Electromagnetism

Kevin Gaughan for

Bristol Myers Squibb

Contents

•Magnets and Ferromagnetism

•Domains Theory

•H, B and µ

•The links between electricity and

Magnetism

–Electromagnets

–Induction

•Applications of Electromagnetism

Magnetism

•Permanent Magnetic

Materials have been

known since antiquity.

•They attract “magnetic

materials”–Iron / Nickel

•They can attract or repel

other magnets

•Every permanent magnet

has two poles (North and

South)

•Like Poles repel, Unlike

Poles Attract

Ferromagnetism

•Permanent magnetic materials are said to be ferromagnetic.

•Ferromagnetic materials become magnetised themselvesin the

presence of an external magnetic field and they retain some of

that magnetismwhen the external field is removed. This is the

cause of permanent magnets.

•Ferromagnetic material interact strongly with magnetic fields and the

induced magnetism causes measureable forces of attraction.

•Ferromagnetic materials include Iron (Latin name Ferrous), Cobalt

and Nickel

Other types of Magnetism

•Paramagnetic materialsbecome magnetised in the

presence of a magnetic field but do not retain that

magnetism when the field is removed (Aluminium,

Tungsten).

•Diamagnetic Materials actually resist an external

magnetic field (Carbon, Copper, W

ater).

•NOTE: Paramagnetismand Diamagnetism are much

muchweaker effects than Ferromagnetism. They do not

produce measurable forces. For our purposes these

materials may be considered to be non-magnetic.

Poles and Flux Lines

Flux Lines are Imaginary lines running from North Pole to South Pole of a

magnet. They show the direction of magnet field. We say Magnetic Flux

flows from North Pole to South pole. The stronger the magnetic field the

higher the flux and the more closely spaced the flux lines.

You can plot the path of the flux lines with a little compass orwith iron filings

but the lines are not physically real –they only represent the direction of flow

of magenticflux.

Unlike Poles Attract

•Like Poles Repel each

other and unlike poles

attract each other.

•One method of visualising

this is to imagine that

magnetic flux always tries

to take the path of least

resistance from a North

Pole to South Pole.

•The magenticflux pulls unlike poles

together in order to shorten the path

but it pushes like poles apart as the

fluxes coming from each magnet bash

against one another.

Did you know that the North Pole is

Really a South Pole?

This is a source of much confusion between Geographers and Physicists.

Geographers even refer to the “Magnetic North Pole”when they mean the

south magnetic pole.

How do you think this confusion came about?Im

age from http://129.128.241.207/carismaweb/content/view/69/1/

Distorting the field

A piece of iron or other ferromagnetic material will distort thelocal

magenticfield. The flux will try to take the path of least resistance

(through the iron). This fact is very useful is you want to control the

path of a magenticfield as in a motor or a transformer.

Domains Theory of

Ferromagnetism

•Ferromagnetic materials have

minute magenticdomains

within their atomic structure.

•When the ferromagnetis

unmagnetisedthese domains

are randomly oriented and

there is no net magnetic field.

•When you apply an external

magnetic field the domains

allignand their fields add up –

increasing the total magnetic

field through the ferromagnetic

material.

Domains (cont)

•Permanent magnetsarise when the

domains are “sticky”they stay allignedand

the material retains a net magnetic field

even after the external field is removed.

Hard magnetic materials retain a large

permanent field. Soft magnetic materials

retain less. Hard magnetic materials make

good permanent magnets.

Demagnetising a Permanent

Magnet

•You can demagnetise a permanent

magnet by

–Hammering

–Heating

–Putting in in an external alternating magnetic

field and slowly withdrawing it again.

•Why do the above methods work?

Ferromagnetic Saturation

•When all the domains are lined up a

Ferromagnetis fully magnetised and is said to

be saturated.

•Increasing the external magnetic field above the

point of saturation can not allignany further

domains so the resulting field will only increase

slowly from then on.

•Magnetic saturation in iron alloys occurs at

around 1.5 Tesla and this is the limiting

magnetic flux density for many magnetic

devices.

H B

•Just like electricity magnetism

has a pressure property (like

voltage) and a flow property

(like current)

•The unit of magenticpressure

is called Magnetic Field

Strength H

and is measured

in Amps/Metre

•The unit of magnetic flow is

called Magnetic Flux Density

B and is measured in Tesla

•(Note –sometimes it is useful to consider

total magnetic flux which is B x Cross

sectional area and is measured in Webers)

Magnetic Permeability µ

•Every material has a magenticpermeability

which represents how easily magnetic flux

flows through it. This is called magnetic

permeabiltiyµ.

•The reference permeability is that of a

vacuum and the permeability of a vacuum is

one of the fundamental scientific constants:

metre

Henries/

10

47

0

−×

=π

µ

Relative Permeabilty

•For any other material we say

0.ur

µµ=

•Where µ

risthe relative permeabililtyof the material.

•All non magnetic materials have relative permeabiltiesvery close to 1.0

They behave very like a vacuum.

•Ferromagnetic materials can have relative permeablitiesof 1000 or more.

This means a ferromagnetis 1000 times better at carrying magnetic flux

than a vacuum.

Mathematical Explanation of µ

B = µH

The bigger µthen the more magnetic flux you will

get for a given magnetic field strength.

For non magnetic materials that simple equation

works. Ferromagnetic materials are very non

linear however and the ratio between B and H

changes.

B H curve of a Ferromagnetic

Material

Its actually even a bit more complicated that this because of ….

Air

Saturation

NB NB

The units of B are

multiplied by 10 in

this graph! For

example Carbon

Steel saturates at

around 1.5 Tesla.

Hysterisis

Copied from bhcurve.com–original source unknown.

Some B remains even after H

is reduced to zero. This is

called Remanenceandis

responsible for permanent

magnetism.

Linking Electricity to

Magnetism

Three fundamental principles of

Electromagnetism

•How to create magnetism from electricity

(Fundamental princplebehind electric motors

and transformers)

•How to create electricity from magnetism

(Fundamental principle behind electric

generators and transformers)

•How a current carrying wire experiences a

force in a magnetic field.

(Used in many electric motors)

Electricty-> Magnetism

•Every wire carrying

current generates a

small magnetic field

The picture comes from

http://www.pbs.org/wgbh/nova/magnetic/reve-drives.html

The Right Hand Grip Rule

•If you imagine gripping

the wire in your right

handwiththumb pointing

in the direction of the

current then your fingers

trace the direction of the

magnetic field.

•Notice how there is no

obvious North or South

Pole –the magnetic field

just goes around in a

circle.

Image from http://sciencecity.oupchina.com.hk/npaw/student/glossary/right_hand_grip_rule.htm

The Solenoid: Its just a coil of wire.

•In practise one wire

produces very little

magnetism so we wrap

many turns of wire into a

coil –often called a

solenoid.

•The resulting coil acts like

a bar magnet with North

and South poles.

•The Magnetic Field in the

middle of the coil is given

by

L

IN

H×

=

Electromagnets

•The current creatsthe H but the magnetic flux B also depends on the

permeability of the material in the middle of the solenoid.

•If we wrap a coild

of wire around an iron core (high permeability) we

get a strong controllable magnet. W

e have just made an

ELECTROMAGNET

•Notice how we can reverse the north and south poles by reversingthe

direction of current.

Magnetism to Electricity

•In order to go in the opposite direction you

need a changing magnetic field.

•A changing magnetic field will induce a

Voltagein a coil of wire.

•A changing magnetic field can mean a a

stationary field which is growing stronger

and or weaker (egtransformers).It can

also mean a constant magnetic field which

is moving (egelectric generators)

Electromagnetic Induction with a

moving magnetic field

Inducing a voltage in a coil of wire

•The voltage

generated in any one

wire is small so you

really need a coil of

wire

•The induced voltage

is given by dt

AB

dN

V)

(.

×=

Michael Faradays Laws of

Electromagnetic Induction

Force on a current in a Magnetic

Field

•A wire carrying current in

a magnetic field

experiences a force

which is proportional to

the level of current (I), the

Flux Density (B) and the

length of the wire

exposed to the field (L)

Force =IxBxL(Newtons)

•Many electric machines

utilise this principle to

generate mechanical

force from electricity

Force on a current –another view

•Remember that every current carryignwire

generates its own magnetic field.

•The force that the wire experiences in an

external magnetic field is due to the

interaction of the wires magnetic field with

the external magnetic field.

Left Hand Rule

•The force on a current

caryingwire experiences

a force that is

perpendicular to the

current and perpendicular

to the original magnetic

field.

•The Left Hand Rule

allows you to predict the

direction of the force.

A bit of Scientific History

•We have seen that electrictyand magnetism are closely

linked. In fact it is likely that all magnetic fields are

generated by microscopic currents.

•The crowning achievement of 19thcentury science came

about when James Clerk Maxwell produced a unified

theory of electricity and magnetism. This theory even

allowed him to predict the existence of travelling

electromagnetic waves (electromagnetic radiation).

•Electromagnetic radiation include radio waves, micro

waves, x-rays, Ultra violet, Infra Red and even visible

light. Maxwells

equations allowed him to calculate the

speed of light purely from physical constants.

Applications of Magnetism and

Electromagnetism

•The Magnetic Compass

was hugely important to

historical navigation. A

freely rotating magnetised

needle will allignwith the

Earths magenticfield so

that the North Pole of the

needle (usually coloured

red) will point towards

the geographic North

Pole.

Magnetic Switches

Commonly used in burglar alarms to detect opening windows and

doors.

Electromagnetic Recording on Disk

and Tape

Magnetic Tape is not very popular any more but most hard disks still use

magnetic recording. The resulting magnetised pattern is read using another

coil to detect the magnetic field.

Solenoid Actuators

Solenoid actuators are very commonly used in automation where linear

movement is required. For example electric control of pneumatic and

hydraulic valves.

Relays

•Relays allow a small

electric current in the coil

to turn on and off a much

larger current in the

contacts circuit.

•Relays provide safety

isolation.There is no

direct contact between

coil and contacts so a low

voltage circuit may safely

control a hazardous

volatgecircuit.

Loudspeakers

•Alternating current in the

coil interacts with the

permanetmagnet to

generate an oscillating

force on the coil.

•The oscillating coil

pushes the air back and

forth and generates

sound waves.

•A reversal of this principle

can be used as a

microphone.

Generators and Motors

ELectricmotors and generators use electromagntismto either

produce a force from electricity or to produce electricity by

electromagnetic induction.

Transformers

Transformers work because the primary winding generatsan

alternating magnetic field in the core which then induces a voltage in

the secondary winding.

Leakage and Fringing

•Most electromagnetic devices

use iron or other ferromagnetic

material to force the magentic

flux to go where it is wanted.

•Some useful flux is still lost. In

the diagram we are using an

iron core to try and focus the

flux through an airgap. Some

flux escapes the core

altogether (a) –This is called

leakage. Also some flux

spreads out at the airgap–this

is called fringing.

•In a typical electric machine up

to one quarter of the flux may

be lost through leakage.