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!

Homopolar generator / Faraday disc

  1. Nov 6, 2014 #1
    Hello everyone , as for starters I would like to ask two questions. As i've been reading about the homopolar generator I have the basic understanding of all the workings yet here are my questions.

    Since the rotatig disc produces a current within while rotating in an axial magnetic field tipically shown coming from a round type permanent or electromagnet held stationary while the disc in wich the current is induced rotates , now since a current flowing in the disc creates a magnetic field around the disc itself , could the homopolar generator work without anything more than just a disc and two connections and once being spun a little kick start current once added would create a magnetic field which using the rotational energy from the disc supplied by an external force would continue to generate current ?

    like the same disc used for current generation would also be used for current induction as it would then become the rotor and stator at the same time and no real stator would be needed just a rotating disc , a little startup current and brushes for contacts would this work?

    also what would happen if I added a capacitor to the two outputs of the generator , wouldnt it become an generating oscillator because the current would flow only as long as the capacitor is charging and then stop so the b field would drop to 0 and the current generation would stop , then as the capacitor would discharge through the disc in the opposite direction a b field of opposite polarity would emerge and the disc would now generate in the opposite direction until again the cap is charged in the opposite polarity and so this would repeat in cycles as long as an external source of energy provides the rotational energy needed to spin the disc ?
  2. jcsd
  3. Nov 8, 2014 #2
  4. Nov 10, 2014 #3
    Question 1: no because the secondary field will be in the opposite direction and act against the primary field. Think about what is forcing the charges to move in the magnetic field: it is some kind of drag force in the moving solid, which will manifest as friction. Energy lost in providing a torque to rotate the wheel against this resistance is converted to electrical energy and dissipated. It can't go back into motion to drive the wheel forever once the heat is dissipated.

    Question 2: yes. The generator is inductive and you're describing the oscillations of an LC circuit.
  5. Nov 10, 2014 #4
    Ok I got the answer to my second question but about the first one I kinda think I got what you were thinking but maybe not so I ask again differently.

    I wasn’t thinking about using the induced current to let it flow through the disc one more time to spin the disc even stronger and so cause a self running machine which is not possible as would violate the laws. I was rather thinking about a situation where I have a constant rotational torque input from an external source like a gas turbine for example, then I have my disc and two brushes for sake of simplicity , now the disc is spinning but there is no permanent magnet next to it and no B field around it. Now once the disc is spinning I take a charged capacitor and discharge the capacitor through the two brush contacts of the already spinning disc , that would send a current down the disc from center to periphery.

    This current would create a B field , and now the situation would be that of a faraday disc spinning in a B field. As with the faraday disc the b field could be either stationary or co-rotating as in this situation yet the current should be induced anyway so an extra current should be induced in the disc, the energy for that induction coming from the rotational torque supplied by the gas turbine.
    That’s why I asked about the current for the B field could be run through the same disc that further generates the current from the B field that created it , or are you saying that the direction of the current generated would be opposite of the current that made the B field in the first place and so would cancel out ?
  6. Nov 13, 2014 #5
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Homopolar generator / Faraday disc
  1. Homopolar Motor Help (Replies: 1)