Electromagnetism - Skill Bank

8/3/2019 Electromagnetism - Skill Bank

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Electromagnetism

BADI Year 2

John Errington MSc


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Coils, Motors, Generators and

Transformers


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History of developments in electromagnetism

1800: Volta develops the voltaic pile the first electrochemical cell

and battery capable of producing continuous electric current.

1820: Oersted discovers a current flowing in a conductor causes a

magnetic field.

1820: Ampere discovers a force between two wires carryingcurrents.

1831: Faraday showed electricity could be produced by magnetism.

Sets basis for electric motor and generator.

1860 James Clerk Maxwell produces a set of equations that puts the

theory of electromagnetism on a mathematical basis.


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Michael Faraday

The Prime Minister of the day is said to

have asked him (Faraday) what use could

be made of his discoveries. Faradayallegedly responded, "Someday it might be

possible to tax them."


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Michael Faraday

Faraday's first law of electromagnetic induction An electromotive force (voltage) is induced in a conductor when

the magnetic field surrounding it changes.

Faraday's second law of electromagnetic induction The magnitude of the electromotive force is proportional to the

rate of change of the field.

Faraday's third law of electromagnetic induction The sense of the induced electromotive force depends on thedirection of the rate of the change of the field.


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Maxwell

The first equation is really Faraday's Law ofInduction. It states that an induced electric field (E)is created by a changing magnetic flux density(dB/dt) with a polarity that opposes the changingmagnetic field (-). The faster the flux densitychanges, the greater the induced electric field.

In the second equation, Oersted and Ampere andGauss showed that a current (J) would create amagnetic field (H). However, Maxwell took it further

and showed that a magnetic field (H) is created by acurrent (J) and a changing electric field (dD/dt). In the third equation, Coulomb and Gauss showed

that an enclosed electrical charge (p) will create anet electric field (D). In other words, if you were toenclose an electron within a soap bubble, therewould be a net electric field created by that electronwhich is a single negatively charged particle.

The fourth equation, also by Gauss, states that anenclosed magnet will have a net magnetic flux (B)of zero. In other words, every magnet has a north

pole and a south pole, so that if you were to encloseeven a part of a magnet within a soap bubble, thetotal number of magnetic field lines entering thebubble would equal the total number of magneticfield lines exiting the bubble, with a net ofzero. Thus there is no monopole, or particle, whichhas just one magnetic pole without the other. Thiswould be like having a magnet with just a northpole, but no south pole.


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Conductors carrying current

generate a magnetic field.A straight wire carrying a current generates

a magnetic field around the wire.

The direction of the magnetic field obeys thecorkscrew rule

Current flow


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Electromagnets

If wire is wrapped around a plastic former to form a coil it

will generate a magnetic field when a current is passed

through it. The magnetic field strength is proportional to

the number of turns, and the current.

If you grasp the coil with your right hand with your fingers

pointed in the direction of current flow your thumb will point

toward the N pole of the coil.


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Magnetic field of a coil of wire


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Magnetic field around

a single loop

Magnetic field in

an air cored coil

Magnetic field in a coil with

an iron core

The size of the air gap has the biggest

influence on the strength of the magnet


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Basic circuit

+

-

S

Coil of wire

Battery

Switch

Car sidelight bulb limits current

12V 5W


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Inductance of coil

The inductance of an electromagnet is

L (in Henrys) = n2 Q a / l where:

n = no of turns

Q = absolute permeability of corea = area of coil in sq metres

l = length of coil in metres

The absolute permeability of airQ0 is 4T X 107

Relative permeability Qrranges from 1 for air, wood,

aluminium & plastics to 3000 for soft iron, silicon steel and

ferrites. Absolute permeability is just Q0 * Qr


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Permeability Q

Permeability is a magnetic property of a material, and is

often expressed as Q0 x Qr , where

Q0 is a physical constant equal to exactly 4*pi*10-7 Henries /meter and

Qris the relative permeability. Qris equal to 1.0 for free space.

Relative permeability Qrrefers to a material's ability toattract and conduct magnetic lines of flux. The more

conductive a material is to magnetic fields, the higher its

permeability.

Most materials, including copper aluminium and goldhave Qrnear 1.0. The metals that are notable exceptions

are nickel, cobalt, manganese, chromium and iron.

These are called ferro-magnetic materials, and can have

permeabilities as high as 100 or more.


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The strength of the magnetic field can be

increased by putting an iron rod inside the coil.

The shorter you make the air path the stronger the

field will be. (This is why horseshoe, button andpot magnets are more popular than bar magnets.)

The permeability of iron is about 2000 times higher

than that for air, so the more iron and the less air

there is in the path the better.


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Permeability of common materials

Cobalt 68 245

Nickel 1240

Copper 1.0 1.2

Austenitic stainless steel 1.02 max

Martensitic stainless steel 900

Cast iron 100 750

Mild steel 380

Alloy 49 Iron-Nickel High Permeability Alloy 150,000

Special Metals NILOMAG Alloy 77 60,000 - 300,000

Supermalloy 1,000,000


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Horseshoe electromagnet

To keep the air gaps small the coils can be

wound directly onto iron rods. (e.g. nails or

bolts) (Its best to put a layer of paper first to

protect the insulation)

Short air pathfor field


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Relay, contactor, or buzzer

The electromagnet can be used to operate a switch with

one or more sets of contacts this is called a relay

Pivot and

common contact

Armature

Contacts

normally open normally closed


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Vibration annunciatorHere the iron slug slides freely inside the coil. When a current flows

through the coil it pulls the slug into the coil. The contact is broken,

and the current stops. The spring pulls the slug out again and the

process repeats. The frequency of vibration is determined mainly

by the strength of the spring and mass of the slug.

+

-

Coil of wire

Battery Sliding

contact

Car sidelight bulb limits current

12V 5W

SpringIron slug


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Motor


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DC Machines (Motors & Generators)

Permanent magnet

Series coil

Shunt coil

DC motors have the same construction as

DC generators. Apply a current and theshaft will rotate; rotate the shaft and you will

generate a current.


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Generator DC

brush

split ringcommutator

field

time

VThe coil (which may be

many turns) is usually

wound onto a soft iron core


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Generators DC (other types)

A

+

N

S

-

Series

A

+

N

S

- +

A

N

S

-

Shunt Compound

i V i or i

V is const


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AC Generator (Alternator)

N

S

AC output

Electro

magnet

wire

coils

commutators

brush


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TransformerA device for changingAC voltages

Vin

primary

Vout

Ns =12

secondary

soft iron laminated core

Np =6

Vout = Vin * Ns / Np


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Transformers

Can step ac voltages

up and down Provide electrical

isolation between

circuits to protect

patient safety etc. Dont work with dc


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Transformer exampleA mains transformer provides 12 V ac for a 48W halogen

light. The voltage ratio needed is 240 V / 12 V so the turns

ratio needed is 20 (primary) : 1 (secondary) In practice

the transformer would have many more turns than this to

provide the right ratio.Note that as the voltage is stepped down (240 V : 12V) so

the current is stepped up (0.2A to 4 A)

48W = 12V * 4A48W = 240V * 0.2A

L

NIron cored transformer


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Resources

www.gcsescience.com basic electromagnetism etc.

www.magnetsinfo.com/ bumf about metal and ceramic magnets

www.4qd.co.uk/ a thorough introduction to motors of all types

www.psigate.ac.uk Physics gateway

scienceworld.wolfram.com

Electromagnetism - Skill Bank

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