Magnetism

2000 years ago, rocks in Magnesia, Greece were found to have weird properties. Later called “lodestones.”

Hans Christian Oersted (Danish Science Professor, 1820) Discovered relationship between electricity and magnetism accidentally while demonstrating electric currents. These caused a compass needle to tip.

Similarities to Electricity• Exert non-contact force

– Has a Field• Strength of force depends on distance• Attract and Repel ∴ there are 2 parts (north & south)

– Opposites attract -- Like pole repel

Differences from Electricity• You can get 1 e by itself. You can never get a

North pole without a South Pole and vice versa.

S NS S NN S NN SS NN S

Magnets & Electricity

• Electrons & magnetic fields– Remember electron spin?

• Charge in motion causes a small magnetic field.

– Many electrons spinning in same direction produces …

• Magnetic Domains– Clusters of aligned electrons spinning in the

same direction– How is a weak magnet different from a strong

magnet?• Stronger magnets have more aligned domains

Magnetic Domains

How strong are these magnets? Why?

Magnetic Field (B) Area around a magnet that affects other magnetic objects.

Measured in Teslas (T) Vector or Scalar?Rules for drawing

Leaves Object perpendicularOut of North into Southmore lines stronger field

Field is strongest at the polesMagnetic Force caused by Magnetic Field

F = q v B– qelectron & proton = 1.6 x 10-19 C

N S

1st Right Hand RuleDirection of the magnetic force is perpendicular to

the velocity and the field

1st Right Hand Rule (3 Dimensions) Tells you the direction of the force. • Place your fingers in the direction of B • your thumb in the direction of v• palm tells direction of the force

• out of + into –

Since this deals with 3 Dimensions…mean going into the page (feathers of an arrow)

mean coming out of the page (arrowhead)

• Find the direction of the force for each of the following on a proton.

Bin

v B

v

v

BoutB

v

What direction would the force be on a proton with velocity v in a field B?

Sketch the path of the electron once it enters the magnetic field.

v

Bout

Because it goes in a circular path, the magnetic force becomes the centripetal force (sum of all the forces pulling an object into a circle.)FC = FM

m v2/R = q v B

What direction would the force be on a proton with velocity v in a field B?

Sketch the path of the electron once it enters the magnetic field.

v

Bout

Because it goes in a circular path, the magnetic force becomes the centripetal force (sum of all the forces pulling an object into a circle.)FC = FM

m v2/R = q v B