Magnetic FieldsChapter 24

24.1 Magnets: Permanent & Temporary

Electric generators Television sets Cathode-ray displays Computer hard drives Compass

General Properties of Magnets Polarized-magnets are polarized. They

have two distinct and opposite ends. North pole and South pole Like electric charges, likes repel and

opposites attract However, charges can be separated

magnetic poles cannot.

The Earth itself is a large magnet.

The north end of a compass needle (a magnet) points to the geographic north pole.

The geographic north pole is the magnetic south pole

How Do Magnets Affect Other Material?

A magnet can cause another metal to become polarized and have magnet properties.

Permanent Magnets Because of the microscopic nature of

the material it keeps the magnetic properties

ALNICO V-a permanent magnet alloy aluminum, nickel, and cobalt

Rare earth elements neodymium and gadolinium produce very strong permanent magnets for their size

Magnetic Fields Around Permanent Magnets

Magnetic forces can be describe by the existence of a field around the magnet

Much like gravitational and electric fields

Can be non-contact forces Magnetic fields are vector quantities

that exist in a region in space where a magnetic force occurs.

Magnetic Field Lines Magnetic field lines are imaginary lines used to

help visualize a magnetic field. Direction of field lines are defined as the

direction that a compass points when placed in the magnetic field.

Outside the magnet field lines leave the magnet from the north pole and end the south

Inside the magnet from south to north to form a closed loop.

Magnetic Flux The number of field lines passing

through a surface is the magnetic flux The flux per unit area is proportional to

the strength of the magnetic field.

Electromagnetism In 1820, Danish physicist Hans

Christian Oersted experimented with electric currents in wires.

Found that when a current was in a wire a compass needle rotated until it was perpendicular to the wire.

If the compass needle rotated it must have been because of a magnetic field.

Circular line indicate that magnetic field lines around a current carrying wire for closed loop in the same way that field lines about a permanent magnet for closed loops.

First Right-Hand Rule A method to determine the direction of

a magnetic field relative to the direction of conventional current

Pretend to hold the wire with your right hand

Point you thumb in the direction of conventional current

Your fingers point in the direction of the magnetic field.

Magnetic Field Near a Coil A long coil of wire consisting of many

loops is called a solenoid. The field of each loop adds to the fields

of the other loops and creates a greater total field strength.

Second Right-Hand Rule A method used to determine the

direction of the field produced by an electromagnet relative to the flow of conventional current.

Curl your right hand fingers around the loops in the direction of the conventional current

Your thumb points toward the north pole of the electromagnet

A Microscopic Picture of Magnetic Materials

Electrons in an atom acts like a tiny electromagnet

Domain is when the magnetic fields of the electrons in a group of neighboring atoms are all aligned in the same direction

When a piece of iron is not in a magnetic field the domains point in random directions and their magnetic fields cancel each other out.

In the case of a temporary magnetic and external magnetic field aligns the domains and when the external magnetic field is removed the domains return to their random arrangement

In a permanent magnet the iron keeps the domains aligned after the external magnet is removed.