TOPIC 3 ELECTROMAGNETISM

TOPIC 3:
BASIC PRINCIPLE OF ELECTROMAGNETISM

Relationship between current flow and magnetism :

i. Current flow in a single
conductor

ii. Current flow in two conductors
iii. Factors that affect

electromagnetic strength in
terms of

a. Current strength
b. Conductor length
c. Coil length

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Characteristics of magnetic quantities in electromagnet :

i. Electromagnetic induction and the factors that
affect the value of induced current.

ii. Problems related to magnetic quantities in
electromagnet

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Introduction on ELECTROMAGNETISM:

• This chapter is explaining about the relationship between current flows
in conductor, classify factors that affect electromagnetic strength and
understand the characteristics of magnetic quantities in electromagnet.

• The learning outcomes for this chapter are the students should be able
to explain clearly the relationship between current flow and
magnetism.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

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 produced
by the current in the solenoid can be determined using two methods:

i) Right Hand Grip Rule
ii) Maxwell's Screw Law

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Right Hand Grip Rule

Right Hand Grip Rule is a physics principle applied to electric current
passing through a solenoid, resulting in a magnetic field. By wrapping the
right hand around the solenoid, thumb is pointing in the direction of the
magnetic north pole and fingers in the direction of the conventional current.
This rule can also be applied to electricity passing through a straight wire.
The thumb points in the direction of the conventional current from positive to
negative. Meanwhile the fingers points of the magnetic lines of flux.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Maxwell's Screw Law

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

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

ELECTROMAGNETIC EFFECT

• A flow of current through a wire produces a magnetic field in a circular
path around the wire.

• The field patterns of a current flow in a conductor can de determine
using both rules of right hand grip or Maxwell’s Screw.

• Note that, conventional current flow towards or inside the conductor is
marked by cross (X) and current flow away or outside the conductor is
marked as dot ( · ).

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Single Conductor

The direction of the field pattern in going and out going produced by a
current flowing through a single conductor can be determine by applying
both rules as illustrated below.

Current flow:
(a)in going
(b)out going

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Two Conductors

• If the two conductors where the current flow in the same direction, the magnetic flux
pattern will produce around both conductors and combine to create attraction between
them as shown in Figure (a).

• If the current in conductors flow in opposite direction, the field pattern will repulse
each other. The effect is shown in Figure (b).

Two closed current-
carrying conductors flow:
(a) in same direction
(b) in opposite direction

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

ELECTROMAGNETIC STRENGTH
There are 4 factors that affect electromagnetic strength:

1. Number of turns;
The strength of the electromagnet is directly proportional to the number of turn in the coil. By
varying the number of turns in its coil can produce very strong magnetic fields and its
strength.
2. Current strength;
The strength of the electromagnet is directly proportional to the current flowing in the coil.
Greater the current flow through the coil, stronger will be the magnetic fields produced.
3. Length of coil;
The strength of the electromagnet is directly proportional to the length of the coil. By coil up
the wire can increasing the length and increase the force of magnetic field.
4. Types of conductor;
Depend on the nature of the core material. The use of soft of core can produces the strongest
magnetism.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

ELECTROMAGNETIC INDUCTION

• When a conductor is move across a magnetic field, an
electromagnetic force (emf) is produced in conductor.

• This effect is known as electromagnetic induction. The effect of
electromagnetic induction will cause induced current.

• There are two laws of electromagnetic induction:
i. Faraday’s law
ii. Lenz’z Law

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Faraday’s law

• Faraday’s law is a fundamental relationship which comes from Maxwell’s
equations.

• 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 two methods i.e. flux
cuts conductor or conductor cuts flux.

• Flux cuts conductor
Flux cut conductor is when the magnet is move towards the coil
as shown in figure given, a deflection is noted on the
galvanometer showing that a current has been produced in the
coil.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

• Conductor cuts flux
Conductor cut flux is when the conductor is moved through a magnetic field as shown in figure
given. An emf is induced in the conductor and thus a source of emf is created between the ends of
the conductor. This is the simple concept of AC generator. This induced electromagnetic field is
given by

If the conductor moves at the angle θ° to the magnetic field, then

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

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 Ω resistance.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

When a conductor moves in a magnetic field it will have an emf induced in it but
this emf can only produce a current if there is a closed circuit.
Induced emf ;
E = Blv = (1.25)(300/1000)(4)

(a) If the ends of the conductor are open circuit, no current will flow even
though 1.5 V has been induced.

(b) From Ohm’s law

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

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. This
effect is shown in figure below.

Bar magnet move in and move out from a solenoid

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

MAGNETIC QUANTITY CHARACTERISTICS
There are many magnetic quantities in the System International (SI) unit. This chapter is
only going to discuss on magnetomotive force, magnetic field strength, magnetic flux,
flux density, permeability and reluctance.

• Magnetiomotive Force, Fm

Magnetomotive force is a cause of the existence of magnetic flux in a magnetic
circuit. The total flux produced is depends on the number of turn (N) made in the
circuit. It is also proportional to the current (I) passing through the coil. Then, the
magnetomotive force is the product of current and the number of turns.

Fm = NI ampere turn

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Magnetic Field Strength, H

Magnetic field strength or magnetizing force is defined as magnetomotive force, Fm
per meter length of measurement being ampere-turn per meter.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

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

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Magnetic Flux and Flux Density

• Magnetic flux is the amount of magnetic filed produced by a magnetic source.
• The symbol for magnetic flux is phi (Φ). The unit for magnetic flux is the weber, Wb.
• Magnetic flux density is the amount of flux passing through a defined area that is

perpendicular to the direction of flux:

The symbol for magnetic flux density is B.
The unit of magnetic flux density is the
tesla, T, and the unit for area A is m2
where 1 T = 1 Wb/m.

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

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Permeability

• Permeability is the measure of the ability of the material to allow the magnetic field
to exist in it.

• Absolute permeability, µ of a material is the ratio of the flux density to magnetic
field strength.

µ = µ 0µ r

• If the magnetic fields exist in the vacuum, the ratio of the flux density to the magnetic
field strength is a constant called the permeability of free space.

• For air or any other non-magnetic medium, the ratio of magnetic flux density to
magnetic field strength is constant , H/B = a constant.

• The equation for permeability of free space in non-magnetic medium is as shown in
equation below;

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

µr varies with the type of magnetic material. The approximate ranges of relative
permeability for some common magnetic materials are as follows:
Cast iron: µr = 100 – 250
Mild steel: µr = 200 – 800
Cast steel: µr = 300 – 900

Therefore the permeability for all media other
than free space in a magnetic medium or
material is as shown in equation below.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

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.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Reluctance

Reluctance, S is the magnetic resistance of a magnetic circuit to
presence of magnetic flux. The equation for reluctance is as equation
below;

The unit for reluctance is 1/H or H-1 or A/Wb. The
ferromagnetic materials have low reluctance and can be
used as magnetic screens to prevent magnetic fields
affecting materials within the screen.

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Determine the reluctance of a piece of metal
with length 150mm, when the relative
permeability is 4 000. Find the absolute
permeability of the metal. Cross section area,
A 1800 x10−6 2 .

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering

END OF TOPIC 3
Q&A SESSION

Prepared by : Gordon Kechendai Anak Nyanggau
Department of Electrical Engineering