Class 10TH Physics Notes Chapter 15 Electromagnetism

Class 10TH Physics Notes Chapter 15 Electromagnetism

Class 10TH Physics Notes Chapter 15 Electromagnetism. Electromagnetism is a branch of physics that involves the study of electromagnetic force, a type of physical interaction that occurs between electrically charged particles. Electromagnetic force is carried by electromagnetic fields made up of electric fields and magnetic fields, and is responsible for electromagnetic radiation such as light. It is one of the four fundamental interactions in nature, along with the strong interaction, the weak interaction, and gravitation. At high energy, the weak force and the electromagnetic force unify as a single electroweak force.Class 10TH Physics Notes Chapter 15 Electromagnetism.

Class 10TH Physics Notes Chapter 15 Electromagnetism
Class 10TH Physics Notes Chapter 15 Electromagnetism

Class 10TH Physics Notes Chapter 15 Electromagnetism. Electromagnetic phenomena are defined in terms of the electromagnetic force, sometimes called the Lorentz force, which includes both electricity and magnetism as different manifestations of the same phenomenon. The electromagnetic force plays an important role in determining the internal properties of most objects encountered in daily life. The electromagnetic attraction between atomic nuclei and their orbital electrons holds atoms together.Class 10TH Physics Notes Chapter 15 Electromagnetism.

Class 10TH Physics Notes Chapter 15 Electromagnetism
Class 10TH Physics Notes Chapter 15 Electromagnetism

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Chapter :15

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Major Concepts

• Magnetic effect of steady current

• Direction of magnetic field

• Fleming’s left hand rule

• D.0 motor

• Electromagnetic induction

• Faraday’s law of electromagnetic induction

• Len’s law

• A.0 Generator

• Mutual induction

• Transformer

• Relay circuit

• Solution of problems

Comprehensive Question

Q1. What is the magnet? Write properties of a magnet. Also define magnetism.

Ans. Magnetism and Electromagnetism:


“An object which attracts small pieces of iron and pointing towards north south direction when freely suspended is known as magnet”.

The end of a magnet pointing towards north is called North Pole while the end pointing towards the south is known is South pole. Properties of a Magnet:-

Following are the some important properties of a magnet.

i. A magnet attracts small pieces of iron.

ii. Like poles of two magnets repel while unlike poles of two magnets repel each other.

iii. A freely suspended magnet always points in the north south direction.

iv. The magnetism of a magnet concentrated in its poles.

v. The two poles of a magnet cannot be separated from each other. If a magnet is broken in to two pieces, then two new magnets are obtained.

vi. There is a magnetic field around a magnet. A test magnet or a current carrying conductor experiences force in a magnetic field.


The study of the properties associated with a magnet is called magnetism.”

Magnetism derives its name from magnesia (turkey), where it was found in the form of lumps of certain iron ore. These lumps have the property of attracting small pieces of iron. It was also known a lode stone.

Q2. Define compass needle, magnetic field and electromagnetism?

Ans. Compass Needle:

“A magnetized iron needle which is so pivoted that it can turn freely in a horizontal plane is called a magnetic compass needle”. This needle comes to rest along the north south direction.


A compass needle is often used to investigate a magnetic field. It can also be used to detect a current in a conductor.

Magnetic Field:

“The space around a magnet in which its magnetic effect can be felt by another magnet is called magnetic field”. Electromagnetism


In general any region in which magnetic effect is felt is a called magnetic field. Magnetic field is represented by the magnetic lines of force which can be traced by means of a small test magnet such as compass needle.

Intensity and Direction:

The intensity and direction of magnetic field at any field point is determined by the force which the field exerts on a test magnet placed at that point.

The magnetic field of a bar magnet is represented by lines of force. The magnet field lines are directed from North Pole to South Pole.


“A magnet formed due to flow of electric current in a conducting coil is called electromagnet”.

The study of the properties associated with an electromagnet is known as electromagnetism.

As we know that the electric and magnetic effects are produced by electric charge.

• The static charge produces electrostatic effects.

• While a moving charge that is current produces both electric and magnetic effect (electric and magnetic fields are produces around it).

Q3. Discuss magnetic field due to current .Describe an experiment to show that the steady current carrying wire produce a magnet field around. What is the direction of this magnetic field?

Ans. Magnetic Field due to current:-

A magnetic field produced around a conductor due to flow of electric current in it, is known as magnetic field due to current.

In 182, it was Hans Forested who first of all determined that a magnetic field is produced around a wire carrying current “I”. He also got the following conclusion.

i. The magnetic field is strong near the conductor and become weaker and weaker as we move away from the conductor.

ii. The magnetic field exists as long as the current is flowing through the conductor.

iii. The magnetic lines of fore are circular around the conductor. Magnetic Field of a straight current carrying wire:

“A magnetic field produced around a conductor due to flow of electric current in it, is known as magnetic field due to current”.


The magnetic field due to current demonstrated by considering a straight conductor passed vertically through a horizontal e current I is allow to pass through the themselves in concentric circles around agnatic field by taping the cad board tic field is produced around a current led lines are circular around the current d due to current can be determined by card board. The iron filings are sp conducting wire. Inkle over the card board around the


When the switch is on and Conductor, the iron filings arrange t the wire in the direction of the m gently. This shows that the magnet carrying conductor. The magnetic fi carrying conductor as shown;


The direction of magnetic fie

If “Right Hand Rule”. If the current flowing upward in the wire, the magnetic field will be anticlockwise. But if the current is flowing downward in the wire. the magnetic field will be clockwise.

Q4. What are magnetic lines of force? Represent the field pattern of a bar magnet by lines of force.

Ans. Magnetic Field lines:

“The path along which an isolated north pole of a magnet moves in a magnetic field is called a magnetic field line of force”.


The magnetic lines of force are curve path originating from the North Pole and terminating at the South Pole outside a magnet. Inside the magnet they start from South Pole and ends at North Pole.

In this way the magnetic lines form a close path. The magnetic field lines of a bar magnet are shown; the magnetic field strength at a certain point can be determined by using the density of the magnetic field lines. The density of magnetic lines of force near the poles of a magnet is greater. The magnetic field lines don’t intersect each other.

Tracing of magnetic lines:

We can trace the magnetic lines of force by placing compass needles. When a compass needle is moved around a current carrying conductor, its direction changes from point to point. If we put dots in front of the ends of a compass needle, we shall get magnetic field lines. The lines of force are anticlockwise when current is flowing upward and vice versa.

Q5. State the right hand rule of electromagnetism. Illustrate by a diagram.

Ans.Right Hand Rule:


According to Right Hand Rule;

“If the current carrying wire is grasped by the right hand with the thumb in the direction of the current, then the fingers encircle the wire is the direction of magnetic field”.


As we know that when current is flowing through a conductor, magnetic field is produced around it. Right hand rule relates the direction of the magnetic field with the direction of the current flowing in the wire. If the current is flowing upward in the wire then the magnetic line of force are anticlockwise according to the right •hand rule. But when the current is flowing downward then the magnetic line of force is clock Wise.

Q6 Write a note on atomic magnetism?

Ans. Atomic Magnetism:-

It was first of all amperes who gave the idea that all magnetic effects may be due to circulating currents (moving charges).

IM9covenof the internal structure of the atom after discovery the internal structure of atom the amperes view appear s to the basically correct. As atom is composed of three kinds of basics particles i.e. electron, Proton and neutrons. Since protons and neutrons resides inside the mcleus while the negatively charged electrons are revolving around the nucleus in different orbits. Moreover the electrons also have spinning motion.

Magnetic field Due to motion of Electrons:-

The rotation and the spin motion of electron both given rise to a magnetic field. Since there are a number of electrons in an atom, their spin may be so oriented as to cancel the magnetic effects mutually or strengthen the effect of each other.

An atom in which there is a resultant magnetic field, behaves like a fitly magnet, and is known as magnetic dipoles.

Q7 Explain the force on a current carrying wire in a magnetic field?

An RT e Cinram alcartrin ri iron i.e. regarded as a drift of charge (electrons) in The same direction. Each electron moves with a drift velocity V in a Conductor. When this Conductor is placed in a magnetic field of strength B, every electron is acted upon by a magnetic force Fm. Since the Electrons are confined inside the wire. Therefore, the magnetic force Experienced by a current carrying conductor in a magnetic field is the Resultant of the forces acting on the charges constituting the current. The magnetic force F,-,, on a wire of length L carrying current places perpendicular to the magnetic field of intensity B is given by;

Fm = ‘LBO

If the conductor makes an angleflwith the magnetic field then the above equation can be written as;

Fm= ILB sin

Demonstration of magnetic Force:

The magnetic force on a current carrying wire can be demonstrated experimentally, as shown a conducting wire is placed between the poles of U-shaped magnet, while its ends are connected to the terminals of a battery through switch s.

When the current is allowed to pass through the wire, the current carrying wire is immediately ptished to a side. Increasing the current in the wire increases the push. Reversing the current in the wire, the direction of the push is also reversed. Similarly exchanging the North and South poles of the magnet, the magnetic force on the wire is also reversed.

Q8 State Fleming left hand rules frog, I and 1?

Ans. Fleming left hand rules.,


This rule states that; “If we stretch the thumb, fore finger and the middle finger of our left hand in such a way that they are mutually perpendicular. If we point the fore finger in the direction of magnetic fields “It the middle finger in the direction of the current “I”. Then the thumb will point in the direction of the magnetic force” that move the current carrying conductor.”

Q9 Illustrate by a diagram, the magnetic field of a current carrying solenoid. Identify the polarity of a magnet so formed. Discuss the intensity and the direction of the field

Ans. Magnetic field due to current in a solenoid:

A solenoid is a closely wounded cylindrical coil of insulated wire. It is constructed when a conducting wire is wounded on a certain cylindrical support as shown;

Field (North)


114 $0$0, Air YIN

And. Are-.


When current is allowed to field produce is uniform and strong weak outside the solenoid. The file sum of the field due to current in ea Using right hand rules on ea inside the coil the magnetic lines shown in figure.

The magnetic field strength given by flow through a solenoid the magnetic in the middle of the turn and negligibly d due to solenoid current is in fact the ch circular turn of the turns of the solenoid, we can find of force are in the same direction as at a certain point inside a solenoid is

Intensity of the field:

The intensity of the field with in the solenoid increases with increasing the current in a solenoid. The intensity also increases with the increase in number of turn of the solenoid.

Identification of polarity:

The polarity of the magnet depends upon the direction of the current passing through the solenoid. The polarity can be determined by means of a compass needle. It can also be determined by right hand rules, stated below

Grasp the solenoid in a right hand by circulating the finger in the direction of the current. The stretched thumb will be towards the north pole of the solenoid.

Q10. Describe the phenomena of electromagnetic induction. List the factors effecting electromagnetic induction.

Ans. “The current produced by the relative motion between the loop of wire and the magnetic field is called induced current and the emf thus produced is known as induced emf’.

i. Faraday observed that a galvanometer connected to the end of a coil deflects when a magnet is moved towards or away from the coil. This deflection of the galvanometer shows that current is set up in the coil. ii. In the figure; when north pole is moved towards the coil the current flows in one direction. But when the magnet is moved away from the coil, the current flows in the opposite direction. Now if South Pole the coil, the current is in the opposite direction. Now if South Pole the coil, the current is in the away from the coil, the current flows in opposite direction.

iii. If the magnet is held stationary and coil is move towards the magnet, the current flow is one direction. If the coil is moved away from the stationary magnet, the current flows in the other direction.

iv. When magnet and the coil are held stationary with respect to each other. The galvanometer shows no deflection.

v. Thus induced current and induced emf are produced by the relative motion of the magnet and the current carrying loop.


If “I” is the induced current produced and “R” is the resistance of the loop when magnet is pushed towards it, then the induced emf denote by “s”is given by,

E = IR


a) The induced current I depends upon the strength of the magnetic field provided by the magnet.

b) The value of the induced emf depends upon the relative motion of the loop.

Q11. State faraday law of electromagnet induction?

Ans: Faraday’s Law of electromagnetic induction:


This law states that,

“The emf induced in a close circuit is equal to the rate of change of magnetic flux through it”.



E = —


For N-loops;


E = —N



Q12. Explain the torque on curer t carrying coil in magnetic

Ans. Turning effect on a current c

When a current carrying coil field it experiences a newt force, the loop of wires. Consider a rectangular sides of length a and b carrying a c magnetic field B directed parallel t figure. Field?

Carrying coil in a magnetic field:

Or loop is placed in uniform magnetic is force can exert torque on a coil or lar coil having four [1, 2, 3 and 4] zurrent “I” in the presence of a uniform o the plane of the loop as shown in Side 3 view


No magnetic forces act on si des 1 and 3 because these Wres are parallel to the field8=0°; the lengi: h of sides 1 and 3 is b; therefore magnetic force F1 and F3 is,

F1 = F2 = I (L x B)

F1 = F2 = ILB Sine

19= 0

F1 = F2 = ILB Sine (Sino = 0)

Since sin0°=0

Therefore, F1 = F3 =0

However, maximum magnetic force acts on sides2 and 4. Because these sides are perpendicular to the field8 =90°, the length of sides 2 and 4 is a, therefore magnetic force is,

F2 = F4 = I (L x B) Sine

F2 = F4 = I (L x B) Sine (… Sine = Sin90° = 1)

F2 = F4 = BIL Sin90° (Sin90° = 1)

Therefore, F2 = F4 = BIL ————- (2)

We see the view shown in figure (b) the two magnetic force F2 and F4point in opposite directions having same magnitude (Bia) such that they form a couple . If the loop is pivoted so that it can rotate about point 0, these two forces produce a torque to rotate the loop (in this case clockwise)

Q13. Explain the working of a D.0 motor?

Ans. D.0 motor:

“A device which converts an electrical energy in to mechanical energy is called D.0 motor”.


When a current carrying coil is placed in a magnetic field, a torque acts on it which caused the coil to rotate.


An electric motor consists of a coil of N- turns wound on an iron core which is known as armature. The armature can rotate about an axis perpendicular to the magnetic field provided by an electromagnet.

The terminals of the coil are connected to split rings (Commentators). Brushes ID, and b2 supplying current from the battery or D.0 source. The split ring system reverses the sense in which the current is supplied to the rotating coil in every half revolution.

Working: When current I is passed through the coil, the coil is acted upon by the torque until the plane of the coil is perpendicular to the field where torque becomes zero. At this instant the split ring system reverses the direction of the current and the armature continues to rotate. The rotating armature provides mechanical power.


Electric motor is used as the basic device for converting electrical energy in to mechanical energy. Its applications are fan, water pumps, electric drill and washing machines.

Electric motors are also used in communication devices such as telephone, television, radar and tape recorders.

Q14.Sketch and describe the construction and working of moving coil loud speaker?

Ans. Moving Coil Loud speaker:


The basic principle of moving coil loud speaker is “a current carrying conductor experiences a force in a magnetic field.”


Moving coil loud speaker consist of a permanent magnet with a central cylindrical pole and a surrounding ring pole. This arrangement creates a strong magnetic field in the gap between the poles. It has a short cylinder with a coil wounded on it, which move backward and forward. Also a paper cone in attached to the coil which produced sound when moves backward and forward Casing Ring pole.


When an alternating current from a radio or a record player passes through the coil. It forces the paper cone to move in or out, depending on the current direction. This motion of the paper cone thus sets up sound waves in the surrounding air of the same frequency as that of the alternating current.

Q15. Explain the direction on induced emf and show its relation to conservation of energy?

Ans. Lenz’s Law:

The direction induce emf can be determined by using lens’s law. Statement:

This law states that

“The direction of an induced emf will be such that it opposes the cause producing it”.

Direction of induced emf and conservation of energy:

Whenever there is a change of magnetic flux, there will be induced emf and current will flow in such a direction so as to oppose the cause producing it. The induced current around a loop produces its own magnetic field. This field may be weak Compared with the external magnetic field. It cannot prevent the magnetic flux through the loop from changing, but its directions are always such that it “tries” to prevent the flux from changing. Consider pushing the bar magnet’s North pole in to the loop. it causes the magnetic field to increase in the upward direction. To oppose the change, the loop itself needs to generate the downward- pointing magnetic field (acting as a magnet with its North Pole at the bottom which repels the North Pole of the bar magnet moving towards it) as shown in figure (a).

Magnet pushed into the loop

  1. Magnet pushed out of the loop


Magnet pushed Into the loop


M t pushed

Out of tn. loop


Direction of induced current:

The induced magnetic field at the center of the loop will point downward (using right hand rule II) only if the current is clockwise (cw) as seen from above. Now suppose the north pole of bar magnet is pulled away from the loop, as shown in figure (b). There is a upward magnetic field trough the loop, but the magnetic field is decreasing as the magnet is moving away. Thus, the induced magnetic field of the loop opposes this decrease, therefore, the induced field needs to point in the upward direction, and the induced current is counter clockwise (CCW) as seen from above.

Similar effects can also be observed with the south pole of a magnet pushed in and out of the loop figure (c, d),it is seen that the induced current setup a magnetic field of its own. From the above discussion, it is clear that the motion of magnet is always opposed by the magnetic field generated from induced current. The mechanical energy spent in overcoming this opposition is converted in to electrical energy.

If the induced current were in the opposite direction, the magnetic force would have accelerated it with no external energy source, even though electric energy would have been dissipated in the circuit. This would have been a clear violation of the law of conservation of energy, which doesn’t happen in nature. 189 Electromagnetism construction and working of A.0

Cheinical energy in to electrical energy on the principle of electromagnetic educed by changing the magnetic flux

Insists of a closely wound Ictangular rotate about an axis 00′, per lenticular connected to slip rings R and R’. The axis of the coil and rotates with it. The he rings and connected the coil to the field of intensityllis applied by gnet. The coil is wound on an iron core


When the coil ABCD is rotated by any means the magnetic flux linked with it changes and an induced emf is produced in it. Now to investigate the direction and magnitude of the produced emf when the coil rotates, consider the figure; in figure (a) the plane of coil is perpendicular to the magnetic field. The sides AB and CD are moving parallel to the magnetic field hence induced emf is zero.

ii. As the coil notates, the sides AB and CD cross the field and their velocity perpendicular component increases that becomes maximum when the coil attains the position (b). So the induced emf increases from zero to maximum. I.e. emf=LVB.

iii. When the coil further rotates the sides AB moves down while the side CD moves upward. So their velocity components perpendicular to the field decreases and becomes zero when the coil gains the position (c). Thus the induced emf decreases from maximum to zero in the same direction i.e. emf= 0

iv. When the coil further rotates the side CD moves downward, the induced emf increased from zero to maximum in opposite direction, when the coil attains the position (d). Emf = -LVB

v. But when the coil rotates from position d to position e, the induced emf reduces to zero.

Thus it is clear that at the beginning of rotation, the emf is zero and reaches to maximum value in +vedirectionduring1/2 rotations.

After 1/2 rotations the emf again reduces to zero in the same direction. After % rotations the induced emf increases to maximum but in -ve direction.

The emf reduced to zero after one rotation. In this way the rotation of the coil in a magnetic field produces an alternating induced emf.


The uses of A.0 generator are countless. A.0 generators are used to convert kinetic energy of water falls to electrical energy. Huge A.0 generators are installed at Turbella, Mangle, and Worsak etc.

In petrol and diesel operated A.C. generators, that convert chemical energy is converted in to electrical energy.

Q17. Describe the phenomena of mutual induction?

Ans. Mutual induction:

“The phenomenon in which induced emf is produced in secondary he primary coil is known as mutual coil when current is changing tl induction, denoted by M” Mathematically; Es At

M Alp/At


To describe mutual induction consider the coils placed near each



The coil in the battery circuit is called primary coil while coil in a galvanometer circuit is known as secondary coil. The magnetic flux induced due to flow of current in the primary coil is linked with secondary coil. When a current in the primary coil is change by varying the resistance of the rheostat, the magnetic flux through the secondary coil is also changed. So according to Faradays law induced emf is produced in the secondary coil.


The unit of mutual induction is henry [H] .Mutual induction of two coil will be on a henry [H] if the current is changing at the rate of one ampere per second in the primary coil and causes an induce emf of one volt in the secondary coil .

Lvt Is





Q 18: Describe the phenomenon of self-induction?


The emf induced in a coil when magnetic flux linked with it is changed by changing the current flowing through it is known as self-induced emf and the phenomenon is called self-induction. It is denoted by L.


Consider a coil connected shown in figure; o a battery through a rheostat R as When a key is closed current flowing through the coil produces magnetic flux which links with the coil itself.

Now if the current through the coil is changed by rheostat, the magnetic flux linked with the coil is also changed. Thus according to faraday law induced emf is produced in the coil.

Q19. What is transformer? Explain n its construction, working principle and types.

Ans. Transformer:

“Transformer is an electrical device which is used to increase or decrease the value of alternating voltage”.


A transformer has two coils, electrically insulated from each other, but wound around the same iron core. One coil is called the primary coil. The other coil is called the secondary coil. Number of turns on the primary and the Secondary coils are represented by Np and Ns respectively. Cted to a source of A.G. Voltage, the g magnetic field, which is carried I. In the secondary coil, the changing ct is called mutual inductance Ondary coil is called the secondary ry voltage Vp. the secondary voltage ber of turns on the seconds coil to ectromagnetic induction.

Working principle:

When the primary coil is comic changing current creates a changer through the core to the secondary coi field induces a varying e.m.f. This effect Voltage and number of turns of the e.m.f induced in the sec voltage Vs is proportional to the prime also depends on the ratio of the numb the number of turns on the prima expressions.

According to Faraday law of

Es = -N 6t


ii. Step-down transformer:

If Np > N5, then the secondary voltage is smaller than the primary voltage, then transformer is called a step-down transformer.

Step Down Transformer

Input Voltage

Iron Core Output


Secondary %


%Primary Coil (Input)

An ideal transformer:

In an ideal transformer, the electric power delivered to the secondary circuit equals the power supplied to the primary circuit. An ideal transformer dissipates no power itself and for such a transformer we can write:

Pp. = Ps

Vp1p = V515

Np: Ns I


= P

Uses of transformer:

Transformer are used to increase or decrease AC voltages. Usage of transformers is common because they change voltages with relatively little loss of energy. In fact, many of the devices in our homes, such as game systems, printers and stereos have transformers inside their casings or as part of their connecting cords

Q20. How high voltage transmissions reach from power station to consumer? (OR)

Why alternating voltage i station? S stepped up at the generating station?

Ans. Whv heat dissipated in transient Electric power is usually generated at places where it is the power is transmitted over long consumed distances at high voltage the loss of energy in the form onto minimize. Dissipated in the transmission cabl heat during transmission. As heat reducing the current trough the cable of resistance R is I2RT.Hence by dissipation can also be reduced. Sable, power loss in the form of heat at generating station. The alternating voltage is stepped up how voltages are stepped down?

High voltages are transmitted tt) the main sub- station. This voltage is stepped down and is transmitted Tc) the switching transformer station or the city sub-station. At the city sub) the switching transformer station or 220v and supplied to the consumer station it is further stepped down to transmission is shown in fig.

A Schematic diagram of high voltage

I Examples I

Example 15.1: Can the magnetic force on a current- carrying wire be large enough to suspend the wire against gravity? Consider a wire having mass of 6.24×104 kg and length 3.5cm placed perpendicular to a 2.4 T magnetic field. How large a current must the wire carry in order to be suspended against gravity?


Mass ‘m’=6.24x 104 kg

Length 112=3.5cm=0.035m

Magnetic field ‘B’=2.4T

Acceleration due to gravity ‘g’=9.8m/s2


Current ‘N.?


F8= W Here FB=ILB and W=mg

Therefore, ILB =mg or l=”9


Putting values,

I= 6.24×10-4kgx9.8 ms-2

0.035m x2.4T

I =6.24×10-4k4x9.8141-2

0.035n4 x2.4k4st2


I = 0.073A =73mA —— —-Answer


The current passes in the wire is 73mA.


If the mutual inductance of two coils is 3.5mH and the current through primary coil changes from OAto 10A in 0.03s, how much emf is induced in the secondary coil?


Mutual inductance ‘m’=3.5mH = 3.5x 10-3H

Initial current i=OA

Final current i = 10A

Time taken for change ‘ile=0.03s


Induced emf in secondary coil ‘E.51=?


The emf induced in the secondary coil is given by the equation


E =771 LIP


F —1 i

Es = —7tt X


Putting values

Es = -3.5 X 10-



3 Vs X 10A a 0.03sec

Es = -3.5 x 10-

Es = -1.17V Answer


Induced emf in secondary coil ‘Es’ is -1.17V.

Example 15.3:

A portable x-ray unit has a step -up transformer, the 220v input of which is transformed to the 100kv output needed the x-ray tube. The primary has 50 loops, what is the number of loops in the secondary?


Primary voltage VP’220V

Secondary voltage ‘W=1 00kv =105v

Number of turns in the primary ‘Np’=50


Number of turns in the secondary ‘N,’=?


By transformer equation

Vs Ns



Vs— Ns



N = —vs N s v

Putting values

22100V5 „

NS- X SO turns

– V


Ac= 2.27 x 104 turns Answer


Number of turns in the secondary ‘N,’ 2.27 x 104turns.


Assignment 15.1:

A wire carrying a steady (dc) 30A current has a length of 0.12m between the pole faces of a magnet. The wire is at an angle 0= 60° to the field. The magnetic field is approximately uniform at 0.90T. Determine the magnitude of the force on the wire.


Current =i=30A

Length of wire =L=0.12m

Angle =0=600

Strength of magnetic field =B=0.90T


Magnitude of magnetic force=Fm=? Calculations:

We know that;


Putting the values;

Fm= (30) (0.12)(30) sin 60°

Fm = 2.8N Answer



Magnitude of magnetic force is 2.8N.

Assignment 15.2:

If the current through the primary coil changes from OA to -10A in 0.02s, such that the induced emf is4.3v. What is the mutual inductance?


Inductance enisle=4.3V

Initial current!;’ =0A

Final current 11/ =10A

Time ‘At’=0.02Sec


Mutual inductance =M=?


We know that;

£5. =-Al X –111


I f-li

Es r- —M X


M= _ Esar

Putting values




M = -4.3) (0.02


M = 0.0086H

M = 8.6mH Answer



Mutual inductance = M=8.6mH

Assignment 15.3:

A step —up transform 200 turns and a secondary coil consist supplied with an effective AC voltage secondary circuit?


Rmer has a primary coil consisting of ting of 3000turns. The primary coil is of 90.0V. What is the voltage in the

Number of turns in Primary coil ‘ Np’200V

Number of turns in Secondary Coil ‘Ns1=3000

Primary voltage (Input) ‘Vp=90v


Secondary Voltage (output) lVs’=?


According to transformer equation;

V, = N, V NP Vb. =

s= X V


Putting values

Vs = 312000° x 90

tis= 1350V


Secondary voltage (output) is 1350V.


1. Choose the correct answer for each of the following;

i. A current carrying wire in which current flow in northward direction is deflected towards the east by a magnetic force. The direction of the magnetic field is

(a) Straight up (b) straight down

(c) South (d) west

ii. Which derived unit is equivalent to tesla T?

(a) Nm/A (b) NA/m

(c) N/Am (d) Am/N

iii. The unit of inductance, the Henry, is equivalent to

(a) Vs/A (b) VA/m

(c) AsN (d) V/A

iv. When the speed at which a magnet is move through a coil is increased, the induced voltage

(a) Increase (b) remain the same

(c) Decrease (d) goes to 0

v. Slip rings are a part of

(a) DC motor (b) AC generator

(c) Transformer (d) magnet

vi. A transformer is used for

(a) Both DC and AC (b) AC voltage

(c)DC voltage (d) farming

vii. A step-up transformer increases

(a) Power (b) energy

(c) Voltage (d) current

viii. A certain transformer has a primary winding with 500 turns and a secondary winding with 250 turns. The turn’s ratio is

(a) 0.5 (b) 2

(c) 250 (d) 750

ix. If the turn’s ratio is 5, the secondary voltage is greater than the primary voltage by a factor of

(a) 0.2 (b) 0.5

(c) 2.5 (d) 5


Given: Current in the wire=1=1

Length of wire=L=5m Strength of magnetic fi Andle”0″ =90°


Magnetic force =FB=? Calculation:

We know that;


Putting the values:

FB = (5×10 -5) (5) sin 90

FB = 250 x 10 5 x1 sin 90 = 1

FB = 2.5 x 10 -3 N



Magnetic force = FB = 2.5 x 10 -3 N

P3: A 10 cm wire at 30 to unniformmagetic field of 0,06T force of 0.024 N .What is the current flowing in the wire?



Magnetic force = F6 = 0.024N

Length = L = 10cm = 0.1m

Angle 0 = 30

Magnetic field = B = 0.06T

Angle 0 = 30


Current through wire=l=?


We know that;

Fm=BILsin 0 I = Fm


Putting the values;


(0.06)(0.1) xsin30°


1— =8


= 8A



Current flowing through the wire 1= 8A

P4: If the current through the primary coil changes from -5A to +5A in 0.05 s, such that the induced emf is 2.8V.what is the mutual inductance?



Initial current =1; =-5A

Final current =If= 5A

Induced emf=Eind=2.8V Time=t1t= 0.05


Mutual inductance =M=?


We know that;


Find=- M at

M _-E,na at


Putting the values;


5-(-5) -2.8×0.05

M –


M = -0.014H

M= -14mH



Mutual inductance =14mH

P5: A transformer connected to a 120-V AC line to supply 9600 V for a neon sign. (a) What are the ratio primary and secondary turns of the transformer? (b) If the transformer consisted of 275 primary winding, how many secondary winding would there be?


Primary voltage (input) ‘Vp’120v

Secondary voltage (output) ‘W=9600V

Number of turns in the primary ‘Np’=275


(a) Turns ration=L=?

(b) Number of secondary loops ‘N,’=?


(a) Turns ration=—N=?

We know that N, Vs

= re V

N, 9600

Np 120

Ns n, — = 86U Np

(b) Number of secondary loops ‘N,’=? We know that

N, =Vs



N, == x N


Putting values

N, = 9600 120 x275

N5= 22000



(a) Turns ration= fre.-5=80

(b) Number of secondary loops ‘N,’ = 22000

P6: How many turn would you want in the secondary coil of a transformer having 400 turns in the primary, if it wire to reduce the voltage from 220V AC to 3.0 VAC?


Primary voltage (input) bF = 220v

Secondary voltage (output) Vs’=3V

Number of turns in the primary ‘Np’=400


Number of secondary Coils IN,’=?


We know that,

Ns =



N, =— x N

T, P


Putting values

Ns= thx 400

N,= 5.45



Number of secondary Coils 4N,’=5.45

P7: A transformer steps down a main supply of220 V AC to operate a 12 V AC lamp. Calculate the turn’s ratio of the weddings.


Primary voltage (input) VP’220v

Secondary voltage (output) ‘V,’=12V


Turns ration = 12==e?



We know that

N, = ✓1

NP v

Ns = 12 N 220

NS 0.05

Ny Answer



Turns ratio= N—s = 0.05


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