Download Physics 1220/1320

Physics 1220/1320
Electromagnetism
&
Thermodynamics
Lecture Magnetostatics, chapter 27-29
Magnetism
- Contrary to common opinion, magnetism is just as common
as electricity.
-Magnetic fields are due to the motion of electric charges
-All moving charges create magnetic fields
-Electric and magnetic fields will turn out to be ‘coupled’ and the
expression of the more general phenomenon of ‘electromagnetism’
-This phenomenon will explain the whole range of radiation and its
ways of spreading.
- Unlike electric charges, magnetism always comes in the form of two
opposing poles (usually called North and South pole)
-The magnetic force, magnetic field lines behave differently than the
electric counterparts
Unlike poles
attract,
Like poles
repel
Unit ‘Tesla’ [T] = [N/(Am)] , 10 k[G] = 1[T]
Many metals can be
‘magnetized’ when brought
in contact with a magnet.
The molten material
inside the earth rotates
and creates a small
magnetic field.
Earth field near surface
varies, ~ 1/3- 1/2 Gauss
Field strength which occur in nature:
Sun 6[kG]
pulsars 10^8 [T], magnetars [GT]
b/w two atoms ~ up to 70 [T]
… in technology:
50 ft from powerline 40[mG]
6’’ hair dryer 300[mG], microwave oven 6’’ 200[mG]
State of the art
-Permanent magnets have
field strength ~ 24[T]
-Electromagnets up to
100[T]
Magnetic field B and magnetic force FB
Unit ‘Tesla’ [T] = [N/(Am)] , 10 k[G] = 1[T]
Magnetic Field Lines
Magnetic flux FB,
Gauss’s Law
!
Mass Spectrometers:
Magnetic fields can act as ‘velocity selectors’ for charged particles:
v = E/B
ie only particles with the right speed can pass through
(condition: SFy =0)
In the famous Thompson
experiment, this effect was used
to determine the ratio e/m for
electrons.
In the mass spectrometer, the effect
is used to determine the mass of unknown
particles with high precision.
http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=53
http://www.pk-applets.de/phy/thomson/thomson.html
Hall Effect
Force on charge
carrier in B

qEz+qvdBy=0
Jx= nqvd
nq= (-JxBy/Ez)
Transverse E builds through charge accumulation
Due to FB until FE equal+opposite to FB  Hall voltage
Force on Current-Carrying Conductor
Force and Torque on a Loop
Torque is zero if dA parallel B and max
if perpendicular to B
Net force is zero
Magnetic dipole moment m =IA
Loops are important because electrons often perform loops,
so material properties can be understood if one understands
B for conductor loops.
A potential energy is associated with the dipole moment in B.
In B, coils will tend to turn toward
their position of Umin.
A case of practical importance is the energy of a coil in B:
Consider a coil which rotates from an initial position into one
where its m is parallel to B.
Note:
t = NIABsinf
How magnets work:
Magnets in non-uniform fields –
If free to move, all magnets will orient
such that their axis // B
Forces on current loops in non-uniform B
dF = I dl x B
Permanent magnets:
Random order
Aligned atomic
m’s
Presence of B makes net m
t tends to align
m’s with B
Non-uniform B  attractive force
http://ist-socrates.berkeley.edu/~cywon/Curie.html
http://ist-socrates.berkeley.edu/~cywon/Stripe.html
Magnetic Field of moving charge
Unit Tesla [T] = [(Ns)/(Cm)] = [N/(Am)]
[m0] = [N/A2] = [Tm/A]
and c2 = 1/(e0m0)
‘permeability’ of free space
Forces between two moving electrons
Magnetic Field of a Current Element:
Biot-Savart
B of Current Carrying Straight Conductor
Magnetic field of two wires
28.24
Find I4 to make B at center of square zero:
Magnetic Field of a Circular Loop
(atoms & electrons!)
Ampere’s Law
A more general integration path gives the
same result, as long as the wire is included
and the surface of integration is closed:
Field Inside a Long Cylindrical Conductor
Magnetic Field of a Solenoid
http://webphysics.davidson.edu/applets/BField/Solenoid.html
http://www.falstad.com/vector3dm/index.html