Re: Physics QOTD
 

sorry this is not about the physics question, but I will post up my research once I finish.

I am investigating what it would take and cost to adapt European robotic parking structures (mainly the Trevipark underground approach). For example, it would need larger spaces and a more heavy duty lift for the US's larger cars.

Send me a PM if you would like more info, I'd hate to get this thread off topic..


I think the wires dont move...they are too far apart..btw

Re: Physics QOTD
 

The answer should be that in addition to the acceleration of gravity, a force acts between them with magnitude u_0*L*i_a*i_b/(2*pi*d), where d is the distance between them, L is the length in question, and i_a and i_b are the currents in the two wires. This force causes them to ATTRACT, not REPEL... think about it: the two magnetic fields act clockwise around the wires as you go in the direction of the current, so the force is i*L X B, or i*Forwards X Down for the right wire, which pushes it towards the left wire... hard to explain this without a diagram though. But the answer requires the "mass density" which is mass per unit length of the wire... call this k, and our final answer becomes an acceleration of 25Amp^2*k*u_0/(2*pi*3m) towards each other and 9.8m/s^2 down.

Re: Physics QOTD
 

If you don't buy my solution above, test it with the following circuit:

|--l|l|l|---V^V^V^---------------------|
|.............................|________|.......... |
|................................................. .....|
|---------------------------------------|
Where a "." means nothing is there and a "V^V^V^" is a resistor and l|l|l| is an emf source.
A simple resistor on a loop, but important is to observe whether the two branched wires fly towards or away from each other. Careful observation should show that they attract each other, with the current running the same way. Run current in opposite direcitons and they will repel.

Re: Physics QOTD
 

If I remember correctly, the formula you've used for calculating the force between the wires only applies when end effects can be neglected (i.e. when the length of the wires is very large compared to the distance between them). Since this is certainly NOT true in this case, with a length of 5 m and a separation of 3 m, the horizontal component of the acceleration will only be a very rough, order-of-magnitude sort of approximation.

Re: Physics QOTD
 

The equation he used is for wires of known length, that is what L is for.

But Grommit is right, when it comes right down to it, you need mass or mass density or SOMETHING like that to get the second law balance.

Re: Physics QOTD
 

That's quite true... actually I didn't even notice the "5m" length given, I had assumed they were infinite wires... :p. But trying to compute an exact solution might be a bit more pain than this problem merits. ;)

On the other hand, here's an interesting problem. You have a straight wire with radius r and electrons traveling through at velocity v, current i. What is the radial distribution of the electron density in the wire, in terms of r?