Transcript Chapter 29 Magnetic Fields - Kennesaw State University

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
FB = 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
X
X
F
X
B
P
X
v
X
FB = q v × B
FB = IqI IvI IBI sinθ
0 when parallel
Must move perpendicular
X
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=IlXB
Il
**Must be a vector to get force!!
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
=qvB
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
Fup = qvB
Fdown = 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
– Proportional to magnetic field, current and area of
the loop perpendicular to the field
IA × B