Magnetism

Force of Mystery

Presentation Text ©2001 Philip M. Dauber as modified by R. McDermott

Magnetic Poles

North and SouthLike poles repel N-N S-SUnlike poles attract N-SPoles are named for which way they point if suspended (so they rotate freely)A magnetic north points to geographic north

Magnetic Poles Are Not Charges

A north pole is not a positive charge.

A south pole is not a negative charge.

Magnets do not exert forces on stationary charges.

Charged objects are not affected by magnets.

Magnetic Poles Are …. Poles

Single poles cannot be isolatedMagnetic monopoles do not exist in nature

N S

S NN S

Get two smaller ones:

Break a magnet:

Ferromagnetic Materials

Show strong magnetic effectsIronCobaltGadoliniumNeodymium

Ferromagnetism

Ferromagnetic material contains “domains”1 mm in length and normally random in directionEach acts like tiny magnetGenerally, domains cancel – no magnetic effectsAn external field aligns domains (non-random)A strong magnetic field can make other ferromagnetic materials into permanent magnets

Electrons and Magnetism

Magnetism is electrical in origin Magnetic fields are produced when charges moveEven permanent magnets owe their strength to electron “currents”There is no way to “divide” a current to get N or S poleStationary charges are unaffected by magnetic fields, and do not generate magnetic fields.

Permanent Magnets

Hi tech Neodymium iron boron magnets

Units of Magnetic Field

Tesla (SI Unit)1 Tesla = 1 Weber/m2

Earth’s magnetic field = 5x10-5 Tesla

Earth’s Magnetic Field

Very weakLike a bar magnetNorth magnetic pole is at the south geographic poleSouth magnetic pole is at the north geographic pole

Direction of Magnetic Field

Demo

The direction the north pole of a compass would point when placed at that location

Magnetic Field

Magnetic field lines (flux) are measured in webersMagnetic lines of flux go from North to South

Electric Currents = Magnetism

Magnetic field around long straight wire

I

Right hand rules determine the direction of magnetic fields

Demo

–Point thumb in direction of current–Fingers wrapped around wire point in direction of magnetic field

Electric Currents = Magnetism

Magnetic field due to circular loops (solenoid)

Demo

– Grab loop with thumb of right hand in the direction of conventional current.– Fingers point in direction of field at center of loop.

Right Hand Rule(s)

Long Straight Wire– Point right thumb in direction of conventional current– Fingers wrapped around wire point in direction of

magnetic field

Circular loop(s) of Wire– Grab loop with thumb in current direction– Fingers point in direction of field at center of loop

Force on a Charged particle in a Magnetic Field

F = qvB sinMagnetic force is perpendicular to both particle direction and field (vector cross-product)

Demo

Charged Particle Path in Uniform Magnetic Field

Circle or helixF = maqvB = mv2/r (centripetal acceleration)r = mv/qB (mass spectrograph)Direction follows third right hand rule

Demo

Force on Current-Carrying Wire

I

F = BIL sin

is angle between

field and wire

Force is perpendicular to both current and field directions

How can F = BILsinbe Used to measurea Field?

Hint: use a rectangular loop of wire

Demo

Magnetic Field Due to Straight Wire

B = 0I/(2r)

*04x10-7 T-m/A

I

* Permeability of free space

Force Between Parallel Wires

F/ L = oI1I2/(2d)

Force per unit length of wired is distance between wiresUnidirectional currents attractBidirectional currents repel

Induced EMF

Moving a wire across magnetic field lines produces an EMF (potential difference) across the wire. This potential difference increases with increasing wire speed.The maximum EMF occurs when the wire is moved perpendicular to the field lines.

= LvB sin