Q29 (b)

Criteria	Marks
Evaluates the statement with thorough reference to relevant physics principles	4
Evaluates the statement using some relevant physics principles	3
Demonstrates some understanding of relevant physics principle(s)	2
Provides some relevant information	1

Sample answer: Rod $B$ does have a larger displacement from the original position than rod $A$. However this is not due to the current in rod $A$ being larger. The force on rod $B$ due to electromagnetic force from rod $A$ is the same magnitude as the force on rod $A$ due to electromagnetic force from rod $B$, according to Newton’s third law. The magnitude of electromagnetic force between both rods is given by

$$\frac{F}{l} = \frac{\mu_0}{2\pi}\frac{I_1I_2}{r}$$

Since the larger displacement of Rod $B$ cannot be explained by a larger electromagnetic force acting on it, it must be due to Rod $B$ having a smaller mass. A smaller mass would mean that it has a larger acceleration ($a = \frac{F}{m}$), causing it to have a larger displacement during time $t$ ($s = \frac{1}{2}at^2$).