MAGNETIC FIELDS

Compass needles can be used to trace the magnetic field lines of a bar magnet.

(a) Magnetic field pattern surrounding a bar magnet as displayed with iron filings. (b) Magnetic field pattern between unlike poles of two bar magnets. (c) Magnetic field pattern between like poles of two bar magnets.

The earth is a large magnet.

Response of compass needles to a current in a straight wire.

Response of compass needles to a current in a straight wire.

Response of compass needles to a current in a straight wire.

The notation for vectors and currents perpendicular to the page.

The magnetic field exerts forces on the poles of a compass, causing the needle to align with the field.

The magnetic field around a current-carrying wire

Quick QuizThe magnetic field at position P points in which direction?

The magnetic field of a moving point charge.

Two views of the magnetic field of a moving positive charge.

Quick QuizThe positive charge is moving straight out of the page. What is the direction of the magnetic field at the position of the dot?

The magnetic field of a long, straight wire carrying current I.

A current loop.

Magnetic field lines surrounding a current loop.

The magnetic field of a current loop.

A coil of wire.

A current loop has magnetic poles and generates the same magnetic field as a flat permanent magnet.

Quick QuizWhat is the current direction in this loop? And which side of the loop is the north pole?

The direction of the magnetic force FB acting on a charged particle moving with a velocity v in the presence of a magnetic field B. (a) The magnetic force is perpendicular to both v and B. (b) Oppositely directed magnetic forces FB are exerted on two oppositely charged particles moving at the same velocity in a magnetic field.

The right-hand rule for determining the direction of the magnetic force FB = qv × B acting on a particle with charge q moving with a velocity v in a magnetic field B. The direction of v × B is the direction in which the thumb points. (a) If q is positive, FB is upward. (b) If q is negative, FB is downward, antiparallel to the direction in which the thumb points.

(a) A wire suspended vertically between the poles of a magnet. (b) The setup shown in part (a) as seen looking at the south pole of the magnet, so that the magnetic field (blue crosses) is directed into the screen. When there is no current in the wire, it remains vertical. (c) When the current is upward, the wire deflects to the left. (d) When the current is downward, the wire deflects to the right.

The relationship among v, B and F.

Magnetic forces on moving charges.

Quick QuizAn electron moves perpendicular to a magnetic field. What is the direction of B?

Quiz1. What is the current direction in the wire in the figure?

2. What is the initial direction of deflection for the charged particles entering the magnetic field shown in the figure?

Quiz3. What is the initial direction of deflection for the charged particles entering the magnetic field shown in the figure?

4. Determine the magnetic field direction that causes the charged particles shown in the figure to experience the indicated magnetic force.

Quiz5. What is the sign of the charge moving in the magnetic field shown in the figure?