Homework Help: Current in a straight wire

B is a term for magnetic field. Look up "right-hand rule"...

 

Hint 1: What two forces are acting on the second wire, and in which direction does each force point?

Hint 2: If the first wire creates a magnetic field with strength B = (m_0*I)/2pi*r how much magnetic force does it exert on the second wire, if theey are both carrying a current I, in the same direction?

Hint 3: If the second wire is "floating", what is its acceleration? What does that mean the net force on the wire has to be?

 

If its acceleration were 9.8m/s2 then it would be accelerating....doesn't floating mean stationary?

 

Yes, the net force on the second wire is zero...so what forces are acting on the second wire?

 

If the net force is zero, there are either no forces or at least two forces.

If there were only the magnetic force, Fnet wouldn't be zero.

So, what else besides the magnetic force acts on the wire?

 

Yes, gravity.

Next step is to set up an equation for Fnet, given that the forces are are gravity, and the magnetic force due to the other wire.

Hint: after setting up the equation, think about how the wire current enters into it.

 

The net force is zero, but that doesn't mean that each of the individual forces acting on the second wire are zero; it just means that they add up to zero.

Do you follow this?

 

Well, you know that the individual forces acting on the second wire are gravity and the magnetic force from the first wire.... correct?

That means that [itex]F_{net}=F_{grav}+F_{mag}=0[/itex]...But what are the equations for [itex]F_{grav}[/itex] and [itex]F_{mag}[/itex]?

 

You must mean: Fnet = -mg + ILBsin(theta) where g is the acelleration to to gravity (9.8m/s^2) right?

Remember, gravity is pulling downward, so you need the negative sign in front of mg.

As for B, it is the magnetic field that is created by the first wire, at the location of the second wire... The magnetic field created by the first wire at a distance 'r' from the wire is just B = (m_0*I)/2pi*r isn't it?....but what is 'r' in this case?

Also, you want the wires to be attracted to each other rather then repulsed, so does that mean the currents should be in the same direction, or in opposite directions?

And what is the angle 'theta' between the magnetic field of the first wire and the direction of the current in the second wire in this case?