Question

A wire carries current directed out of the page. A proton travels near the wire as shown in the picture. What direction is the magnetic field at the location of the proton at the instant shown in the picture?

Zero 

Down 

To the left 

Out of the page 

Up To the right 

Into the page 

In the same scenario as the previous question, what is the direction of the magnetic force on the proton? 

Into the page 

Out of the page 

Up 

Down 

To the left 

To the right 

Zero 

A negatively charged particle is moving to the right, directly above a wire having a current flowing to the right, as shown below. In which direction is the magnetic force exerted on the particle? 

Out of page 

Into the page 

Up 

Down 

No force 

A conducting hoop is held at rest above the South pole of a bar magnet; the axis of the bar magnet lies along the axis of the circular hoop. The bar magnet is then released from rest What is the direction of the induced current in the metal hoop as the bar magnet descends? As viewed from above, the induced current in the hoop is 

Clockwise 

Counterclockwise 

No induced current

 Not sure 

The figure shows a region of uniform magnetic field directed out of the page. Outside the region, the magnetic field is zero. Rectangular loops of wire move as indicated. At the instant shown in the diagram, indicate the direction of the induced current flow (cw, ccw, or no current flow) in loop 1, 2, 3, 4 below. 

Answer 1

1) The magnetic field circulates around the wire. When the current is coming out of the page (as indicated by the dot), the right hand rule tells us the magnetic field is counterclockwise, so it will point up at the proton's position.

2) Since the proton is travelling parallel to the magnetic field, it will feel zero force.

3) The right hand rule again tells us the magnetic field is point out of the page at the position of the negatively charged particle. The right hand rule for forces tells us the force would thus be down, but since the charge is negative we reverse that direction, so the force is up.

4) The external magnetic field, which points down through the loop toward the south pole, decreases as the magnet moves away, so the induced magnetic field points downward to make up for this loss. The right hand rule for induced currents tells us the current must be clockwise.

5) Blank 1: the loop loses B external on the left, so B induced points in the same direction as B external (out of the page), so the current is CCW

Blank 2: B external increases, so B induced is into the page to oppose this increase, thus the current is CW

Blank 3: Same reasoning as blank 2, so CW

Blank 4: Same reasoning as blank 1, so CCW