1. Magnetic and Electrical Fields in materials Flashcards Preview

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Flashcards in 1. Magnetic and Electrical Fields in materials Deck (37)
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1
Q

What is (BH)max?

A

This point occurs in the second quadrant of the B-H loop and corresponds to maximum magnetic potential energy. This translates to the highest possible B for a given volume of material. (also thought as the energy stored in the magnetic field around a magnet, and is the optimum working point for the battery)

2
Q

What is Hc?

A

Hc is the coercive force and corresponds to the field at which the material demagnetises completely.

3
Q

What is Br?

A

Br is the remanent magnetic field i.e. that when H is reduced to zero (hence can only be measured easily in long rods).

4
Q

What is Bsat?

A

Bsat is the saturation field and occurs when all the individual magnetic dipole are aligned by H. The gradient of the B-H curve is µ0 at higher fields (negligible increase compared to Bsat.).

5
Q

What is

µ?

A

µ is the permeability and is given by the gradient of the B-H curve in the linear region.

6
Q

Define the electric field E

A

The electric force per unit charge, and is a vector field, units N/C

7
Q

What does Gauss’s law state

A

The surface integral of the electric field through a surface gives you the charge enclosed

8
Q

What does Coulomb’s law state?

A

That like charges repel and opposite charges attract, with a force proportional to the product of the charges and inversely proportional to the square of the distance between them.

9
Q

What is a dielectric

A

A dielectric (or dielectric material) is an electrical insulator that can be polarized by an applied electric field.

10
Q

What are polar dielectrics

A

Dielectrics where the dipoles are permanently present but disordered

11
Q

What is polarisation

A

The contribution to the electric field from the charges between the plate of a capacitor, ie. in the dielectric.

12
Q

What is the equation for the Displacement field

A

D = ε0*E + P where P is the polarisation/electric field from charges between the plates, E - electric field

13
Q

What does the displacement field allow us to do?

A

Determine what electric field will result from the combination of the electric fields generated from charges in the capacitor and in the dielectric between the plates. It represents the free charges (as opposed to bound within the dielectric) and thus the field developed by the charges in the plates. If there is no dielectric then D = ε0*E.
In free space, the electric displacement field is equivalent to flux density, a concept that lends understanding to Gauss’s law.

14
Q

Why is the there the distinction between the B and H field

A

The H field is devised so that the magnetisation current can be removed from amperes law inside a material, as this is not possible to measure. This leaves only the conduction currents in the amperes law expression: ∇ x H = μ0 * jc (jc = conduction currents). Therefore it is the component of the megnetic field due to conduction currents. However this definition does not mean that permanent magnets with no flowing currents do not have an H field, they do.
There is no distinction between B and H outside a material as they have the simple relation B = μ0H.

15
Q

What is the equation for total H field?

A

H total = H0 + Hd
H0 = externally applied H field
Hd = H field fue to demagnetisation

16
Q

What is the magnetisation of a material?

A

The total dipole moment / Volume

17
Q

What is the equation for the torque on a dipole?

A

Torque = magnetic dipole moment x B (Magnetic field)

18
Q

What is the magnetic suscepibility

A

Measures how easily the atomic dipole moments may be rotated, thus the Magnetisation = susceptibility x H (auxilary field)

19
Q

What is the susceptability of Diamagnetic materials?

A

Susceptability < 0 (they expel magnetic field lines)

20
Q

What is the susceptability of Paramagnetic materials?

A

Suscepability > 0 (they concentrate magnetic field lines)

21
Q

Why does a sufficiently large magnetic field need to be applied to a paramagnetic material to align the dipoles?

A

As the tendency for dipoles to align with the field competes with thermal effects

22
Q

What are ferromagnets?

A

Materials which exhibit spontaneous alignment of their magnetic dipoles

23
Q

What are antiferromagnets?

A

Adjacent dipoles are oppositely directed leading to no overall magnetisation

24
Q

What are ferrimagnets?

A

where anti-ferromagnetic couplic occurs but the two dipoles have different strengths, therefore some ferromagnetic materials are actually ferrimagnets

25
Q

What is the magnetic dipole moment of aligned spins?

A

1 Bohr magneton

26
Q

What is the curie temperature?

A

the Curie temperature (TC), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism.

27
Q

What are the 3 known ferromagnetic elements at room temp?

A

Fe (bcc), Ni (fcc), Co (hcp)

28
Q

Why does smelted bbc iron not exhibit magnetisation

A

Due to the formation of domains

29
Q

How does a magnetic field affect domain sizes in bcc iron?

A

The domains that are alligned with the applied magnetic field grow in size.

30
Q

What affects the ease at which domains can grow on the application of an external magnetic field?

A

The ease at which domains grow and their walls move depends on the presence of pinning centres such as impurities, grain boundries, or other inhomegeneities in the material. Pinning causes hysteresis and loss linearly with frequency.

31
Q

Why are B-H curves measured for long rods with the field applied parallel to the rod axis?

A

So that the material can’t demagnitise itself, so H=H0 as there is no magnetising currents to generate a demagentising field Hd

32
Q

Where does BH(max) occur for linear materials?

A

At the mid-point of the second quadrant on the BH curve

33
Q

Why do permanent magnets operate in the 2nd quadrant of B-H curve?

A

As when there is no externally applied H0 field, the demagnetising field Hd is the only H field present as this is thought of as negative

34
Q

Differences between H and B fields within a magnet and at the boundary:

A

H and B fields act in opposite directions inside the magnet, and B much be continuous across the boundary.

35
Q

What aplifies the demagnetising field

A

The shape of the demagnetising field (N = demagnetising factor, N=0 for infinite cylinder (field parallel to axis, N=1 for flat plate (field perpendicular to the surface))

36
Q

What is the equation for the H field in terms of demagnetisation factor?

A

H = -NM (M= magnetisation)

37
Q

What is the permeability of a magnetic material?

A

The permeability of a material is a measure of its ability to support a magnetic field within itself. It can be thought of as the effective multiplication of an external magnetic field by the alignment of the magnetic domains within the material.