the flow of energy will be the same all the time and the bulbs will be dim.

Student 3

Ammeter 2: 0.3 ampere

Ammeter 3: 0.3 ampere

Because all the current has been shared equally.

Student 4

Ammeter 2: 0.3 ampere

Ammeter 3: 0.3 ampere

I predicted these values because the current stays the same speed all the time.

Student 5

Ammeter 2: 0.3 ampere

Ammeter 3: 0.3 ampere

No explanation given.

Thinking about the teaching

Although four out of five students give correct predictions of the value of the current, the explanations do not inspire confidence! At least some student answers are off down the wrong tracks.

Students 1 and 2 refer to energy in their explanations, possibly getting mixed up with ideas of voltage. The way in which student 3 refers to the current being shared seems closer to an explanation for a parallel circuit. The answer given by student 4 is more encouraging, although it would be preferable to say the charged particles drift along at the same rate everywhere.

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