2

For a bar magnet dropped through a solenoid, we know that the graph of the induced emf looks like this:

enter image description here

This source says that position A is just before the magnet enters the solenoid, B is when the magnet is in the middle of the solenoid, and C is just as the magnet exits the solenoid. ( Source: http://www.physics.usyd.edu.au/~khachan/PTF/magnet_and_solenoid.pdf )

So this source is saying that the peak voltage occurs some place inside the solenoid, right?

However, another website says that the peak emf corresponds to when the just magnet enters and leaves the solenoid, NOT inside the solenoid.

enter image description here

(Source: https://mammothmemory.net/physics/magnets-and-electromagnetism/electromagnetism/drop-a-magnet-through-a-solenoid.html )

Which one is correct??

Thank you...

Freddie
  • 342
  • The graphs look as if they are for a magnet dropped through a 'flat' coil rather than through a solenoid (tube-shaped coil). – Philip Wood Feb 04 '22 at 21:20

3 Answers3

0

What you need to know is that the induced e.m.f. is proportional to the rate of change of magnetic flux: $$\varepsilon = \frac{d\phi}{dt}.$$ Intuitively, this rate is going to be the largest when the magnet is entering the coil and when it is leaving the coil. That is because before entering the coil, the flux through the solenoid is almost negligible in comparison to the flux when the magnet is passing through the entrance. This gives rise to a large change in flux in a short time, i.e. a large e.m.f. A similar argument applies when the magnet is leaving the coil -- once the magnet is outside the coil, the flux through the solenoid is negligible compared to what it was right before the magnet fell out.

Yejus
  • 2,106
0

The voltage versus time graph will depend on the lengths of the magnet and solenoid. The flux and voltage will increase as the magnet enters the coil (with a polarity which depends on which pole comes first). With the magnet completely inside, the voltage will stay close to zero until it leaves. Since the speed increases as the magnet falls, the two voltage pulses will not have the same shape. If the magnet is long compared with the coil, the response will be similar.

R.W. Bird
  • 12,139
0

The peak occurs when maximum field lines cut through the coils. This would happen when the magnet is just about to enter the solenoid. So the second one would be correct. The emf starts to increase(is non-zero) as soon as there is some changing flux with respect to the coil. That starts to happen on approach of the magnet, from somewhere outside the solenoid.

R.R
  • 1