Ch.8 Magnetic Fields

8.1 Magnets and

Electromagnets

The Law of Magnetic Poles

Opposite magnetic poles attract.

Like magnetic poles repel.

The area around a magnet in which

magnetic forces are exerted.

The magnetic field at any point is a vector

quantity, represented by 𝛽 .

Magnetic Force Field

Principle of Electromagnetism

Accidental discovery by Hans Christian Oersted

(1777-1851); he observed the deflection of a

compass needle due to an electric current flowing

in a nearby wire

Moving electric charges produce a magnetic field.

+ I

Magnetic Fields Around Bar Magnets

Magnetic field lines are ALWAYS drawn

from North Pole South Pole.

Permanent Magnets

Magnetic field lines

always form closed

loops – you can

never have just a

south pole or just

a north pole.

Where is the

magnetic field, B

the strongest?

N

Domain Theory of Magnetism

Not all substances are magnetic … so

what makes something magnetic?

N S

N S N S

N S

N S

N S

N S

N S

S S N

Domain Theory of Magnetism

This theory describes how tiny magnetically

homogeneous regions or “domains” behave

such that a material can become magnetized

as each domain acts like a bar magnet.

Unmagnetized material Magnetized material

Earth’s Magnetic Field

magnetic declination

Earth’s Magnetic Field

The convention is to

refer to the south

magnetic pole as Earth’s

north magnetic pole.

Geological studies

show that Earth’s

magnetic field has

completely reversed

direction many times

during the planet’s

history.

Right Hand Rule (Straight Conductor)

If your right thumb points in the direction of

current, your curled fingers indicate the

direction of the magnetic field (circular

shape or concentric circles).

+ I

current convention = conventional

current, not electron flow direction Current, I = measured in

A or C/s

RHR VS LHR

B

Magnetic Field of a Current Loop

If the conductor forms a loop, the magnetic

field is very strong inside but weak outside.

magnetic

fields

current

Solenoids

A solenoid is a long conductor wound

into a coil of many loops that produces a

strong magnetic field inside the coil.

Right Hand Rule (Solenoids)

If your right fingers

curl in the direction of

current, your thumb

points in the direction

of the magnetic field

lines in the core.

There is a very strong

magnetic field inside

the solenoid

I

ELECTROMAGNET = a magnet

produced from electric current

I

S

N

Electromagnets

Centers of solenoids are

often filled with iron to

strengthen B.

Adding carbon to iron

“locks” domains in place,

to create permanent magnets.

Electromagnetics can be

turned ON/OFF

using electricity.

2-D Drawings

Current is towards you /

out of the page

Current is away from you /

into the page

+ I

Example 1

Find the unknown (either magnetic field

direction or current direction). Assume

conventional current direction.

N S I

B

B

B

I