1. This question is a short free-response question. Show...

1. This question is a short free-response question. Show your work for each part of the question. N S The figure above shows a bar magnet and five points near the magnet. (a) The arrow on the rightmost point represents the needle of a compass. On each of the remaining four points, draw an arrow to show the orientation of a compass needle at that point. XXXX N S S Side View End View (b) A coil of wire is initially held perpendicular to and concentric with the north pole of the magnet and then slowly moved to the right until it reaches the other end of the magnet, as shown above. In a paragraph-length response, predict the direction and magnitude of the current in the coil at the three positions shown. Indicate the direction with respect to the end view shown. Justify each prediction using physics principles.

Transcript

00:01 This question involves the magnetic field of a bar magnet and also involves electromagnetic induction.

00:09 Now with induction, if we want to know the direction of the induced field and therefore the direction of the induced current, we have to use what's called lensis law.

00:21 Lensis law tells us that the magnetic field that gets induced by a changing magnetic flux will be pointed in the direction so as to oppose the change.

00:38 Let us say if the existing magnetic field is pointing to the left, let us say, and it is decreasing.

00:45 Lenz's law says that the induced magnetic field will also point to the left to kind of reinforce it so as to oppose the change.

00:53 The questions that we usually ask are these here that i've listed on my little whiteboard here.

00:59 You first have to ask yourself, which way is the existing field? and then you ask, how is the magnetic flux changing because of that field? and then from there, you figure out, well, what then would oppose, what kind of induced field would oppose that change, which way would it point? and lastly, if you're asked about the current, you can say, well, if the induced field points in the given direction, then using the right -hand rule, which way is the current going to flow? so as i have drawn, on here in my little diagram, this is the answer to question one, which is already kind of shown on the question as it was given to me.

01:40 But i just want to verify that it is correct.

01:43 Near the north end of the magnet, the magnetic field is pointing to the left.

01:49 Above the magnetic field points to the right, and it is pointing slightly in an elevated direction at this point over here.

02:01 Up like this and then it points basically horizontal right here at this point right here and then over here it starts pointing slightly downward curve downward so that matches the arrows that have been drawn on this diagram already and then on the south end of the magnet the magnetic field it does go in as the way they already drew it so now since we know the direction of magnetic field and we know based on the magnetic field lines where it gets weaker, the farther part of the magnetic field lines are the weaker the field is, we can start answering the question for part b.

02:41 If we put a coil right here, let us say, and well right now, the existing magnetic fields, answering question one, the existing magnetic field going through that area of that coil would be going to the left.

03:02 It's these fields right here going off here to the left.

03:07 It is getting bigger because as we approach the magnet, the magnetic field gets stronger, and so the magnetic field, which points to the left, is getting bigger, so the magnetic flux is increasing.

03:20 Therefore, which way of the induced field point? well, the induced field is going to oppose the change.

03:26 So if the existing magnetic field is getting bigger and it points to the left, the induced magnetic field will point to the right.

03:33 And the only way to get the induced magnetic field to point to the right would be for the current to be going counterclockwise as seen from the end.

03:43 And the other picture, you know, we're given this picture here where we have the bar magnet and it's the south -the -south end.

03:50 The field then must be going this way to produce a magnetic field that is going to the right, towards the south then.

04:01 So that's that first position.

04:03 Now, as the loop goes over the magnet, you can see that the magnetic field now is pointing to the right.

04:17 The magnetic field up here points to the right, magnetic field down here points to the right...

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