Chapter 29 Magnetic Fields Physics 2212. Differences Between Electric Fields and Magnetic Fields...
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Transcript of Chapter 29 Magnetic Fields Physics 2212. Differences Between Electric Fields and 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/dt
l 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– 1 parallel, 1 perpendicular– Makes spin
F X F