Video Transcript
Parts (a), (b), (c), and (d) in the
diagram show a straight piece of copper wire moving through a magnetic field. The magnetic field is uniform, and
in each part the wire is moving in a different direction through the magnetic
field. Which of (a), (b), (c), and (d)
show a motion of the wire that would lead to an electric potential difference being
induced in it?
To answer this question, we need to
work out how the direction of the copper wire’s motion will affect the magnitude of
the potential difference that’s induced in the wire. Let’s start by clearing some room
on screen and thinking about how a potential difference can be induced across the
wire.
Imagine we have a straight piece of
copper wire. We can look at the wire from a
different angle, like this, so that the length of the wire is pointing into the
screen. Currently, there is no potential
difference between the two ends of the wire. The wire is not connected to a
circuit, and there’s nothing else near the wire that could create a potential
difference.
Now, let’s think about what would
happen if the wire was then placed into a uniform magnetic field. When the wire is stationary in the
magnetic field, there’s still no potential difference between its two ends. But if the wire then started to
move through the magnetic field, it is possible that a potential difference could be
created across the wire. Imagine that the wire begins to
move in this direction. We can see that as the wire moves,
it will cross these magnetic field lines. When the wire moves in such a way
that it crosses magnetic field lines, a potential difference will be induced across
it. This is electromagnetic
induction.
This question is asking us to work
out which parts of the diagram show the wire moving in such a way that a potential
difference will be induced across the wire. To do this, we simply need to
recognize which of the parts show the wire moving so that it will cross magnetic
field lines. Let’s take a closer look then.
In part (a), the wire is moving
diagonally, up and to the right. If we extend the arrow to show the
path that the wire will take, we see that the wire will cross several of the
magnetic field lines. So we know that a potential
difference will be induced across the wire.
Next, in part (b), the wire is
moving antiparallel to the magnetic field lines, following this path. This means that the wire will never
cross any of the field lines, and therefore no potential difference will be induced
across the wire.
Similarly, part (c) shows the wire
moving parallel to the field lines, along this path. Here, like we saw with part (b),
the wire can never actually cross the field lines. So no potential difference will be
induced.
Finally, part (d) shows the wire
moving perpendicular to the magnetic field lines, following this path. This means that as the wire moves,
it will cross magnetic field lines at a great rate. And because the wire is crossing
the field lines, a potential difference will be induced across the wire.
Thus, we’ve seen that only parts
(a) and (d) show the wire moving in such a way that it will cross magnetic field
lines. So we know that for (a) and (d), a
potential difference will be induced across the wire. In parts (b) and (c), the wire
never crosses any field lines. So no potential difference would be
induced. Therefore, parts (a) and (d) show a
motion of the wire that would lead to an electric potential difference being induced
in it.
