Video Transcript
The direction of the current
induced in the conductor by a changing blank is such that the magnetic field created
by the induced current blank the initial change in the magnetic field.
Now, this question is talking about
induced currents in a conductor by a changing something. We are told that the magnetic field
created by the induced current is somehow related to the initial change in magnetic
field. This statement should seem
familiar. It looks very much like the
definition of Lenz’s law.
Lenz’s law tells us that when
current is created through electromagnetic induction, the direction of the current
is such that it generates a magnetic field opposing the change in the original
magnetic field. To see how exactly this works,
let’s draw a diagram, starting with a loop of some conductor, like a wire. Then, let’s introduce a magnet to
our setup with the north pole facing downwards, which means that the magnetic field
lines from this magnet should look something like this.
We can clearly see that some of the
magnetic field lines are passing through the coil. This is basically a direct
indicator of the amount of magnetic flux passing through the coil. However, just because there’s
magnetic flux passing through the coil doesn’t mean that there should be an electric
current in the coil. It is the change in magnetic flux
through the coil that results in the induction of a current, not the simple presence
of a magnetic field by itself.
One way to do this is by moving the
magnet relative to the coil. If we move the magnet closer to the
coil, then more of the magnetic field lines will pass through the coil, indicating a
larger magnetic flux. This change in flux will produce a
current in the wire loop. How quickly we move the magnet
determines the current that’s produced in the coil. This induced current will generate
its own magnetic field, which will oppose the change in the original magnetic
field. Since we moved the original magnet
in the downward direction towards the coil, then the change in magnetic flux must
also be in the same direction, downwards.
Lenz’s law then tells us that the
opposing magnetic field produced by the induced current in the wire loop should be
in the upwards direction, as it must be in the opposite direction to the original
change. The induced current that would
generate such a magnetic field must be traveling in this direction then, using the
right-hand rule. The right-hand rule is simple to
use. Basically, position the right hand
so the thumb points in the direction of the current. Then, when we curl our fingers, the
fingers point in the direction of the magnetic field. So we can use this right-hand rule
on any one of these generated magnetic fields to see which direction the current is
flowing in.
Now, since Lenz’s law depends on
the change in the original magnetic field, if we instead move the same magnet in the
opposite direction, upwards, then even though the orange magnetic field lines still
point downwards, the current in the coil will flip around. Subsequently, the magnetic field
due to that current will also reverse direction. So it’s not the direction of the
field lines from the magnet that matter, but it’s the direction of the change in
flux, in other words, the change in the magnetic field. This is what Lenz’s law states,
which we can use as a basis for our answer.
The direction of the current
induced in a conductor by a changing magnetic field is such that the magnetic field
created by the induced current opposes the initial change in the magnetic field.
