magnets ph 203 professor lee carkner lecture 26. magnetic materials the moving charges in a magnet...
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Magnetic Materials
The moving charges in a magnet are
spinning or orbiting electrons
Problem: The most common and well-known examples of
magnetism result from extremely complex physics
Dipoles
= B sin U = -B cos
When aligned with the field the energy is –B, at right angles to the field the energy is 0, when opposite the field the energy is +B Dipoles that are not aligned with the
field will turn until they are
Spin Magnetic Dipole
We can write this as
s=B = (eh/4m) = 9.27X10-24 J/T
The electron can feel a torque and has magnetic potential energy just like a normal magnetic dipole
Orbital Magnetic Dipole
The orbital angular momentum is quantized
We can write the orbital magnetic dipole moment as
orb = -mlB
where B is again the Bohr magneton and ml is the orbital magnetic quantum number (0, 1, 2, 3 …)
Magnetic Energy
Like any magnetic dipole (e.g., a loop of current)
U = -Bext
where potential energy is minimum
Note that the total energy between parallel and anti-parallel spins is 2 Bext
Diamagnetism
Net orbital magnetic dipole moment is zero Like a bunch of loops with half with clockwise and half
with counterclockwise currents
Speeds up electrons one way, slows down electrons other way
Almost all materials are diamagnetic, but the net
effect is usually very weak
Paramagnetism
But they are usually randomly aligned and so there is no net magnetic field
An external magnetic field will cause the
atomic dipole moments to try to align Thermal motions of the material try to
randomize the atoms
Curie’s Law
For low temperatures M can be found from Curie’s Law
M = C(Bext/T) Where C is the Curie constant
Strong magnetic fields and low temperatures
produce more paramagnetism
Ferromagnetism
Due to an effect called exchange coupling, the electron spins of nearby atoms align with each other
Appling an external magnetic field can cause these domains to permanently align
The Earth’s Magnetic Field The Earth has a magnetic field produced by the
dynamo effect
We can find the direction of the Earth’s magnetic field with a small bar magnet called a compass
The Earth’s “north magnetic pole” is really a the south pole of a magnet!
Planets and Magnetism A planet will have a magnetic field if:
It has a conducting liquid interior It has a fast enough rotation
This interaction region is known as the magnetosphere
To step down 120 household current to 12 volts, we would need a transformer with a ratio of turns between the primary and secondary transformer of,
A) 1 to 1
B) 10 to 1
C) 12 to 1
D) 100 to 1
E) 120 to 1
What is the direction of the magnetic field in the PAL at a point due east of the center of the region?
A) North
B) South
C) East
D) West
E) Up
Consider a standard 6-sided die. Suppose the magnetic flux through each side 1-5 is equal to the number of spots on the side (in Wb) and points outward for even sides and inward for odd. What is the flux magnitude through the side with 6 spots?
A) 1 Wb
B) 2 Wb
C) 3 Wb
D) 6 Wb
E) 9 Wb
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