Question

In an Ampere

Answer 1

1) By DEFINITION two parallel wire that carry PARALLEL currents attract each other. If the currents are ATNTIPARALLEL the two wires repel each other.

This can be explained with the help of the figure below.

The lower wire creates a field B according to the rotation of a right screw (screw is advancing in the direction of the current and its rotation gives the direction of B).

The upper wire feels a magnetic force F = L*(I x B),   where x is the vectorial product. Hence the magnetic force is attractive.

2) Still in the wires N and G the currents are peprendicular to the field B (so the cross product I x B should be different from zero), but in this case the currents in the wires N and G are opposing one to the other so in total the two magnetic forces on the wires N and G will cancel each other. The figure with this case is below

c) The magnetic field of a current I1 through a wire is given at distance d from the wire by:

B = mu*I1/(2*pi*d)

The magnetic force of field B on current I2 is

F =| L*(I2 x B) |= B*I2*L

since the angle between B and I is 90 degree.

Therefore F = mu*I1*I2*L/(2*pi*d) = mu*I^2*L/(2*pi*d)

where mu is the magnetic permeability of the air mu=4*pi*10^-7 H/m

d) m*g = mu*I^2*L/(2*pi*d)

I = sqrt [mu*L/(2*pi*d*m*g)] =sqrt [4*pi*10^-7*0.26/ (2*pi) / (0.004*2*10^-6*9.81)] =0.814 A