Magnetics- Intro Flashcards
Uses of magnetic materials
Speakers, data storage, heavy lifting, transformers, credit cards, fridge magnets, motors
Describe right hand rule for current through a wire
Wrap hand around wire and stick thumb out. Thumb is direction of current. Fingers show direction of field lines
Ampere’s law
H=I/2πr
H is magnetic field (A/m)
r is distance from wire
I is current
Formula for field produced from current in solenoid and its direction
H=nI
n is number of turns
Field inside solenoid goes to where current goes into wire (North) and comes back outside solenoid to where current exits (South)
What is magnetic moment?
m=IA for a single current loop
I is current
A is loop area
Direction is perpendicular to area
How are magnetic fields created?
Moving electrical charge. Electrical current creates a magnetic field. Atoms have moving electrical charges and the electrons have spin which are equivalent to circulating electric currents. This is the origin of magnetic field (and magnetisation) in materials. Results in atomic magnetic moments. Each magnetic atom has N and S pole. Atomic moments in magnetic material combine to create the simple pole of a bulk material.
Magnetisation
The magnetic moment per unit volume
M=m/vol
Units A/m
Compare saturation magnetism Ms of iron, nickel and cobalt
Iron greatest, cobalt next, nickel last
Magnitude around 10^6
Strength of magnetic pole
p=m/l
l is distance between centres of opposite poles from which a magnetic field arises
Force between two poles
F=μ0p1p2/4πr^2
μ0 is permeability of free space=4πx10^-7 H/m
Magnetic field produced by pole
H=μ0p/4πr^2
Force exerted on second pole by one pole
F=Hp2
What happens when magnetic field applied to bar magnet?
Magnet rotates in the field due to the torque applied
Formula for torque
τ=μ0mxH
m and H are vectors so do cross product
Energy of magnetic dipole
E=-μ0m•H
m and H vectors and do dot product
Aka Zeeman energy
What is magnetic induction?
The response when a magnetic field is applied to free space or a material
Aka magnetic flux density
B=μ0H
B and H vectors
If H is 0, what is magnetisation, M?
Also 0
For a magnetic material with linear response to applied magnetic field, what is magnetisation for non-zero H?
M=χH
Where χ is magnetic susceptibility
For a magnetic material with linear response to applied magnetic field, what is magnetic induction, B for non-zero H?
B=μ0μrH=μH
Where μr is relative permeability =1+χ
B results from all sources of magnetic interaction
Where to find conversion from SI to cgs units
Intro lecture page 28
What do Teslas (T) measure?
Magnetic polarisation
Magnetisation multiplied by μ0 to give more convenient numbers
