The diagram shows a four-terminal box connected to a battery and two ammeters.

Question 37

The diagram shows a four-terminal box connected to a battery and two ammeters.

The currents in the two meters are identical.

Which circuit, within the box, will give this result?

Reference: Past Exam Paper – November 2012 Paper 13 Q35 & November 2015 Paper 12 Q38

Solution:

Answer: D.

The ammeter connected close to the battery gives the total current in the circuit while the ammeter connected between terminals 3 and 4 give the current passing through terminal 3 to 4 (current in this one will only be equal to the one close to the battery only if all components are connected in series in the whole circuit, including within the box).


For circuit A, the section from terminal 2 to 4 and finally to 3 is short-circuited (since 1 and 3 are not connected). So, no current flows through the ammeter.


For circuit B, the ammeter connected to 3 and 4 is short-circuited since wires (which have negligible resistance) are connected between 1 to 3, 3 to 4 and 4 to 2.


For circuit C, a parallel connection is formed between the 2 resistors between 1 to 3 and 1 to 2. So, the current splits up at junction 1. The second ammeter is connected in series with one of the resistors, so it only gives the current flowing in one resistor.


For circuit D, the 2 ammeters are in series with the resistor between terminals 1 and 3. The same current flows through components connected in series.