chapter 29 magnetic fields
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Chapter 29 Magnetic Fields. Physics 2212. Differences Between Electric Fields and Magnetic Fields. What creates the field a) Electric - charge created just by sitting b) Magnetic – current ultimately charged at source, but charge must be moving - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 29 Magnetic Fields
Physics 2212
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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
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Magnetic Versus Electric Field Lines
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Magnetic Field Lines
• Implies no magnetic charge (no North)• No magnetic charges• Always closed loop surrounding charge• But still makes dipole with same shape
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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
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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
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• This is a CROSS PRODUCT• Tricky – 3 mutually perpendicular vectors
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• Magnetic fields on Earth point North towards the magnetic South pole– Going into page X– Coming out of the page
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Example
X X
XXX
X
P v
F
B
FB = q v × BFB = IqI IvI IBI sinθ
0 when parallelMust move perpendicular
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Right Hand Rule
• V – Thumb• B – Finger • F – Palm
• For point charge or current
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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
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**Must be a vector to get force!!
Il
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Units
• Units of B: Tesla (T)• T = Ns/Cm• A 1T magnet is BIG!
A 19T magnet
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• 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
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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
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m = Br q v
curving at different speeds
Speed, strength, measure curvature
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• 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
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qvB = qEv = E
B
Curve up too fast, curve down too slowly, straight = right speed
Charge is quantized (have same q)
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Loop of Current
• No force on top and bottom• Causes torque– 1 parallel, 1 perpendicular– Makes spin
F X F
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