Video Transcript
Each of the following diagrams
shows a copper ring moving near to a stationary bar magnet. In each case, the red arrow shows
the direction in which the copper ring is moving. In which case, if any, will no
current be induced in the copper ring? (A), (B), (C), (D), or (E) a
current will be induced in the ring in all of the cases shown.
To answer this question, we need to
determine whether a current is induced in the ring in each of these scenarios. In each option, the bar magnet is
stationary and the copper ring is moving. Let’s start by clearing some space
on screen and reminding ourselves about electromagnetic induction.
Imagine a stationary copper ring,
on its own, with no magnets around it. The ring isn’t connected to any
kind of circuit, and there’s nothing else near the ring. So there is no current in this
ring, because there is nothing that could cause the charges in the ring to flow. Now consider what would happen if
we placed a stationary bar magnet near to the ring. In this case, there is still no
current in the ring. But some of the bar magnet’s
magnetic field lines now pass through the ring, like this. If the ring started to move, the
magnetic field that passes through the ring would change. When the ring is in a different
position, the number of magnetic field lines that pass through the ring might be
different, or the direction of the field lines might be different. If either of these things change,
the magnetic field passing through the ring changes.
So, when the ring is moving, the
magnetic field that passes through the ring is changing. If the ring was stationary and the
bar magnet was moving, this would also cause the magnetic field that passes through
the ring to change. The change in magnetic field
creates a current in the ring. This process is called
electromagnetic induction. When the ring moves, the magnetic
field that passes through the ring changes, and a current is induced in the
ring.
To answer this question, we need to
work out whether any of the options given show a case in which no current will be
induced in the ring. For this to be the case, we would
need both the ring and the magnet to be stationary. But we know that each of the
diagrams shows a ring that is moving near to a bar magnet. So a current will be induced in all
of the rings shown. Note that it doesn’t matter which
direction the ring is moving in. As long as the ring is moving, a
current will be induced in the ring.
So a current is induced in the ring
in all the cases we have been given. The correct answer is therefore
option (E).
