Video Transcript
Each of the following diagrams
shows a bar magnet moving near to a stationary copper ring. In each case, the red arrow shows
the direction in which the bar magnet is moving. In which case, if any, will no
current be induced in the copper ring?
To answer this question, we need to
determine whether a current is induced in the ring in each of these scenarios. In each option we are given, the
bar magnet is moving and the copper ring is stationary.
Let’s start by 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 magnet started to move, the
magnetic field that passes through the ring would change. When the magnet 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 magnet is moving, the
magnetic field that passes through the ring is changing. If the bar magnet was stationary
and the ring was moving, this would also cause the magnetic field that passes
through the ring to change. The changing magnetic field creates
a current in the ring. This process is called
electromagnetic induction. When the bar magnet 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 next to a moving bar magnet. So a current will be induced in all
of the rings shown. Note that it doesn’t matter which
direction the bar magnet is moving in. As long as the magnet 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).
