Source: High school physics (Chinese)
Problem Sets:
Problem
A primary coil carrying a battery and a switch $S$, and a secondary coil connected to a resistor $R$, are wound on the same core. Consider the current through $R$ in each of the following cases.
- At the instant the switch $S$ is closed: is there a current through $R$, and in what direction?
- Some time after $S$ has been closed: is there a current through $R$?
- At the instant $S$ is opened: is there a current through $R$, and in what direction?
(1) Yes -- an induced current flows through $R$, opposing the rising flux. (2) No current (the flux is constant). (3) Yes -- an induced current flows through $R$, in the direction opposite to case (1).
A current flows in $R$ only while the magnetic flux through the secondary coil is changing (Lenz's law). At the instant $S$ closes, the primary current rises from zero so the flux increases, driving an induced current through $R$ that opposes the increase. After $S$ has been closed for a while the primary current is steady and the flux is constant, so no current flows in $R$. At the instant $S$ opens, the primary current falls to zero so the flux decreases, driving an induced current through $R$ opposite in direction to the closing instant.