Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The magnetic force and work

  1. Mar 26, 2008 #1
    My professor brought this up in class the other day, but has yet to elaborate on it and i can't stop thinking about it. I don't know if my wording is correct either so correct me if i'm wrong:

    The magnetic force will always act perpendicular to the displacment of a particle. Therefore, the magnetic force never actually does any work because the angle between this force and the displacment of a particle is 90 degress and the cos(90)=0.

    If this is the case, why is that in a junkyard the large cranes with magnets that can pick up a car can do the work of lifting a junk car to a height where it gains energy - mgh? Surely the magnetic force is involved in this somehow, but if it doesn't do the work, what does?

    Obviously there are other things going on, but if someone can enlighten me, that'd be great.

  2. jcsd
  3. Mar 26, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not quite, the magnetic force always acts perpendicular to the velcoity.
    Is it the magnet the actually does the lifting, or is it the crane?
  4. Mar 26, 2008 #3
    Strange as it may sound magnetic fields dun do any work!! In all such cases where where it seems magnetic field is doing work, one can somehow apply Faradays law which says that changing magnetic fields induce an electric field. It is this electric field which does the work. Let me demonstrate by a simple example. Let us say that we hav a charged particle at rest relative to a lab frame (which is our frame) and a magnetic field is applied. The particle although in vicinity of the magnetic field will feel no force (Lorentz law: no velocity no force) Now if the magnetic field is "moved" uniformly (changed by moving the source of the field say a bar magnet uniformly) then in the frame of the bar magnet (which is inertial) the particle should feel a force. This force is real and should be manifested in the lab frame also. It is manifested as an electric field "produced" due to a changing magnetic field which does the WORK ! If you go on to treat electricity and magnetism in SR this is what u get - electric and magnetic fields keep on exchanging roles in different frames and are a manifestation of the underlying electromagnetic field. Magnetic field then becomes the component of this field which does no work and electric field is the component that does all the work (poor sucker!). I hope u gt d idea although cranes are much more complicated.

    P.S. See example 5.3 in Introduction to Electrodynamics by Griffiths
    Last edited: Mar 26, 2008
  5. Mar 26, 2008 #4
    Ok, I was thinking about this, but it still kind of foggy in my mind.

    How about a situation where the crane magnet is held above the crushed car at a distance (i.e.-not touching the car at all), then when it is switched on, with the magnetic field being strong enough, it is able to lift the car up into the air until the car reaches the magnet.

    What is doing the work then?

    Harshant, your post is confusing me a little. Not necessarily the content, but its layout. Perhaps you can reword it.
  6. Mar 26, 2008 #5
    anyone else have any thoughts?
  7. Mar 26, 2008 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Last edited by a moderator: Apr 23, 2017
  8. Mar 28, 2008 #7
    I thought about the same question (the crane lifting up the car) and I think it has something to with alignment of domains. Domains align because of magnetic fields, and this aligned state is more stable leading to a loss of energy which is somehow converted to kinetic energy, so magnetic fields end up doing no work. It sounds weird, I should ask my professor..
    As for my previous post, i am sorry, i wrote everything down that came to my mind, but i really cant figure how to edit posts after they have been posted.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook