# Identifying Magnetic Interactions

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1. Apr 3, 2017

### jlmccart03

1. The problem statement, all variables and given/known data
There are two problems:

1. At the instant the switch is closed determine the direction of the net force exerted by the magnet on the wire segment at the instant that the magnet is in the position shown. Explain.

2b. Suppose the a third wire, carrying another current i0 out of the page, passes through point P. Draw a vector on the diagram to indicate the magnetic force, if any, exerted bon the current in the new wire at P. If the magnitude of the force is zero, indicate that explicitly. Explain your reasoning.

2. Relevant equations
I don't think there is any relevant equations since this is all theoretical actions.

3. The attempt at a solution
For problem 1 I didn't know how to go about it so I used Newton's Third Law. In the previous part they wanted me to find the direction of the current through the wire and by using the right-hand rule I found the current to be ccw so the B-field must point toward the magnet. Now using Newton's Third Law I said that since there must be an equal and opposite force then the net force exerted by the magnet must be towards the wire or the opposite direction of the B-field produced by the wire. Is this reasoning correct? I feel like the wire should have a net force to the right allowing it to be repulsed by the B-field.

For problem 2 I drew the currents around all three wires using the right-hand rule and immediatly got confused on how to determine the direction vectors. I want to say it will be straight down since the left wire will cancel out with the right wire thus leaving only the top wire to exert a magnetic force.

Any help would be great!

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2. Apr 3, 2017

### BvU

I'd have used Lorentz !
Connecting a battery to a loop has little to do with the right-hand rule. Current flows from high voltage to low voltage. If the picture of the battery is halfway realistic, the positive side is at the top.

3. Apr 3, 2017

### jlmccart03

Ok if current flows positive to negative then in this example the B-field is in and the current is clock-wise. I have not learned Lorentz's Law yet and am a bit confused cause I know there is a qv x B, but where does the qE come from. And how do I use it to determine the direction only?

4. Apr 3, 2017

### BvU

Let's make one step back. From the text that you could easily have rendered in the full problem statement but cut off partly in a picture, I can read :
wrong button. If the magnet ...ung by a string moves as the arrows indicate, which way is the magnetic field inside the current loop ?
I think I start to understand your
And I am inclined to agree (provided that what I telepathize is indeed your exercise -1) which would mean the battery picture is mischievously misleading and the top is the negative side.

Can you enlighten us ? I understand you want to go along with exercise 1 and 2 but we want to be sire we are on the same path and that it's the right path ...

But now I don't know how to interpret
Great. What does that mean ?

Last edited: Apr 3, 2017
5. Apr 3, 2017

### jlmccart03

Ok I uploaded an image of the entire first problem with my reasoning to the first problem already written there. I want to believe based on the image that the current is ccw and thus there is a B-field into the page. (Hard to describe through words). Based on that observation then I wanted to believe that there would be a net force from the magnet to the left pointing toward the wire since the wire would exert a force to the right at that position, but I am really now just confused on what the overall problem is looking for me to do.

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6. Apr 3, 2017

### Staff: Mentor

Your description looks reasonable. To solidify your understanding you might google: "Oersted experiment"

7. Apr 3, 2017

### jlmccart03

So how do I go about solving for part 2 on the net force? Is it toward the wire or not?

8. Apr 3, 2017

### Staff: Mentor

The question asks for the direction of the net force exerted by the magnet on the wire. So of course any force exerted on the wire must act towards the wire. But "towards" does not come with any particular direction. To get your answer, look at the motion that the magnet is going to make. What kind of force or forces must act on the magnet to make that happen? Then think about Newton's Third Law.

9. Apr 3, 2017

### jlmccart03

I figured it out by using F=qv x B and the fact that qv is The current I. I then did right and rule and it's basically into the page but kinda angled due to the image but if it was front faced the force would be into the page. So how do I go about doing the second problem?

10. Apr 3, 2017

### Staff: Mentor

You should know that we generally do not allow multiple separate problems in a single thread. This is to avoid having the confusion of separate discussions taking place at the same time. It would be preferable for you to start a separate thread for the second problem.