# Magnetic Fields question

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#1

Hey - for part (b) (i), and (ii), I am quite confused - could someone please explain this to me? Thanks!
0
3 years ago
#2
Do you know what Lenz's law is?

It says that the direction of induced current in a conductor by a magnetic field is such that it will induce a field to oppose the change that produced it. It is a restatement of the conservation of energy.

In (i), an emf will be induced to oppose the increasing flux through the loop, so the motion will be opposed, hence acceleration will be less than g

In (ii), an emf will be induced to oppose the decreasing flux through the loop, so the motion will be opposed, hence the acceleration will be less than g
1
3 years ago
#3
The acceleration is less than g in both cases. This is because of Lenz's law: the direction of an induced current opposes the magnetic field that produces it. In this example, the falling magnet has induced a current within the coil, therefore producing a magnetic field which opposes the magnetic field of the permanent magnet and its motion

i) As the magnet enters, a current is induced, therefore a magnetic field is set up to oppose the magnetic field of the permanent magnet entering the coil. Essentially, the top of the coil becomes a North pole thus opposing the motion of the permanent magnet (since the 'front' of the falling magnet is also North), causing it to slow down (as it's repelled) so acceleration is less than g.

When the permanent magnet is in the middle of the coil, the magnetic poles instantaneously switch and there will be no pd. Somebody please correct me if this is wrong!

ii) When it has left the coil, due to g acting on the magnet, a current is induced that, again, creates a magnetic field that opposes that of the permanent magnet. That is, the top of the coil is now South, the bottom is a North, hence the top of the magnet is attracted to the coil (as the 'tail' of the falling magnet is South) and so its acceleration is less than g.

Oh god, I waffled so much. Sorry!!
0
3 years ago
#4
(Original post by Lauren-x-)
When the permanent magnet is in the middle of the coil, the magnetic poles instantaneously switch and there will be no pd. Somebody please correct me if this is wrong!
There is a point in the middle of the wire where the rate of change of flux through the wire is instantaneously zero, and so the emf induced is zero at this point too by Faraday.
1
3 years ago
#5
(Original post by crashMATHS)
There is a point in the middle of the wire where the rate of change of flux through the wire is instantaneously zero, and so the emf induced is zero at this point too by Faraday.
0
#6
(Original post by Lauren-x-)
(Original post by crashMATHS)
There is a point in the middle of the wire where the rate of change of flux through the wire is instantaneously zero, and so the emf induced is zero at this point too by Faraday.
(Original post by Lauren-x-)
The acceleration is less than g in both cases. This is because of Lenz's law: the direction of an induced current opposes the magnetic field that produces it. In this example, the falling magnet has induced a current within the coil, therefore producing a magnetic field which opposes the magnetic field of the permanent magnet and its motion

i) As the magnet enters, a current is induced, therefore a magnetic field is set up to oppose the magnetic field of the permanent magnet entering the coil. Essentially, the top of the coil becomes a North pole thus opposing the motion of the permanent magnet (since the 'front' of the falling magnet is also North), causing it to slow down (as it's repelled) so acceleration is less than g.

When the permanent magnet is in the middle of the coil, the magnetic poles instantaneously switch and there will be no pd. Somebody please correct me if this is wrong!

ii) When it has left the coil, due to g acting on the magnet, a current is induced that, again, creates a magnetic field that opposes that of the permanent magnet. That is, the top of the coil is now South, the bottom is a North, hence the top of the magnet is attracted to the coil (as the 'tail' of the falling magnet is South) and so its acceleration is less than g.

Oh god, I waffled so much. Sorry!!
(Original post by crashMATHS)
Do you know what Lenz's law is?

It says that the direction of induced current in a conductor by a magnetic field is such that it will induce a field to oppose the change that produced it. It is a restatement of the conservation of energy.

In (i), an emf will be induced to oppose the increasing flux through the loop, so the motion will be opposed, hence acceleration will be less than g

In (ii), an emf will be induced to oppose the decreasing flux through the loop, so the motion will be opposed, hence the acceleration will be less than g
Thank you all so much!! <3
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