Electromagnetic induction question

[aznking1]

Homework Statement

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This is the picture. Sorry i can't add a link cause i do not have 10 posts.

The part that i don't understand is why the aluminium ring takes a longer time to fall when an alternating current is used

The Attempt at a Solution

Take this scenario. Let's say the alternating current is flowing in the coil, and a North pole is induced at the top of the coil and South pole induced at the bottom of the coil.

Since it is alternating, when the current in the coil decreases, the magnetic field and hence magnetic flux through the aluminium ring decreases since it is linked with a soft iron rod.

By lenz's law, the aluminium ring would want to oppose the change, thus a north pole is induced at the top and south pole induced at the bottom of the ring to increase the flux through the ring.

Since unlike poles attract, why does the ring take a longer time to fall?? I know when the current increases the aluminium ring will be repelled but since it is attracted when the current decreases why does it still take a longer time to fall?

Your help is greatly appreciated. Thanks!

 

[anathema]

Hello,

Thank you for bringing up this interesting question. The phenomenon you are describing is known as Lenz's Law, which states that an induced current will always flow in a direction that opposes the change in magnetic flux that caused it. In this scenario, the alternating current in the coil is causing a changing magnetic flux through the aluminium ring, which in turn induces a current in the ring.

Now, let's break down what is happening in each half-cycle of the alternating current. In the first half-cycle, the current in the coil is increasing, which causes a magnetic field to be induced in the ring that opposes this change. This means that the north pole of the ring will be attracted to the south pole of the coil, causing the ring to move upwards. However, during this time, the magnetic field in the coil is also increasing, which means that the induced current in the ring is also increasing. This means that the force of attraction between the ring and the coil is also increasing, causing the ring to accelerate upwards.

In the second half-cycle, the current in the coil is decreasing, which causes the magnetic field to also decrease. This means that the induced current in the ring will also decrease, causing the north pole of the ring to now be repelled by the south pole of the coil. This will slow down the upward motion of the ring. However, during this time, the magnetic field in the coil is also decreasing, which means that the induced current in the ring is also decreasing. This means that the force of repulsion between the ring and the coil is also decreasing, causing the ring to decelerate upwards.

So, in summary, the alternating current causes the ring to both accelerate and decelerate in each half-cycle, resulting in a longer overall time for the ring to fall. I hope this explanation helps to clarify the phenomenon. If you have any further questions, please let me know.