C H A P T E R   21Magnetic Forces and

Magnetic Fields

Outline •Magnets, Magnetic fields, and Magnetic field of the Earth

•Force on a moving charge in a magnetic field

•Motion of a charge particle in a magnetic field

•The mass spectrometer

•Force on a current in a magnetic field

•Magnetic fields produced by currents

•Magnetic Resonance Imaging (MRI)

•Cathode Ray Tube (CRT)

•Magnetic Tape recording and Maglev Trains

21.7. Magnetic

Fields Produced by

Currents

Right Hand rule 2:Long straight current-carrying wire

• Point your right thumb in the direction of the current and then wrap your fingers around the wire. The magnetic field will form concentric circle in the direction of your fingers.

21.7. Magnetic

Fields Produced by

Currents

Right-Hand Rule No. 2. Curl the fingers of the right hand into the shape of a half-circle. Point the thumb in the direction of the conventional current I, and the tips of the fingers will point in the direction of the magnetic field B.

A LONG, STRAIGHT WIRE

Two Current-Carrying Wires Exert Magnetic Forces on One

Another

SOLENOID

Right hand rule 3: Magnetic field due to a current loop

• The magnetic field will form like a bar magnetism's field perpendicular to the plane of the loop. The north pole can be found by wrapping your right four fingers in the direction of the current your thumb will now point in the direction of the north of the pole.

Electric Currents & Magnetism

Winter 2007 UCSD: Physics 121; 2007 12

Electromagnet Coil• By arranging wire into a loop, you can

make the magnetic fields add up to a substantial field in the middle

looks just likea magnet

Magnetic Resonance Imaging (MRI)

Cathode Ray Tube (CRT)

21.9. Magnetic Materials

Most substances are nonmagnetic, the magnetism produced at the atomic level tends to cancel out.

However, there are some materials, known as ferromagnetic materials, in which the cancellation does not occur for groups of approximately 1016 – 1019 neighboring atoms, because they have electron spins that are naturally aligned parallel to each other.

This alignment results from a special type of quantum mechanical interaction between the spins. The result of the interaction is a small but highly magnetized region of about 0.01 to 0.1 mm in size, depending on the nature of the material; this region is called a magnetic domain.

Each domain behaves as a small magnet with its own north and south poles. Common ferromagnetic materials are iron, nickel, cobalt, chromium dioxide, and alnico (an aluminum-nickel-cobalt alloy).

INDUCED MAGNETISM

Magnetic Grippers

MAGNETIC TAPE RECORDING

MAGLEV TRAINmagnetically levitated train