From: Stephen A. Lawrence <sa...@pobox.com> >To: vortex-l@eskimo.com >Sent: Wednesday, October 5, 2011 5:25:31 PM >Subject: Re: [Vo]:Free Work > > > >On 11-10-05 03:55 PM, Harry Veeder wrote: >> >> When the wires have nothing holding them together or apart there is no >> opposition. In any case you do not answer my question which I will rephrase: >> if the electricity is used to power a motor, and the same electricity is >> used to compress or stretch a spring placed between the parallel wires, will >> the movement of the spring slow the motor? > >Well, of course. Mind the details, and it should be obvious. > >With two parallel wires in a horizontal plane, there's a vertical B field >around each wire in the plane. Look at wire A, and its field. When wire B >moves toward or away from wire A it's moving through a vertical B field, and >that's going to exert a force on the charge carriers in B, either slowing them >or speeding them up. Let's use some ASCII art and work it out. I'll look at >the case where the current in the wires is going in opposite directions >(they're the plus and minus wires powering the motor, rather than two stands >of wire in one cable). > >If wires A and B go straight into the page, and A is on the left, and the >current in A is going into the page, then we have this: > >A . B . > >(the wires look like dots, cause they're going straight into the page) In that >case, the B field at wire B (sorry about the double use of "B") is contained >in the surface of the page, and is pointing down the page. (Use the right hand >rule for current in wires to figure this part out: curl your fingers, point >your thumb in the direction the current is going, and the fingers show you the >B field.) > >The current in B is flowing out of the page (opposite direction to A), and the >force on B is to the *right*. (Use a different right hand rule: Hand held >flat, thumb pointing out; point thumb in direction the positive charges are >going, fingers in direction the B field points, and the force sticks out of >your palm.) > >If B moves to the *right* (under the impulse of the force exerted by A's B >field) then there's a force on its (positive) charge carriers directed into >the page, and the current in wire B slows down. (Use the flat-hand RHR again.) > >QED. > >You can work it out for two wires carrying current in the same direction, as >well, and you'll get the same answer: If you let the wires move under the >influence of the B fields which surround them, the current in them will be >slowed by the resulting back EMF. > >The presence of the spring is just a red herring -- the interesting thing is >the motion, not what we're using to restrain the wires.
OK, you have just argued that the spring cannot add to the energy loses from induction, so now consider the system with spring. "Free work" means the work done in compressing the spring is greater than the energy lost during the process of induction. I can imagine it will take less work if you choose wires with the wrong electrical properties. However, except for an invocation of CoE, I see nothing which tells me that the work done in compressing spring cannot exceed the energy loses from induction. Harry
