# Predicting Polarity of capacitor in the given diagram

I have to determine the polarity of the Capacitor in the given diagram.

I Approached the problem as follows:

When Magnet 1 moves with its North pole towards the coil, emf is induced in the coil as the magnetic flux through the coil changes. So, when seeing from the left hand side (i.e. from magnet 1) the direction of induced current appears to be Anticlockwise. Though, on seeing from the left hand side, the South pole of magnet is coming towards, according to Lenz's Law the coil will behave like a South pole, thus the direction of current is Clockwise. I am stuck at this point. How shall I proceed?

My textbook explained it this way which I did not understand:

The direction of induced current when seen from the left hand side is Anticlockwise, and its direction is Clockwise when seen from right hand side.Thus, direction of induced current is in Clockwise sense (why?) . This implies Plate A is positive plate and point B is negative one.

• I can't tell where the capacitor is in the out-of-plane dimension. Is it outside of my computer screen, or inside my computer screen? But aside from that, I don't understand the question. It seems that you and your textbook agree. What's the question? Commented Feb 23, 2015 at 14:34
• @garyp, the capacitor is inside the computer screen, I am unable to understand how my textbook can say that the direction of induced current is clockwise. Basically I am confused in predecting the direction of the induced current in the coil due to motion of two magnets(oriented as shown) towards the coil with the capacitor. I can very well predict the polarity once I know the direction of induced current. Commented Feb 23, 2015 at 14:39
• @Abdullah Olin is right, the electromotive force doesn't have to do with magnetic poles, but with the direction of the magnetic field, so, leave the poles. Commented Feb 23, 2015 at 15:04

The best way to think about this is imagine the capacitor plates A and B to be behind the paper plane and then think about the direction of current induced. Observe it from the right magnet's side whose South Pole is approaching so the face of coil facing that South Pole will itself produce clockwise current in the coil. So the current flows from high potential A to low potential B.

The two magnets are mirrored, so from different sides the same current can appear either clockwise or anticlockwise.

Think if the coil was flat in the page and the north magnet was dropping from above. This creates an increasing $\vec{B}$ field into the page, so the emf must induce a current out of the page, and the current will be anticlockwise.

Now imagine the same north magnet approaching the coil from under the page. The north magnet creates an increasing $\vec{B}$, this time out of the page. The emf must counter this magnetic field, and so from this perspective we see a clockwise current. But both systems are equal, and the only thing that changed was our perspective. If you keep the capacitor with labeled sides A and B in your diagrams of both perspectives, you'll see that in both cases current flows into side A.

When you do the analysis of the current for the south magnet you should find the same thing, that current flows into side A of the capacitor.

• +1. Thanks but there is a similar question where only one magnet with North pole is approaching the coil(with the capacitor exactly as shown) and my textbook says that the current induced in it is in the clockwise direction whereas it should be in anticlockwise direction as North pole is approaching. Where am I going wrong? Commented Feb 23, 2015 at 14:57
• In that question, is the magnet approaching from the right or the left? Is it being viewed from the right or the left? Are you clear about the orientations and directions in all the cases that you are considering? Commented Feb 23, 2015 at 15:00
• In that question, the magnet is approaching from the right. It is not given from where it is viewed. Is the question invalid now? Do we have to take a particular direction of viewing every time we find the direction of induced current? Commented Feb 23, 2015 at 15:32

One way to see that the effect from both magnets is the same is to notice that both of them will create a N-S field left to right thru the coil. Moving either magnet into position from originally being far away will cause increased left-right N-S magnetic field, which will cause induction in the same direction, which causes the cap to charge up the same polarity.

• Ok I agree that moving either magnet into position from originally being far away will cause increased left-right N-S magnetic field, which will cause induction in the same direction, but basically my question was to find this 'same direction'. Commented Feb 23, 2015 at 15:32

As I conceive u have a problem expressing the directions...let me make some things clear to u.

In a loop wherever is the direction of current marked or cell marked or capacitor marked is the outward arm of loop.

Secondly books prefer to see every direction from top right corner. So try to deduce what direction it is from top right in your questions and mark direction of current accordingly on OUTER ARM. Hope it helps a bit for you.

I think the coil is in y-z plane ( taking standard Cartesian directions) So to the left of the coil direction of current appears anticlockwise and to the right of coil it appears clockwise Although from both references Current is flowing from Ato coil then to B Therefore A is at high potential and B is at lower potential