Magnetic Materials

Question 1

What is the symbol and definition for a magnetic dipole moment?

Answer 1

> Symbol: μm
A magnetic dipole moment is a vector for the direction and magnitude of magnetism produced by a very small current loop

Question 2

What is the equation for a magnetic dipole moment? What are all the symbols?

Answer 2

μm = I × A × μn
> μm = Magnetic dipole moment
> I = Current
> A = Area
> μn = Unit vector perpendicular to loop

Question 3

What will happen do the magnetic dipole moment when placed in a magnetic field?

Answer 3

The magnetic dipole moment will align with the magnetic field

Question 4

How can the small current loop inside a material be described as?

Answer 4

A small bar magnet

Question 5

What is a good example of a small current loop?

Answer 5

An atoms electrons and their spin

Question 6

What happens in an atom that causes a NET magnetic moment

Answer 6

> Electrons spinning cause a magnetic moment to be produced.
Because electrons are usually paired and spin in opposite directions this causes the magnetic moment s to cancel out.
A net magnetic moment is produced when there is an unpaired electron around an atom

Question 7

What is the equation for the magnetic moment caused by an electron?

Answer 7

μ[el] = μ[spin] + μ[orb]

Question 8

What is the equation and the symbols for the magnetic field strength in the solenoid?

Answer 8

B0 = μ0 × n × I
B0 = Magnetic field strength
μ0 = Permeability of free space
n = Number of turns per unit length
I = Current

Question 9

What happens to the magnetic field strength of a solenoid when a material is placed inside the loop?

Answer 9

The magnetic field strength changes from B0 to B

Question 10

What is the vector M?

Answer 10

> The total magnetic dipole moment per unit of volume.

> Also known as the magnetisation vector

Question 11

What is the equation for the magnetisation vector?

Answer 11

M = n[at] × μ[av]

Question 12

How is the magnetisation vector similar to polarisation?

Answer 12

M = n[at] × μ[av]
P = n[at] × p[av]

Question 13

What is the equation for the total magnetic moment?

Answer 13

μ[tot] = M × Az
M = Magnetic moment per unit volume
A = Area
z = Length

Question 14

What is the other equation for the total magnetic moment?

Answer 14

μ[tot] = I[m] × n × z × A
I[m] = Current on the surface
n = Number of turns per unit length
A = Area
z = Length

Question 15

What is the equation for the magnetisation vector?

Answer 15

M = n × I[m]

Question 16

What is the equation for the magnetic field inside a solenoid when a material has been added? What does this show?

Answer 16

> B = B0 + μ0M

> The material in the field contributes to the magnitude of the magnetic field strength

Question 17

What is an equation for the magnetic field inside a solenoid when there is a material added with regards to the currents for these materials?

Answer 17

B = μ0 × n × ( I + I[m] )

Question 18

What is magnetic field intensity?

Answer 18

This is the contribution of the solenoid to the overall magnetic field divided by the permeability of free space

Question 19

What are the equations for magnetic field intensity? (3)

Answer 19

H = nI
H = B0/μ0
(B - μ0×M) / μ0 = H

Question 20

How does it all work with the solenoid and magnetic material inside?

Answer 20

The solenoid induces a magnetic field inside the material which causes all the magnetic dipole moments to align which further increases the magnetic field

Question 21

How can the relationship with the magnet field intensity be related to the magnetic dipole moment per unit volume?

Answer 21

M = χm × H
M = Magnetisation vector
H = Magnetic field intensity
χm = Magnetic suceptability

Question 22

What is the magnetic susceptibility?

Answer 22

This is how well the material accepts the magnetic field

Question 23

What is the exceptions for magnetic susceptibility?

Answer 23

Ferromagnetic materials

Question 24

How is the relative permeability related to the magnetic susceptibility?

Answer 24

μr = 1 + χm

Question 25

What is a diamagnetic material?

Answer 25

> This is a material with a small and Negative value of magnetic susceptibility (χm)
Reduces the magnetic field within the solenoid because it is expelling it

Question 26

4-5 Marks on a diamagnetic material

Answer 26

> Small and Negative value of magnetic susceptibility (χm)
Try to reduce the change of the magnetic field
The magnetisation vector is in the opposite direction to H; the magnetising field
Material trying to expel the magnetic field
When placed in a non-uniform magnetic field, it will tend to move in to an area of lower magnetic field strength.
Material examples
> Atoms with closed shells
> Organic polymers
> Covalently bonded solids
> Copper, Gold etc
No permanent magnetic dipole moment
Its properties are not affected by temperature. Except superconductors

Question 27

What is a paramagnetic material?

Answer 27

> This is a material with a small and Positive value of magnetic susceptibility (χm)
Increases the magnetic field within the solenoid
Atoms have a magnetic moment

Question 28

4-5 Marks on a para magnetic material

Answer 28

> Atoms have a net magnetic dipole moment due to an unpaired electron
Each magnetic moment is independent on it neighbours
The directions of these magnetic dipole moments will be scattered but when placed in a magnetic field they will align with it
This increases the magnetic field of solenoid.
Increasing temperature of the material will make it worse at increasing the magnetic field because the increased energy will scatter the magnetic dipole moments because the atoms will move more so less will align and more will cancel each other out
This follows the curie temperature equation:
χm=C(T-T_c)
Small positive magnetic susceptibility (χ_m)
It will move towards the regions of higher magnetic field
Attracted
Material examples
> Oxygen (Liquid or gas)
> Ferromagnetic materials
> Antiferromagnetic materials
> Ferrimagnetic materials

Question 29

What is the curie temperature equation?

Answer 29

χm = C ( T - Tc )
C = Curie coefficient
Tc = Curie temperature

Question 30

What is a ferromagnetic material?

Answer 30

> A special type of paramagnetic material

> Large magnetic moment in the absence of a magnetic field

Question 31

4-5 Marks on a ferromagnetic material

Answer 31

> A special type of paramagnetic material
Large magnetic moment in the absence of a magnetic field
Magnetic susceptibility (χm) is positive and VERY large and depends on the applied magnetic field intensity
The relationship between M and H is very non-linear with magnetisation saturating at high fields
All atoms and their neighbours align with each other. Due to quantum mechanics. They are not independent of their neighbors. Even at low temperatures they will align.
At very high temperatures it will become paramagnetic because this alignment will be disrupted. T> T_c. They are therefore temperature dependent
Maximum magnetization occurs when all the possible magnetic moments have been aligned
Maximum magnetization is called saturation magnetization
As the temperature increases, more lattice vibrations occur and disrupt the alignment of spins so magnetization falls
At a high temperature (curie temperature), the thermal energy is larger than the exchange interaction energy (which aligns the spins) and this will cause it to act paramagnetically

Question 32

What is domain behaviour?

Answer 32

This is the way that a ferromagnetic material may have no net magnetic field and also how the magnetic field of a ferromagnet can be changed

Question 33

Describe how domain behaviour works?

Answer 33

> The ferromagnetic material is split into different sections called domains.
In each domain the direction of the magnetic dipole moments of all the atoms in that domain may be different from neighbouring domains.
This can cause domains to cancel out and so no magnetic field can be seen.

Question 34

How can applying an increasing magnetic field to a ferromagnetic material change its domain? (4 steps)

Answer 34

1. Low magnetic field - The domain moves slightly (This is reversable)
2. Medium Low magnetic field - The domain moves suddenly and irreversibly changing the domains shapes (jerk)
3. Medium High magnetic field - The domain will align in the easy direction
4. High magnetic field - The materials magnetic dipoles will allign with the magnetic field

Question 35

What will happen to the ferromagnetic material after a High magnetic field has changed the domain to align with it and is then removed?

Answer 35

The domain will align with the easy direction and stay there

Question 36

How will the ferromagnetic material be returned to its original domains?

Answer 36

A high magnetic field in the reverse direction is required

Question 37

How can a high magnetic field in the reverse direction alter the domain of a ferromagnetic material?

Answer 37

> It can return the material to its original domain or it can go so far as to align the domain in the opposite direction

Question 38

What is the graph called for changing the domain of a ferromagnetic material?

Answer 38

Hysteresis loop

Question 39

What are the properties of a soft ferromagnetic material?

Answer 39

> Easy to magnetize and demagnetize
> Require little energy to change them
> Used when magnetization needs to change quickely
> They need to be highly resistive
> Amorphous magnetics are good choices

Question 40

What are the properties of a hard ferromagnetic material?

Answer 40

> Hard to magnetize and demagnetize
> Generally used in permanent magnets
> Often made by compacting powders together
> E.g. ceramic magnets
> Used in magnetic storage