Magnetism (sec. 27.1) Magnetic field (sec. 27.2) Magnetic field lines and magnetic flux (sec. 27.3) Motion of charges in a B field (sec. 27.4) Applications - moving charged particles (sec. 27.5) Magnetic force on

conductor with current (sec. 27.6) Force and torque on a current loop (sec. 27.7) Direct current motor (sec. 27.8) The Hall effect (sec. 27.9)

Magnetic Field & Forces Ch. 27

C 2009 J. Becker

Forces between bar magnets (or permanent

magnets )

Earth’s magnetic field (Note the N-S polls of magnet!)

Compass over a horizontal

current-carrying wire

Magnetic force acting on a moving (+) charge

Magnetic field lines associated with a permanent magnet, coil, iron-core electromagnet, current in

wire, current loop

MAGNETIC FLUX through an area element dA

Orbit of a charged particle in a uniform magnetic field is a

circle

R = m v / q B

The Van Allen radiation belts around the Earth

Velocity selector for charged particles uses perpendicular E and B fields

q v B = q E

v = E / B

Mass spectrometer uses a velocity selector to produce particles with uniform speed.

And from R = m v / q B

we get

q / m = v / B R

Force on a moving positive charge in a

current-carrying conductor:

F = I L x BL

I

I

For vector direction use “RIGHT HAND

RULE”

Magnetic force on a straight wire carrying current I in a magnetic field B

Right hand rule

F = I L x B

Magnetic field B, length L, and force F vectors for a straight wire carrying a current I

Components of a loudspeaker

F = I l x B

Forces on the sides of a current-carrying loop in a uniform magnetic field.

This is how a motor works!

Right hand rule determines the direction of the magnetic moment () of a current-carrying loop

Torque ( x B) on this solenoid in a uniform magnetic field is into the screen

thus rotating the solenoid clockwise

Current loops in a non-uniform B field

Atomic magnetic moments in an iron bar

(a) unmagnetized

(b) magnetized

(c)Torgue on a bar magnet in a B field

Bar magnet attracts an unmagnetized piece of iron; the B field gives rise to

a net magnetic moment in the object

A simple DC motor

The Hall effect – forces on charge carriers in a conductor in a B field.

With a simple voltage measurement we can determine whether the “charge carriers” are

positive or negative.

A linear motor

Electromagnetic pump

See www.physics.edu/becker/physics51

Review

C 2009 J. F. Becker