Question

1 multiple choice 2 points
a student coils a bare copper wire around a metal rod and attaches the ends of the wire to an ampmeter. he quickly moves a magnet past the coil and notes the resulting current. how could the student alter this apparatus to create a larger current?
use thinner wire
use insulated wire
increase the length of the rod that is used
increase the number of times the wire is coiled around the rod
2 multiple choice 2 points
in which case would it take the most effort to make points 1 and 2 on the magnets touch each other?

Explanation:

Question 1

According to Faraday's law of electromagnetic induction, the induced electromotive force (emf) in a coil is given by $\mathcal{E}=-N\frac{\Delta\Phi}{\Delta t}$, where $N$ is the number of turns in the coil and $\Delta\Phi$ is the change in magnetic flux. The current $I = \frac{\mathcal{E}}{R}$ (where $R$ is the resistance of the circuit). Increasing the number of turns $N$ (by coiling the wire more times around the rod) will increase the induced emf and thus the current (assuming resistance changes are negligible compared to the change in $N$).

• Using thinner wire would increase the resistance $R$ (since $R=

ho\frac{l}{A}$, where $
ho$ is resistivity, $l$ is length, and $A$ is cross - sectional area; thinner wire has smaller $A$), and by $I=\frac{\mathcal{E}}{R}$, this would decrease the current.

• Using insulated wire doesn't change the magnetic flux change or the number of turns in a way that would increase the induced current (the key for induction is the change in magnetic flux through the loop, and insulation doesn't affect the flux linkage in a beneficial way for current increase here).
• Increasing the length of the rod doesn't directly affect the number of turns per unit length (if the wire is just coiled around a longer rod without increasing the number of turns, it won't increase the flux linkage per unit time in a way that increases the induced current).

Magnets have the property that like poles repel and unlike poles attract. We need to find the case where the force between points 1 and 2 is repulsive (since repulsive forces require effort to bring the magnets together).

• For the first option: Point 1 is $S$ (from $N - S$ magnet) and point 2 is $N$ (from $N - S$ magnet). $S$ and $N$ attract.
• For the second option: Point 1 is $N$ (from $S - N$ magnet) and point 2 is $N$ (from $S - N$ magnet). Like poles ($N - N$) repel.
• For the third option: Point 1 is $N$ and point 2 is $N$. Like poles ($N - N$) repel. But the magnetic field strength at the end of a bar magnet is stronger. In a bar magnet, the poles are at the ends. When we consider the configuration in the second option (where the "poles" at points 1 and 2 are more "concentrated" as they are at the ends of the bar - like magnet configuration compared to the third option where the magnets are shorter in the direction of the poles at points 1 and 2), the repulsive force is stronger.
• For the fourth option: Point 1 is $N$ and point 2 is $S$. They attract.

Answer:

increase the number of times the wire is coiled around the rod

Question 2