Download - Chapter 29 Magnetic Fields
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