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Physics question

  • 25-04-2011 7:15pm
    #1
    doctorg
    Registered Users Posts: 555 ✭✭✭


    Hey guys, I was doing the physics exam paper's and I came across and awkward question.

    Its 2003 Q12 (d) last bit.

    "If the switch at A is open, the magnet will take longer to come to rest. Explain why. "


    I was wondering if the switch is closed, how will the magnet come to rest quicker?


Comments

  • #2
    Neodymium
    Registered Users Posts: 179 ✭✭Neodymium


    You will find the answer here


  • #3
    doctorg
    Registered Users Posts: 555 ✭✭✭doctorg


    Neodymium wrote: »
    You will find the answer here

    I am already aware of the marking scheme. My question is different "I was wondering if the switch is closed, how will the magnet come to rest quicker?"

    Thanks for the response


  • #4
    brownlad
    Registered Users, Registered Users 2 Posts: 642 ✭✭✭brownlad


    its lenz law......when the magnet meets the coil....... the magnet gives an induced emf and they are both the same charge (sign)...and like charges repel :D


  • #5
    LilMissCiara
    Registered Users, Registered Users 2 Posts: 2,292 ✭✭✭LilMissCiara


    I don't have the question in front of me but I would assume it would be because there would be no force because there is no current?


  • #6
    brownlad
    Registered Users, Registered Users 2 Posts: 642 ✭✭✭brownlad


    LilMissCiara wrote: »
    I don't have the question in front of me but I would assume it would be because there would be no force because there is no current?

    thats the answer on the marking scheme;)


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  • #7
    Michael Collins
    Moderators, Science, Health & Environment Moderators Posts: 1,851 Mod ✭✭✭✭Michael Collins


    Yes, the magnet will induce a voltage across the coil, if the switch was closed this would in turn cause a flow of current in the circuit which, by Lenz' Law, must oppose the change of flux in the coil.

    The magnet is what causes the change in flux in the coil, so the coil opposes the magnet (by generating its own magnetic field in the opposite direction to the magnet's motion) with a force in the opposite direction - this is what causes the magnet to come to rest quicker.

    Incidently if you did this yourself you would find it noticeably harder to push the magnet in and pull it out of the coil when the switch is closed - this idea is put into practice in the DART system where this induced voltage and current are used to slow down the vehicle in a contactless way (less wear-and-tear) and, as an added benefit, this energy can be given back to the DART voltage supply system.


  • #8
    doctorg
    Registered Users Posts: 555 ✭✭✭doctorg


    Thank you for your answers!


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