LESSON PLAN

CLASS: X

SUBJECT: PHYSICS

CHAPTER: Magnetic effect of Electric current

AIM:

*To enrich the Concept of Magnetic effect of current

*To impart in-depth knowledge of Magnetic force
*To develop the ideas of solving numerical with Fuse.

GENERAL OBJECTIVES: Given the content (chapter) the students will be able to understand the
Magnetic effect of current. They will be able to learn its applications and natural phenomena in our day-to-
day Generator and Motor.

TEACHING AIDS:

*Text Book: NCERT text book for Physics.

*Reference Book CBSE Exemplar
*PPT, Video, White board, charts, pictures.

WARM UP ACTIVITY:

Students will be asked to explain how generator generates electricity.

DESCRIPTIVE EXPLANATION/PRESENTATION:

Magnetism :The property due to which a substance attracts iron pieces towards it, is called
magnetism.

The substance having property of magnetism, is called magnet.

Types of Magnets

(A) Permanent (Natural & Artificial magnets)

(B) Temporary (Electromagnets)

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(A) Permanent (Natural & Artificial magnets)

Natural magnets

Definition : Pieces of naturally occurring iron ore lode–stone or magnetic or black iron oxide (Fe 2O3),
are called natural magnets.

Properties :These have following two properties.

(i) Attractive property: They attract small iron pieces towards them.

(ii) Directive property: When suspended freely, their ends would point in geographical north–south
direction. For this reason, the suspended piece is called lode stone or leading stone.

Disadvantage : These have following two demerits :

(i) They have irregular shape.

(ii) They are weak.

Artificial Magnets

Description : These magnets are made of hard steel or special alloys. The substances of these
magnets have many small ‘atomic’ magnets. Ordinarily they are all oriented in random directions
(Fig.). Then the substance is unmagnetised.

(a) Unmagnetised

(b) Magnetised.

When such a piece is put in North–South direction and hammered, the atomic magnets align
themselves in the direction of the earth’s field fig. They retain this alignment and the piece becomes
a magnet with North (N) and (S) pole near ends. This magnet with two poles, is called a magnetic
dipole.

Advantage : These have following two merits

(a)They may be given desired regular shape.

(b)They are strong.

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(B) Electromagnet

Electromagnets are based on the magnetic effect of electric current. An electromagnet is usually
prepared by placing a soft iron core in a solenoid, or by winding a large number of turns of an
insulated wire (generally the insulated copper wire) on a cylindrical soft iron core. An electromagnet
shows magnetic properties only as long as the electric current flows through the solenoid. Thus,
electromagnets are temporary magnets.

Difference between electromagnet and permanent magnet

Electromagnet :

Nature of magnetism :Temporary. An electromagnet shows magnetism only as long as current

flows through it.

Polarity :The polarity of an electromagnet can be changed by reversing the direction of the current.

Strength : The strength of an electromagnet can be increased or decreased by increasing or

decreasing the current.

Permanent magnet (or Bar magnet) :

Nature of magnetism :Permanent (or bar) magnets show permanent magnetism.

Polarity :Polarity of a permanent magnet cannot be changed.

Strength : The strength of a permanent magnet cannot be changed.

Poles of a magnet : When ends of a magnet are dipped in iron filings, the filings stick to its ends
only and not to its sides. It means that in magnets, centres of attraction are located near ends only.
These centres of attraction near the ends of a magnet are called poles.

When this magnet is freely suspended, the two ends point in north–south direction. The pole near the
end pointing towards North (north–seeking end) is called North pole. The pole near the end pointing
towards South (south–seeking end) is called South pole.

Interaction between poles : The magnetic poles exert forces on each other. Like pole repel each
other, i.e., one north pole will repel another north pole or unlike poles attract each other i.e., north
pole attract south pole.

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Lines of Magnetic field (Magnetic lines of force)

Definition : A magnetic line of force, is a line straight or curved, in the magnetic field of a magnetic
pole of magnetic dipole, such that the tangent at any point of this line gives the direction of the
magnetic field at that point.

Properties of Magnetic Lines of Force

These have following properties :

They are always nearly normal to the surface ofmagnet at every point.

They start from a north (positive) pole and end at a south (negative) pole.

Two lines of force do not intersect each other.

They tend to contract longitudinally (longitudinal contraction).

They tend to expand laterally (lateral repulsion) so as to exert lateral pressure on neighboring
lines.

(The above two properties are similar to that of a stretched rubber band).

The number of magnetic lines of force passing normally per unit area about a point, gives the
intensity of the magnetic field at the point.

Earth Magnetism : Earth behaves as a huge magnet (or a giant solenoid). The source of this huge
magnetism is given as the molten charged metallic fluid giving rise to a current flowing inside the
core of the earth. This core has a radius of about 3500 km (earth’s radius is 6400 km).

Some Associated Terms

1. South Magnetic Pole (S) : It is near north geographical pole N.

2. North Magnetic Pole (N) : It is near south geographical pole S.

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Arrangement :A straight wire AB is connected to a battery V and key K. The wire is held
horizontally over a magnetic needle.

Working : When key is closed, current flows in the wire in the direction as shown. The needle deflects
one side. When key is taken out and current in wire becomes zero, needle returns back to its initial
position (S – N). This shows that a magnetic field is associated with an electric current.

When direction of current in wire is reversed, direction of deflection of needle is also reversed. If
direction of current be kept same and wire be put under the needle, direction of deflection of needle
again becomes reversed.

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(A) Straight wire :

It means that when the current flows in a straight wire, the magnetic field produced has circular lines
of force surrounding the wire, having their centres at the wireas shown in fig. The plane of circular
lines is perpendicular to the length of the wire. Their direction is marked by arrows.

Fig. (a) Current straight, magnetic field circular

(B) Circular coil :

fig, (b) Current circular, magnetic field straight

It means that when the current flows in a circular wire (coil), the magnetic field produced has straight
lines of force near the centre of the coil, as shown in figure. The parallel lines are in a plane
perpendicular to the plane of the coil. Their direction is marked by the arrows.

Rule : The direction of the magnetic lines of force is related with the direction of the current by the
right hand thumb rule.

The rule states :

Curl the four fingers of the right hand on the palm, keeping the thumb stretched out at right angles.
The thumb is straight and the fingers are circular.

In case 1.thumb represents the direction of the current in the straight wire and curling of fingers
represents the direction of the circular magnetic lines of force (fig.).

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 In case 2, curled fingers represents the direction of the current in circular wire and thumb represents
the direction of the straight magnetic lines of force. (Fig.)

Right hand thumb rule for direction of magnetic field.

(C)Solenoid :

Definition : A solenoid is a straight cylindrical core on which a large number of turns of a insulated
copper wire are wrapped. It is shown in fig.

Solenoid carrying a current and polarity of its face

Force on a Current Carrying Wire due to Magnetic Field :

Introduction : A current carrying conductor produces a magnetic field around it. When it is placed
in a magnetic field, the two magnetic fields interact. A force acts on the conductor.

Expression : It is found by calculation that if the conductor of, length  be carrying a current I lying
inside a magnetic field of intensity B and making an angle  with it, the force acting on it is given by

F = I B sin 

Fleming’s Left–Hand Rule :

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 Fleming’s left–hand rule is used to find out the direction of motion of a current–carrying conductor
when placed in a magnetic field. This rule states as follows.

Stretch out the thumb, the forefinger, and the second (middle) finger of the left hand so that these are
at right angles to each other. If the forefinger gives the direction of the magnetic field (N to S), the
second (middle) finger the direction of current (+ to –), then the thumb gives the direction of the
force acting on the conductor.

Since the conductor will move in the direction of the force acting on it hence the thumb gives the
direction of motion of the conductor. Force on a moving charge

A current–carrying conductor (e.g., a wire) experiences a force when placed in a magnetic field. The
current represents a collection of charged particles in motion. Therefore, each moving charged
particle in a magnetic field will also experience a force, called Lorenz force.

The direction of the force experienced by a positive charge is the same as that on the current and is given
by Fleming’s left-hand rule.

The force, experienced by a current carrying conductor in a magnetic field is given by,

F=BI

If Q is the charge passed through the conductor in time t, we can write

Q
I= t

The above relationships, when combined give,

BQ ℓ
F = t = BQv

where v is the velocity of the charged particle perpendicular to the direction of the field

(A) Main board :

It is provided outside the building under a covered place (varandah or poarch). It contains the meter
(energy–meter) and the main switch.

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 From the street electric pole a thick rubber insulated cord reaches the main board. It contains two
thick copper or aluminium wires, one covered with red and the other covered with black (or brown)

plastic covering.

They from the live line wire (L) and neutral line wire (N) respectively. Live line has a potential of
220V whereas the neutral wire has zero potential (with respect to the earth). They enter the main
board and are connected to the meter.

Wiring ahead is provided by the house owner himself. These wires are also red and black plastic
covered. From the meter the wires enter the main switch. In the main switch, a fuse F is provided in
the live wire.

A third wire is a thick bare wire of copper, called earth wire E. It is connected to an earth connection
which consists of a thick copper plate P buried deep inside the moist earth.

(B) Inside the building :

It is a well known fact that inside the house, connections to all the devices are made in parallel, each
having independent switch and fuse (if necessary). Thus, whenever some fault occurs in circuit of
one particular device in one room, devices in other rooms do not suffer.

As shown in fig. connection to low power devices like bulb B and fan F are made with lines N and L
only, putting switch in line L. For devices of more power and with whom the body remains in
contact (like electric press or refrigerator), we use connections through a 3 pin plug–socket (show in
fig. b) system.

A three pin plug P and three pin socket S are shown in diagram. The three points of the socket are
connected to the three lines as shown in diagram. (fig. b) A fuse F is also introduced to avoid damage
to the appliance.

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 The three pin plug uses a three wire cord which has three plastic wires inside a single rubber
insulating cover. The wire are coloured red, black and green to serve an extension of live, neutral and
earth wires for the appliance. The three wires are connected to the three holes in the socket as shown
in diagram. When the plug is inserted in the socket, proper lines become connected to the appliance.

(C) Function of earth wire :

Due to long use some covered wires inside the appliance may become bare and make contact with
metallic body of the appliance. In such a case the appliance gives a shock if not earthed. The earth
wire keeps the potential of the appliance zero and shock is avoided.

Earthing

Definition : Connecting the metallic body of a high powered electrical appliance (e.g. electric iron,
refrigerator, oven, etc.) to the earth wire of domestic circuit, is called ‘earthing’.

In three pin plug, has three pins forming a triangle. The upper pin is thicker than the two lower pins.
(fig.) The pins are connected inside the body of the plug to the three different coloured wires of a
three–cored, wire cable [Fig.]. The colour code of the wires is green (Earth–E), Red (Live–L), Black
or Brown (Neutral–N).

Benefits : Due to wear and tear with long use, the live wire inside the appliance becomes bare
(uncovered) and touches the body of the applicance. This contact raises the potential of the body to
the huge potential of the live wire. If we operate that appliance with bare foot, we will be getting a
severe shock.

If the appliance is earthed, its body potential remains zero due to contact with the earth. Nothing is
felt when such an appliance is operated.

We may conclude that we save ourselves from severe electric shocks, by earthing our electrical
appliances.

Over–Loading and Short–Circuiting

The current exceeds the limit under two situations.

(i) over–loading (ii) short–circuiting

They are discussed ahead.

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 Over–Loading : When high powered electric appliances like refrigerator, air conditioner and
electric iron are switched on simultaneously, total current through main wire may exceed the
tolerance limit causing damage and fire. This situation causes over–loading.

Short–Circuiting :Due to long use wear and tear or defective insulating material, the live and the
neutral wires may become bare at some points and come in direct contact. Due to zero resistance, a
large current is produced and a huge sparking is caused at the point of contact. It may cause
damages and fire. This situation is called short–circuiting.

Electric Fuse

Introduction : It is a small, simple and cheap but very useful electric appliance used in domestic
electric circuits.

Working : Whenever there is sudden voltage rise in mains or “over–loading” or short–circuiting’

in domestic electric circuits, the current becomes very large and exceeds current tolerance of the electric
appliance, the fuse becomes hot and melts to break the circuit. The appliance is saved from the damage
to be caused by the large current.

CLASSWORK:

Notes will be given.

ASSIGNMENT/HOMEWORK

Book back exercise questions.

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