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PPT – Magnetic field similar to a bar magnet PowerPoint presentation | free to download - id: 701776-MTkwN

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Magnetic field similar to a bar magnet For a

very long solenoid, the magnetic field can be

considered to be confined to the region inside

the coils.

2a

x

z

2a

z

x components cancel

?

r

r

z

z gtgt a

?

x

a

a

magnetic dipole moment

right hand screw rule

At a point along the axis z gtgt a

i

current loop in xy-plane

Diamagnetic material ?m lt 0 (small) B

?o (1 ?m ) H

Permeability ? ?o (1 ?m ) slope of B-H line

B

Ideal magnetic material or paramagnetic

material ?m gt 0 (small) B ?o ?r H ? H ?

constant slope of B-H curve

H

L11.5 Magnetization

If H is large or substance strongly magnetic

(e.g. ferromagnetic), as H increases, the

magnetization M (and hence B) may increase

nonlinearly

from the graph

Measure

So mr varies with H. Could also use

differential permeability High field region

where slope decreases is called "saturation"

region.

L11.6 Magnetization

Hysteresis Ferromagnetic materials also show a

hysteresis effect, where decreasing the applied

magnetic field, or H, doesnt produce the reverse

effect of increasing the field

Br remanence or residual magnetism

Hc coercivity

L11.7 Magnetization

hard magnetic materials Hc is high, area of

the loop is large, used for permanent

magnets. soft magnetic materials Hc is small,

area of loop is small, used for transformer cores

electromagnets. Material can be

demagnetized by striking or heating it, or go

round the hysteresis loop, gradually reducing its

size. "Degaussing"

L9.1 Magnetic fields due to currents

Magnetic fields are produced by currents.

Biot-Savart law

Example

Amperes law

so

L9.2 Magnetic fields due to currents

A solenoid

(n is number of turns/length)

Therefore

(inside)

L9.3 Magnetic fields due to currents

Use the Biot-Savart law to derive the magnetic

field on the axis of a current loop

and

Therefore

L9.4 Magnetic fields due to currents

Magnetic field of the Earth

L9.5 Magnetic fields due to currents

The magnetic field of a magnetic dipole

(I??, A?0)

This magnetic field has the same shape as the

electric field of an electric dipole do the

exercise in the Exercise Set.

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Y

X

Z

w

I

t

- - - - - - - - -

charge carriers are electrons for copper Right

hand rule ? electrons are deflected down ? bottom

of probe is negative

B

Y

X

Z

w

I

t

B

Saturation of M

retentivity (remanence)

M ? H

coercivity

retentivity (remanence)

Saturation of M

Area enclosed energy dissipated in a cycle in

reversing the magnetic domains

H

G

L

K

I

F

E

C

D

I

J

B

A