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 current 2)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 F B = q v × B q – charge v – velocity VECTOR B – 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 XX XXX X Pv F B F B = q v × B F B = IqI IvI IBI sinθ 0 when parallel Must move perpendicular

Right Hand Rule V – Thumb B – Finger F – Palm For point charge or current

Current I = dq/dt l I = (dq/dt) l l (dq) = q (d l ) l I = q (d l /dt) = qv F B = IqI IvI IBI sinθ F = I l X B

**Must be a vector to get force!! IlIl

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 field If 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 F up = qvB F down = qE

qvB = qE v = 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 – 1 parallel, 1 perpendicular – Makes spin F X F