Chapter 15 ( 10th Physics) Electromagnetism

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1. Define electromagnetism?
Ans. Electromagnetism is the study of magnetic effects of current.
2. What are concentric circle?
Ans. Concentric circles are the circles of different radii having common center.
3. Define right hand rule.
Ans. If we hold the current carrying wire in right hand, such that the thumb is directed in the
direction of current, the curled fingers will show the direction of magnetic field.
4. How can a material be magnetized?
Ans. A material can be magnetized by producing a strong magnetic field inside the solenoid.
5. What is a solenoid?
Ans. Solenoid is a coil with many circular turns of insulated copper wire wrapped closely in the
shape of cylinder.
6. How can magnetic field around the coil be understood?
Ans. Magnetic field around the coil can be understood by placing iron fillings near the coil. The
iron fillings will arrange themselves in the presence of magnetic field.
7. State Fleming’s left hand rule.
Ans. Stretch the thumb, forefinger and the middle finger of the left hand such that they are
perpendicular to one another. If the forefinger and the middle finger indicate the direction of
magnetic field and the current respectively, the thumb would point towards the force acting on
the wire.
8. On which factors the magnetic force depends.
Ans. Magnetic force depends on three factors:
i. Current in the wire (l)
ii. Magnetic field strength(B)
iii. Length of the wire (L)
9. Write the mathematical relation between magnetic force and the factors of a magnetic field.
Ans. The mathematical relation between magnetic force and the factors of a magnetic field is :
F = ILB
B = F/IL
Where “B” is magnetic field strength.
10. What is the unit of magnetic field strength?
Ans. The SI unit of magnetic field strength is tesla (T).
1T = 1N/1 Am
11. Define tesla.
Ans. The magnetic field strength will be 1 tesla when a conductor of length 1m carrying a
current of 1A perpendicular to the field experiences a force of 1N.
12. Define couple.
Ans. Two forces which are equal in magnitude but opposite in direction but not acting on the
same line form a couple.
13. When will the magnetic force be maximum?
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Ans. The magnetic force will be maximum when conductor is perpendicular to the magnetic
field.

Composed by Kashif Mughal

Chapter 15 ( 10th Physics) Electromagnetism

14. Define motor.

Ans. Motor is a device which converts electrical energy into mechanical energy. It works on the
principle of torque.
15. Which device is used to convert electrical energy into mechanical energy?
Ans. An electric motor is a device which is used to convert electrical energy into mechanical
energy.
16. Define electromagnetic induction.
Ans. The process of generating an induce current in a circuit by changing the number of
magnetic lines of force passing through it is called electromagnetic induction.
17. What is armature?
Ans. Armature is a rectangular coil in DC motor. It is connected to DC supply.
18. How total force acting on armature is increased.
Ans. The total force on the armature can be increased by:
i. Increasing the number of turns on the coil.
ii. Increasing the current in the coil.
iii. Increasing the area of the coil.
iv. Increasing the strength of the magnetic field.
19. How is unidirectional torque achieved?
Ans. Unidirectional torque is achieved by connecting the ends of coil to the two halves of a ring
called split ring or commutator.
20. What is magnetic flux?
Ans. Total number of magnetic lines of force passing through any area is called magnetic flux.
21. What is an induced current?
Ans. When an emf is produced in the solenoid, a current flows through the galvanometer. This
current is called induced current.
22. State Faraday’s law of electromagnetic induction.
Ans. “ The value of induced emf in a coil is directly proportional to the number of magnetic field
lines passing through it”.
23. State Lenz's law.
Ans. The Lenz’s law states that “ The direction direction of induced current in a circuit is
opposite to the cause which produces it.”
24. State the law of conservation of energy.
Ans. Law of conservation of energy states that “ Energy is never created or destroyed. It only
changes from one form to another.”
25. On which factors the magnitude of induced emf depends.
Ans. The magnitude of induced emf in a circuit depends on the following factors:
i. Speed of relative motion of the coil and the magnet.
ii. Number of turns of the coil.
iii. Amount of current passing through the coil.
26. When does magnetic flux changes?
Ans. When a coil is moved in a magnetic field, the number of magnetic field lines called
magnetic flux changes.
27. What is an AC generator?

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Chapter 15 ( 10th Physics) Electromagnetism

Ans. AC generator is a device which converts mechanical energy to electrical energy. This energy
is in the form of alternating current (AC).
28. What is the main difference between an electric motor and an electric generator?
Ans. The main difference between an electric motor and an electric generator is that an
electrical motor converts electrical energy into mechanical energy whereas an electrical
generator converts mechanical energy to electrical energy.
29. Briefly explain an AC generator.
Ans. An AC generator consists of a rectangular coil. This coil rotates between the poles of a
permanent magnet. Two slip rings are connected to both ends of the coil. With the help of
springs, two carbon brushes are attached to the slip rings. These carbon brushes conduct the
current produced in the coils. Load is connected between the brushes.
30. Define mutual induction.
Ans. If a current is induced in a circuit, due to a change of current in another nearby circuit, this
phenomenon is known as mutual induction.
31. What is the SI unit of mutual inductance? Define it.
Ans. The SI unit of mutual inductance is Henry(H).
“If the rate of change of current is 1A/s in primary coil and it induces emf of 1 volt in secondary
coil then their mutual induction is one henry.
32. Define transformer.
Ans. Transformer is an electrical device which increases or decreases the value of alternating
voltage.
33. Write the principle of transformer.
Ans. Transformer works on the principle of mutual induction.
34. How is transformer useful in comparison to other such devices?
Ans. A transformer is useful in comparison to other such devices because it converts voltage
with relatively small loss of energy.
35. What is a step-up transformer?
Ans. If secondary voltage is larger than primary voltage, the transformer will be a step-up
transformer. Its mathematical expression will be:
Ns > Np so Vs > Vp
36. What is a step-down transformer?
Ans. If secondary voltage is less than primary voltage, the transformer will be a step-down
transformer. Its mathematical expression will be:
Ns < Np so Vs < Vp
37. What is an ideal transformer? Give its mathematical expression.
Ans. In an ideal transformer, the electric power delivered to the secondary coil equals to the
power supplied to the primary coil. An ideal transformer dissipates no power itself and for such
transformer we can write:
Pp = Ps
VpIp = VsIs
38. Write some uses of transformer.
Ans.Uses of transformer:
i. Major use of transformer is in electric power supply system.

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Chapter 15 ( 10th Physics) Electromagnetism

ii. Transformers are used in most electronic devices such as T.V. and computer to step
down the voltage to some desire level.
iii. X-rays machine, CT scanners, MRI equipments use the transformer to give required
level of Voltage.
39. Define electromagnet.
Ans. An electromagnetic is a type of magnet in which the magnetic field is produced by an
electric current. The magnetic field disappears when the current is turned off.
40. Most of the electromagnet we use are made of iron. Can all metals be turned into magnet.
Ans. No, all metals cannot be turned into magnet.
41. In what type of devices are electromagnets are used.
Ans. Electromagnets are used widely in electric devices such as motors, relays, loudspeakers
hard disk, MRI machine, circuit breakers etc.

Composed by Kashif Mughal