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# Direction of motion in a magnetic field question watch

1. Hey,

Having a bit of trouble imagining the 3D nature of this problem, the answer is B. if anyone can explain why the direction can't be vertical that'll be a big help.
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2. By exhaustion. It could be travelling vertically, but the emf wouldn't be Blv or B/lv, so it's not either of those two answers. What would the emf be if the wire was travelling upwards?
3. (Original post by morgan8002)
By exhaustion. It could be travelling vertically, but the emf wouldn't be Blv or B/lv, so it's not either of those two answers. What would the emf be if the wire was travelling upwards?
Well, I thought that since the wire would be cutting the flux at right angles, it would be, emf = (-) faraday's law

4. The magnetic force tries to push electrons through the wire, and this creates the EMF. If the wire was traveling upwards then the magnetic force would push electrons along the sides of the wire. However, the wire is considered to have negligible thickness.
You can see here the proof that . You do not need to consider the Lenz's law yet. You take this law into consideration when you have to decide about the polarity or the direction of any induced current.
5. (Original post by Nikhilm)
Well, I thought that since the wire would be cutting the flux at right angles, it would be, emf = (-) faraday's law
I'm not completely sure what you mean by (-) Faraday's law. Do you mean or something else?
I think you're on the right lines. So what's ?
6. (Original post by depymak)

The magnetic force tries to push electrons through the wire, and this creates the EMF. If the wire was traveling upwards then the magnetic force would push electrons along the sides of the wire. However, the wire is considered to have negligible thickness.
You can see here the proof that . You do not need to consider the Lenz's law yet. You take this law into consideration when you have to decide about the polarity or the direction of any induced current.
Surely, by your diagram, any movement of the wire in the vertical or horizontal direction would cut the magnet's flux lines and an emf would be induced?
7. (Original post by morgan8002)
I'm not completely sure what you mean by (-) Faraday's law. Do you mean or something else?
I think you're on the right lines. So what's ?
Is it the case where any movement of the wire vertically or horizontally would induce an emf, but it's just the magnitude of the emf/equation you use differ?
8. (Original post by Nikhilm)
Is it the case where any movement of the wire vertically or horizontally would induce an emf, but it's just the magnitude of the emf/equation you use differ?
The equation you always use is Faraday's law. Movement of the wire doesn't have to generate emf. For example if it moves vertically the emf is 0, since the width of the wire is 0, so there's no area, so no change in flux. Horizontal movement will create an emf, as long as the movement isn't parallel or antiparallel to the direction of the magnetic field.
9. (Original post by morgan8002)
The equation you always use is Faraday's law. Movement of the wire doesn't have to generate emf. For example if it moves vertically the emf is 0, since the width of the wire is 0, so there's no area, so no change in flux. Horizontal movement will create an emf, as long as the movement isn't parallel or antiparallel to the direction of the magnetic field.
OHH I understand!! No flux is cut by the wire's width because it's assumed to be zero! Thanks!
10. (Original post by Nikhilm)
Is it the case where any movement of the wire vertically or horizontally would induce an emf, but it's just the magnitude of the emf/equation you use differ?
well in order to have non-zero induced emf the wire's velocity should be parallel neither to the magnetic field lines nor to the wire's direction. The maximum induced emf occurs in the case of the previous diagram of mine. In any other case, you should resolve velocity into the appropriate components. This affects the magnitude of the emf not its polarity, which can be determined correctly using RHR.
I hope the following diagrams will help
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11. Guys I used the right hand rule and I got B but based on the replies eventhough the RHR gave the correct answer, I seems like a bad approach. ls that the case ?
12. x
x
13. the general case for motional emf

EMF=Bul sina cosφ
Thank you.

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