Chapter 29 Magnetic Fields

Physics 2212

Differences Between Electric Fields and Magnetic Fields

1) What creates the field a) Electric - charge created just by sitting b) Magnetic – current ultimately charged at source, but charge must be moving

right hand current2) Field Lines

a) Electric – lines start and end b) Magnetic – circular loop lines

Magnetic Versus Electric Field Lines

Magnetic Field Lines

• Implies no magnetic charge (no North)• No magnetic charges• Always closed loop surrounding charge• But still makes dipole with same shape

Charges

• Both electric and magnetic dipoles: opposites attract

• North side of compass orients itself to face South, so that the dipole is opposite

• Everywhere on Earth, there is a magnetic field– It produces a force

A Magnet’s Effect

• Charges move, producing the magnet’s effects• A force acts on the charges

FB = q v × Bq – chargev – velocity VECTORB – magnetic field

• This is a CROSS PRODUCT• Tricky – 3 mutually perpendicular vectors

• Magnetic fields on Earth point North towards the magnetic South pole– Going into page X– Coming out of the page

Example

X X

XXX

X

P v

F

B

FB = q v × BFB = IqI IvI IBI sinθ

0 when parallelMust move perpendicular

Right Hand Rule

• V – Thumb• B – Finger • F – Palm

• For point charge or current

Current

I = dq/dtl I = (dq/dt)l l (dq) = q (dl)

l I = q (dl/dt) = qv

FB = IqI IvI IBI sinθ

F = I l X B

**Must be a vector to get force!!

Il

Units

• Units of B: Tesla (T)• T = Ns/Cm• A 1T magnet is BIG!

A 19T magnet

• Magnetic field with current pointed at it: bends path into circle or spiral

• Old TV works like this: Cathode Ray– Resistor inside capacitor is heated, free electrons go

into the positive end and shoot out hole, causing a beam

– Current goes opposite of electrons– Must deflect the beam to other parts of screen

magnetic field– Produce 2D field

• 2 sets of charged coils– Side to side produces up and down– Up and down produces side to side

Centripetal Force

= q v B

mv = qB r

For a particle travelling in uniform magnetic fieldIf you know “r”, the particles being bent

measure ration, measure mass

m = Br q v

curving at different speeds

Speed, strength, measure curvature

• Mass spectrophotometer – separates ions of different mass

• Consistent velocity – add electric field • Take beam, pass into magnetic field that pulls

beam up, a capacitor that causes electric field to go straight through

Fup = qvB Fdown = qE

qvB = qEv = E

B

Curve up too fast, curve down too slowly, straight = right speed

Charge is quantized (have same q)

Loop of Current

• No force on top and bottom• Causes torque– Proportional to magnetic field, current and area of

the loop perpendicular to the field

IA × B