Magnetic Currents

Magnetic Fields Produced by Currents

By Rachel Wampler

Ampere's Law

Current through a Straight Wire

Current through a Looped Wire

Multiple Choice

A long straight current-carrying wire runs from north to south.

A. A compass needle is placed above the wire points with its N-pole toward the east. In what direction is the current flowing?

B. If a compass is put underneath the wire, in which direction will the needle point?

• A. North

  B. West

• A. South

  B. East

• A. North

  B. East

• A. South

  B. West

Multiple Choice

A single straight wire produces a B-field.  Another wire is parallel and carries an identical current.

A. If the two currents are in the same direction, how would the magnetic field be affected? 

B. What if the currents are in the opposite direction?

• A. B-fields between wires add; outside of wires cancel

  B. The B-fields between the wires cancel; outside of wires add.

• A. B-fields between wires cancel; outside of wires add

  B. The B-fields between the wires add; outside of wires cancel.

Multiple Choice

Suppose a piece of coaxial cable is made with a solid wire at the center.  A metal cylinder has a common center with the wire and its radius is 1 mm.  A 2 A current flows up the center wire and a 1.5 A current flows down the cylinder.

A. Find the B-field at 4 mm from the center.

B. Find the B-field at 0.5 mm from the center.

C. What current should be in the cylinder to have no B-field outside of the cylinder?

• A. 2.5x10^-5 T

  B. 4x10^-4 T

  C. 2 A

• A. 5x10^-4 T

  B. 8x10^-4 T

  C. 2 A

• A. 2.5x10^-5 T

  B. 8x10^-4 T

  C. -2 A

• A. 5x10^-4 T

  B. 4x10^-4 T

  C. -2 A

Applications: Maglev Trains