Magnetism

MagnetismChapters 36 & 37

MagnetismA brief history

Lodestones were found in Greece some 2000 years ago.

The Chinese later used them for navigating ships.

In the 18th century, Charles Coulomb conducted a study of the forces between lodestones.

MagnetismA brief history:

Until early 19th century, electricity and magnetism were considered to be separate fields.

Hans Christian Oersted, in 1820, discovered a relationship between the two during a classroom demonstration.

This led to new technology that would bring electric power, radio and television.

Magnetic PolesMagnets apply forces on each other similar

to charges.

Magnets can attract and repel each other.

Magnets have poles that are the regions in the magnet that apply forces.

Magnetic poles are not positive and negative, but rather North and South.

Magnetic PolesThere is a rule when it comes to the

poles of magnets:

Like poles repel; opposite poles attract.

Magnetic PolesWhat would happen if you were to cut a bar

magnet in half?

Magnetic FieldEvery magnet produces a magnetic field.

A magnet’s magnetic field is similar to a planet’s gravitational field.

When another magnet is near, or even a compass, it will lie in a line with the magnetic field.

Similarly, iron filings become tiny bar magnets in the presence of a magnetic field.

Magnetic FieldWhat did we learn in our lab about the shape of

a magnetic field around a bar magnet?

Magnetic FieldWhat did we learn about the direction of the

magnetic field lines around a bar magnet?

Field lines go out from the North and into the South.

Magnetic FieldWhat would happen to the magnetic fields

of two like poles placed next to each other?

N N

Magnetic FieldWhat would happen to the magnetic fields

of two like poles placed next to each other?

N N

Magnetic FieldWhat would happen to the magnetic fields

of two opposite poles placed next to each other?

N S

Magnetic FieldWhat would happen to the magnetic fields

of two opposite poles placed next to each other?

N S

Magnetic FieldWhat can you tell about the two magnets in

each of these situations?

Earth’s Magnetic FieldThis is a drawing of Earth’s magnetic field

and its direction. What do you notice?

So which magnetic pole is which?

Magnetic DomainsMagnetic

domains = a microscopic cluster of atoms with their magnetic fields aligned.

Magnetic DomainsIn our lab yesterday, how were you able to pick

up the paperclips with the nail?

What did the magnet do to the nail?

Induced magnetism = metals (particularly iron) exhibiting magnetic properties due to contact with another magnet.

Electric Current & Magnetic Field

Remember Oersted? What did he discover during a classroom presentation?

A moving charge/current produces a magnetic field, deflecting a compass.

No current: Current:

Electromagnetism• First Right-Hand

Rule– Thumb points in

direction of current

– Fingers follow magnetic field lines (direction of magnetic field)

I

I

Electric Current & Magnetic Field

These are examples of a current-carrying wire, a current-carrying loop and a coil of loops.

Electric Current & Magnetic FieldIf a current-carrying wire is bent into a loop, the

magnetic field lines bunch up. If you add another loop and another, the magnetic field becomes more and more concentrated. This coil is called an electromagnet.

Electromagnetism• What about a

coil of wire?–The RHR still

applies!

I

Electromagnets• Coil has a field like any

permanent magnet with N and S poles• Advantage: can be turned off

and on

Electromagnets• 2nd Right-Hand Rule–Determine magnetic field of

electromagnets–Fingers follow current as it curls in the

coil–Thumb points in direction of N pole

Magnetic ForceA magnetic field will also apply a force on a

current-carrying wire.

To determine direction, we use the Right Hand Rule.

Forces caused by Magnetic Fields

• Vectors• Perpendicular to magnetic

field lines and current

Forces caused by Magnetic Fields

• 3rd Right-Hand Rule– Determine direction of Force on a current-

carrying wire in a magnetic field

N S

I

Magnetic ForceLet’s try another one…

Thumb points in direction of current.

Fingers point in direction of magnetic field.

Palm points in direction of force.

Magnetic Force

N

Give this one a try:

S

I

Force: into the page

Magnetic Force

N S

I Force: out of the page

Magnetic Force

N S

I

X

Force: down

Forces caused by Magnetic Fields

• F = BIL–B = strength of magnetic field–I = current in the wire–L = length of wire in magnetic field

–We know how to measure F, I and L, but not B so instead we use…

Forces caused by Magnetic Fields

• B = F / (IL)–Magnetic induction – strength of

the magnetic field• Units: Tesla (T)• 1 T is very strong•Most lab magnets are 0.01 T• Earth’s magnetic field is 5 X 10-5 T

A Simple DC Motor

A Simple DC Motor

Important Definitions•Magnetic flux–Number of magnetic field lines

passing through a surface

Electromagnetic InductionFaraday discovered that

electric current could be produced in a wire simply by moving a magnet in and out of a coil of the wire.

This is called electromagnetic induction.

Electromagnetic InductionThe greater the number of loops of wire that move

in a magnetic field, the greater the induced voltage and the greater the current in the wire.

Magnetic ForceA magnetic field applies a force on a moving charge.

Force on a single charged particle

• Cathode ray tube – TV!–Electrons deflected by magnetic

fields to form pictures

Cathode Ray TubeElectric fields pull electrons off atoms,

then more electric fields gather, and focus electrons into a beam.

• Magnetic fields deflect electrons side to side and up and down across the screen• Screen coated with phosphorous that

glows when struck

Force on a single charged particle

• F = BIL• F = B(qv/L)L• F = Bqv–q = charge of electron–v = particle velocity

Magnetic ForceThe magnetic field of Earth deflects many

charged particles that make up cosmic radiation.

Van Allen Radiation Belts• Electrons trapped in Earth’s magnetic

field– Solar storms send high-energy charged

particles toward Earth–They knock electrons off VA belts–The electrons excite nitrogen and oxygen

in the atmosphere creating a “halo”–The halo surrounds geomagnetic north

Van Allen Belts• The Van Allen radiation belts are formed as a

result of earth’s magnetic field and shield us from radiation. We can see the aurora borealis as a result.