Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    3
    ReputationRep:
    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.
    Attached Images
     
    • Thread Starter
    Offline

    3
    ReputationRep:
    depymak morgan8002
    Offline

    20
    ReputationRep:
    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?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (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
    Offline

    2
    ReputationRep:
    Name:  EMF_0002.jpg
Views: 47
Size:  236.5 KB
    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 \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t}=Bul. 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.
    Offline

    20
    ReputationRep:
    (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\dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t},\ - \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t} or something else?
    I think you're on the right lines. So what's \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t}?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by depymak)
    Name:  EMF_0002.jpg
Views: 47
Size:  236.5 KB
    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 \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t}=Bul. 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?
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by morgan8002)
    I'm not completely sure what you mean by (-) Faraday's law. Do you mean\dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t},\ - \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t} or something else?
    I think you're on the right lines. So what's \dfrac{\mathrm{d}\Phi_B}{\mathrm  {d}t}?
    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?
    Offline

    20
    ReputationRep:
    (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.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (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!
    Offline

    2
    ReputationRep:
    (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
    Name:  EMF2.jpg
Views: 32
Size:  151.9 KB
    Attached Images
      
    Offline

    2
    ReputationRep:
    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 ?
    • Thread Starter
    Offline

    3
    ReputationRep:
    x
    x
    Offline

    2
    ReputationRep:
    the general case for motional emf
    Name:  EMF4.jpg
Views: 32
Size:  303.6 KB
    EMF=Bul sina cosφ
    Thank you.
 
 
 
Poll
Who is your favourite TV detective?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.