28. Magnetic Fields and Forces

Which of the following best describes a magnetic field?

A current-carrying circular coil lies flat in the $xy$-plane with current flowing counterclockwise when viewed from above. In which direction does the magnetic field at the center of the coil point?

The heart produces a weak magnetic field that can be used to diagnose certain heart problems. It is a dipole field produced by a current loop in the outer layers of the heart. What is the magnitude of the heart's magnetic dipole moment?

When a magnet spins near a conducting wire, does the changing magnetic field create an electric field that causes electrons to move in the wire?

Two long, straight, parallel wires carry currents in the same direction. What is the direction of the magnetic field produced by wire $2$ at the location of wire $1$?

A long straight current-carrying wire runs vertically upward. At point $z$, which is located to the east of the wire, what is the direction of the magnetic field produced by the wire at that point?

Which of the following statements about the direction of the $B$ field (magnetic field) around a straight, current-carrying wire is correct?

Which of the following is the primary source of magnetic fields $B$?

In devices that use electromagnets, why is a permanent magnet often included as well?