Question

"A copper wire loop is constructed so that its radius, r, can change. It is held near a solenoid that has a constant current through it.
a. Suppose that the radius of the loop were increasing. Use Lenz' law to explain why there would be an induced current through the wire. Indicate the direction of that current.

b. Check your answer regarding the direction of the induced current by considering the magnetic force that is exerted on the charge in the wire of the loop. "

So, I think I have the answer to part a, but I am having trouble understanding exactly what part b is saying.
"A copper wire loop is constructed so that its radius, r, can change. It is held near a solenoid that has a constant current through it.
a. Suppose that the radius of the loop were increasing. Use Lenz' law to explain why there would be an induced current through the wire. Indicate the direction of that current.

b. Check your answer regarding the direction of the induced current by considering the magnetic force that is exerted on the charge in the wire of the loop. "

So, I think I have the answer to part a, but I am having trouble understanding exactly what part b is saying.
@Physics

Answer 1

More specifically, what exactly is exerting a magnetic force on the charge in the wire of the loop?

Answer 2

For part a, recall Lenz' law:\[\mathcal E = -\frac{d \Phi_B}{dt}\]Even though we're not really working with vectors, the negative is included to indicate that the EMF works to oppose the change. If the loop is getting bigger, then the magnetic flux through the loop is increasing, and thus there will be an induced EMF. The induced EMF will form a magnetic field pointing against the magnetic field as to prevent the magnetic flux from increasing.

For part b, consider a magnetic field pointing out of the page and a ring of wire that is increasing in size in the plane of the page. Since the material that composes the wire is moving with the wire as it gets bigger, that means that the individual free electrons will have a velocity pointing radially outward as the ring gets bigger. Recall now the equation for the Lorentz force.\[\vec F = q\vec v \times \vec B\]Since the velocity is radially outward and the magnetic field is out of the page, the charges will feel a force in the clockwise direction. By the right hand rule, a current in the clockwise direction induces a magnetic field into the page, and thus opposes the change in magnetic flux as caused by the increasing size of the loop.

Answer 3

Thank you. =)