1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnets in Cars-Coils in Roads

  1. Jan 26, 2010 #1
    Magnets in cars passing over coils in roads would definitely generate power. There are a few people who think this would be free power. I don't know how to properly explain why it would NOT be free power. Can someone help?
    Last edited: Jan 26, 2010
  2. jcsd
  3. Jan 26, 2010 #2


    User Avatar

    Staff: Mentor

    The energy transferred to the coils would come from extra gas burned to overcome the extra resistance to motion. You are correct, there is no free energy.
  4. Jan 26, 2010 #3
    Just to be clear, would this resistance come from the attraction of the car's magnet to the coil? Or is it that the coil is resisting the 'charging' by the car's magnet (it comes from relative motion)? Or both?
  5. Jan 26, 2010 #4


    User Avatar

    Staff: Mentor

    Just as in a regular rotating generator, the current that is induced in the coil generates a reverse magnetic field that opposes the field that is inducing the current. This makes a resistance to the motion of the exciting magnet. That's why there is resistance to rotation of a rotating generator, when there is a load connected to its output.


  6. Jan 26, 2010 #5


    User Avatar
    Gold Member

    What if you put them at stop lights or freeway exits or similar places where people are slowing down anyway? It's not free energy, but would it be 'more efficient' energy?
  7. Jan 26, 2010 #6


    User Avatar
    Science Advisor
    Gold Member

    I saw a segment on the BBC a while back where someone had designed a speed bump that "produced" energy, i.e. the energy that would normally be dissipated by breaking the car was somehow turned into electricity when the car drove over it.
    I very much doubt it would be commercially viable, but it was a nice idea.

    Also, there are already all sorts of techniques for recovering energy from cars that are slowing down, most(all?) hybrids use regenerative breaking which does exactly that.
  8. Jan 26, 2010 #7


    User Avatar

    Staff: Mentor

    In addition to f95's comments, remember that adding weight to a car requires burning more fuel. The speedbump idea would be more efficient, though not much fun for the cars.
  9. Jan 26, 2010 #8
    Think about electricity as marbles in a tube- you push on the marble on one end, and almost instantly the marble on the other end moves. You could put a match box on the end of the tube and you can still easily push the marble- but it is a little harder to do, but that is work performed.

    It is really the same thing, Voltage represents a difference in pressure (charge to be specific)- the pressure of electrons (and protons- but they aint budging, unless your talking about inside a plasma field (like a neon sign)) You have one wire with high pressure (dangerous) and one wire with low pressure (less dangerous and depending on design totally safe) but in reality they are both the same wire- just different ends.

    When a magnet passes over say a coil a copper wire- that magnetic field pushes the electrons in the same direction. In the direction it travels becomes the side which is now under higher pressure. In order to make anything under pressure takes some oomph- and that oomph is energy required which will mean more gas is needed to drive the same distance at the same speed.

    Then you have to factor in efficiency- a car is 20-30% efficient, it has to be able to operate in all sorts of ways- sometimes you have to hit the gas to get on the highway, sometimes you are backing out of your driveway- and you dont want to wait 2 hours for your car to warm up. A power generation facility is designed to run one way- optimally their engines are not changing rpms- there is no need for transmissions and other energy robbing but vital components for a car which is why they are closer to 80-98% efficient at producing power from fuel.

    Also the amount of infrastructure in my opinion would NEVER pay off- by the time the system is rusted and falls apart it will not have produced more energy then it took to construct. Mining ore, refining ore, machining metal takes GREAT deals of energy.
  10. Jan 27, 2010 #9
    Thanks to all.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Magnets in Cars-Coils in Roads
  1. Magnetic flux and coil (Replies: 9)