SOUTH

Pole

NORTH

Pole

S N

MAGNET MAGNETIC FIELD

S N

Needle Paper

Copper

Cable

Thumb Nail

MAGNETIC CIRCUIT,

ELECTROMAGNETISM AND

ELECTROMAGNETIC

INDUCTION

The end of lesson, students should be ;

Understand magnetism

Understand the composite series magnetic circuit

Understand the electrical and magnetic quantities

Understand hysteresis

Understand electromagnetism

Determine the magnetic field direction.

Understand electromagnetic induction

iNTRoDUctION

MAGNET is

the material that have two poles NORTH and SOUTH

S N

SOUTH

Pole

NORTH

Pole

iNTRoDUctION

MAGNET can be define as

Material that can attract piece of iron or metal

S N

Needle

Thumb Nail

iNTRoDUctION

MATERIAL that ATTRACTED by the MAGNET is known as

MAGNETIC SUBSTANCES

S

Needle

Thumb Nail

iNTRoDUctION

The ABILITY to ATTRACT the MAGNETIC SUBSTANCES is known as

MAGNETISM

S

Needle

Thumb Nail

iNTRoDUctION

MAGNETIC FIELD is

the force around the MAGNET which can attract any MAGNETIC MATERIAL around it.

FLUX MAGNET is the line around the MAGNET bar which form MAGNETIC FIELD.

S N

TYpEs of MAGNET

There are 2 types of MAGNET

PURE MAGNET

MANUFACTURE MAGNET

PURE MAGNET

Known as MAGNET STONE

The stone ORIGINALY have the

NATURAL MAGNETIC

Basically the stone is found in the form

of IRON ORE

MANUFACTURE

MAGNET

There are 2 types of MANUFACTURE MAGNET

PERMANENT MAGNET

TEMPORARY MAGNET

PERMANENT MAGNET

The ABILITY of the MAGNET to kept its MAGNETISM

The basic shape of PERMANENT MAGNET

U shape horseshoe

ROD Cylinder BAR

PERMANENT MAGNET

U shape Horseshoe Rod

Cylinder

Bar

Permanent magnet can be obtained by:

naturally or magnetic induction

( metal rub against natural

magnet)

placing a magnet into the coil and then supplied with a high

electrical current.

PERMANENT MAGNET

Permanent magnet used in small devices such as:

PERMANENT MAGNET

speakers meter compass

TEMPORARY MAGNET

BECOME MAGNET only when

there is CURRENT SUPPLY to the metal

It has magnetic properties when subjected to magnetic force and it will be lost when power is removed.

TEMPORARY MAGNET

Example :

relay

electric bells

Magnetic flux lines have direction and pole.

The direction of movement outside of the magnetic field lines is from north to south.

CHARACTERISTICS OF MAGNETIC

FORCE LINES (FLUX).

The strongest magnetic field are at the magnetic poles .

DIFFERENT POLES ATTRACT each other SAME MAGNETIC POLES will REPEL each

other

S N S N

S N S N

CHARACTERISTICS OF MAGNETIC

FORCE LINES (FLUX).

FLUX form a complete loop and never intersect with each other.

FLUX will try to form a loop as small as possible.

S N

CHARACTERISTICS OF MAGNETIC

FORCE LINES (FLUX).

MAGNETIC QUANTITY CHARACTERISTICS

Magnetic Flux Magnetic flux is the amount of

magnetic field produced by a magnetic source.

The symbol for magnetic flux is .

The unit for magnetic flux is the

weber, Wb.

MAGNETIC QUANTITY CHARACTERISTICS

Magnet Flux density

The symbol for magnetic flux

density is B.

The unit is tesla, T

the unit for area A is m2 where

1 T = 1 Wb/m.

MAGNETIC QUANTITY CHARACTERISTICS

Magnet Flux density

Magnetic flux density is the amount of flux passing through a defined area that is perpendicular to the direction of flux

MAGNETIC QUANTITY CHARACTERISTICS

Magnetic flux density =

area

flux magnetic

A

ΦB Tesla

MAGNETIC QUANTITY CHARACTERISTICS

Example 3

A magnetic pole face has rectangular section having dimensions 200mm by 100mm. If the total flux emerging from the pole is 150Wb, calculate the flux density.

A

ΦB

Area, A

Flux, Φ

B?

MAGNETIC QUANTITY CHARACTERISTICS

Solution 3 Magnetic flux, = 150 Wb = 150 x 10-6 Wb

Cross sectional area, A = 200mm x 100mm = 20 000 x 10-6 m2

Flux density,

= 7.5 mT

6

6

1020000

10150

A

ΦB

MAGNETOMOTIVE FORCE (MMF) The force which creates the magnetic flux in a

magnetic circuit is called magnetomotive force (mmf)

- The mmf is produced when a current passes through a coil of wire. The mmf is the product of the number of turns(N) and current (I) through the coil.

Unit = Ampere Turns (A.T)

Formula , Fm = N x I

MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)

Defined as magnetomotive force, Fm per metre length of measurement being ampere-turn per metre.

Current

l

NI

l

FH m

magnetomotive force

number of turns

average length of magnetic circuit

MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)

Example 1

A current of 500mA is passed through a 600 turn coil wound of a toroid of mean diameter 10cm. Calculate the magnetic field strength.

l

NI

l

FH m

Current, I

Turn, N

Diameter, d

H?

MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)

Solution 1 I = 0.5A

N = 600

l = x 10 x 10-2m

mATH

H

metreampereturnl

NIH

/81.954

3142.0

5.0600

/

MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)

Example 2 An iron ring has a cross-sectional area of 400 mm2. The coil resistance is 474 Ω and the supply voltage is 240 V and a mean diameter of 25 cm. it is wound with 500 turns. Calculate the magnetic field strength, H

MAGNETIC FIELD STRENGTH,H (MAGNETISING FORCE)

Solution 2 I = V/ R = 240 / 474 = 0.506 A

l = π D = π (25 x10-2) = 0.7854 m

H=

H=

H= 322.13 AT/m

l

NI

7854.0

506.0500

PERMEABILITY

For air, or any other non-magnetic medium, the ratio of magnetic flux density to magnetic field strength is constant ,

This constant is called the permeability of free space and is equal to 4 x 10-7 H/m.

H

B

µ0

PERMEABILITY

For any other non-magnetic medium, the ratio

For all media other than free space

r

rH

B0

PERMEABILITY

r is the relative permeability and is defined as

r varies with the type of magnetic material.

in vacuumdensity flux

materialin density flux r

PERMEABILITY

r for a vacuum is 1 is called the absolute permeability.

The approximate range of values of

relative permeability r for some common magnetic materials are :

Cast iron r = 100 – 250 Mild steel r = 200 – 800 Cast steel r = 300 – 900

PERMEABILITY

Example 4 A flux density of 1.2 T is produced in a piece of cast steel by a magnetizing force of 1250 A/m. Find the relative permeability of the steel under these conditions.

HB r0

Flux density,

B

H

µr?

PERMEABILITY

Solution 4

HB r0

764

)1250)(104(

2.17

0

H

Br

RELUCTANCE

Reluctance,S is the magnetic resistance of a magnetic circuit to presence of magnetic flux.

Reluctance, The unit for reluctance is 1/H or H-1 or A-T/Wb

AAHBBA

HlFS

r

m

0)/(

RELUCTANCE

Example 5

Determine the reluctance of a piece of metal of length 150mm and cross sectional area is 1800mm2when the relative permeability is 4 000. Find also the absolute permeability of the metal.

S?

Length, l µr µ?

RELUCTANCE

Solution 5 Reluctance, = = 16 579 H-1

Absolute permeability,

=

AS

r0

)101800)(4000)(104(

1015067

3

r 0)4000)(104( 7

= 5.027 x 10-3 H/m

ELECTROMAGNET

Is a magnetic iron core produced when the current flowing through the coil.

Thus, the magnetic field can be produced when there is a current flow through a conductor.

The direction of the magnetic field can be determined using the method:

1. Right Hand Grip Rules 2. Maxwell's screw Law. 3. Compass Three rules may be used to indicate the

direction of the current and the flux produced by current carrying conductor.

Right Hand Grip Rule

is a physics principle applied to electric current passing through a solenoid, resulting in a magnetic field.

Right Hand Grip Rule

When you wrap your right hand around the solenoid

your fingers in

the direction of

the conventional

current

your thumb points

in the direction of

the magnetic

north pole

Right Hand Grip Rule It can also be applied to electricity passing through a straight wire

the thumb points in the direction of the conventional current (from +ve to -ve)

the fingers point in the direction of the magnetic lines of flux.

Another way to determine the direction of the flux and current in a conductor is to use Maxwell's screw rule.

MAXWELL’S SCREW LAW

a right-handed screw is turned so that it moves forward in the same direction as the current, its direction of rotation will give the direction of the magnetic field.

MAXWELL’S SCREW

LAW

Electromagnetic Effect Direction of Current

going INside

Solenoid

Direction of Magnetic

Flux around Solenoid

Direction of Current

going OUTside

Solenoid

Direction of Magnetic

Flux around Solenoid

Right Hand Grip

Rule

Electromagnetic Effect Direction of Current

going INside

Solenoid

Direction of Magnetic

Flux around Solenoid

Direction of Current

going OUTside

Solenoid

Direction of Magnetic

Flux around Solenoid

Same Direction Different Direction

Maxwell Screw Law

Electromagnetic Effect

Factors that influence the strength of the magnetic field of a solenoid

The number of turns

The value of current flow

Types of conductors to produce coil

The thickness of the conductor

ELECTROMAGNETIC INDUCTION

Definition : When a conductor is moved across a magnetic field so as to cut through the flux, an electromagnetic force (emf) is produced in the conductor.

This effect is known as electromagnetic induction.

The effect of electromagnetic induction will cause induced current.

ELECTROMAGNETIC INDUCTION

2 laws of electromagnetic induction:

i. Faraday’s law

ii.Lenz’z Law

Faraday’s law It is a relative movement of the magnetic

flux and the conductor then causes an emf and thus the current to be induced in the conductor.

Induced emf on the conductor could be produced by 2 methods

flux cuts conductor or

conductor cuts flux.

Faraday’s law

Faraday’s First Law : Flux cuts conductor

When the magnet is moved towards the coil, a deflection is noted on the galvanometer showing that a current has been produced in the coil.

Faraday’s law

Faraday’s Second Law :Conductor cuts flux

When the conductor is moved through a magnetic field . An emf is induced in the conductor and thus a source of emf is created between the ends of the conductor.

Faraday’s law

This induced electromagnetic field is given by E = Blv volts

B =flux density, T l =length of the conductor in the magnetic

field, m v =conductor velocity, m/s

If the conductor moves at the angle to

the magnetic field, then E = Blv sin volts

Faraday’s law

Example A conductor 300mm long moves at a

uniform speed of 4m/s at right-angles to a uniform magnetic field of flux density 1.25T.

Determine the current flowing in the conductor when :

a. its ends are open-circuited b. its ends are connected to a load of 20

resistance.

Faraday’s law

Solution a. If the ends of the conductor are open

circuited no current will flow .

Faraday’s law Solution b. E.m.f. can only produce a current if there is a

closed circuit. When a conductor moves in a magnetic field it will have an e.m.f. induced.

Induced e.m.f. , E = Blv =(1.25)(0.3)(4) = 1.5 v From Ohm’s law

mAI

I

R

EI

75

20

5.1

Lenz’z law

The direction of an induced emf is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that emf

Formula

AS

r0

l

NI

l

FH m

A

ΦB

MAGNETIC FIELD STRENGTH

RELUCTANCE

MAGNETIC FLUX DENSITY

PERMEABILITY H

Br0

AAHBBA

HlFS

r

m

0)/(

MAGNETOMOTIVE FORCE (MMF), Fm = N x I

Composite magnetic circuit

A series magnetic circuit that has parts of

different dimensions and material is called

composite magnetic circuit.

Each part will have its own reluctance. Total

reluctance is equal to the sum of reluctances

of individual parts.

Total reluctance

Comparison between magnetic and electric circuit

Similarities & dissimilarities between magnetic circuit and electric circuit

Similarities & dissimilarities between magnetic circuit and electric circuit

Hysterisis and hysterisis loss

Figure 7.6