NCERT CLASS 10 CHAPTER 12 MAGNETIC EFFECT OF ELECTRIC CURRENT
NCERT CLASS 10 SCIENCE
CHAPTER 12 – MAGNETIC EFFECTS OF ELECTRIC CURRENT
IN-TEXT QUESTIONS AND ANSWERS
1. Why does a compass needle get deflected when brought near a bar magnet?
A compass needle is a small magnet. When it is brought near a bar magnet, the magnetic field of the bar magnet exerts a force on the compass needle, causing it to deflect.
2. Draw magnetic field lines around a bar magnet.
Magnetic field lines around a bar magnet emerge from its North pole and enter its South pole outside the magnet. Inside the magnet, they travel from the South pole to the North pole, forming closed curves.
3. List the properties of magnetic field lines.
The properties of magnetic field lines are:
Magnetic field lines emerge from the North pole and enter the South pole outside a magnet.
Inside the magnet, their direction is from the South pole to the North pole.
Magnetic field lines form closed and continuous curves.
The tangent at any point on a magnetic field line gives the direction of the magnetic field at that point.
No two magnetic field lines intersect each other.
The closer the magnetic field lines, the stronger the magnetic field.
4. Why don't two magnetic field lines intersect each other?
If two magnetic field lines intersected each other, the compass needle at the point of intersection would have to point in two different directions simultaneously. This is impossible. Therefore, two magnetic field lines never intersect each other.
5. Consider a circular loop of wire lying in the plane of a table. Let the current flow through the loop clockwise. Apply the right-hand thumb rule to find the direction of the magnetic field inside and outside the loop.
According to the right-hand thumb rule, if the fingers of the right hand curl in the direction of current, the thumb points in the direction of the magnetic field.
For a clockwise current:
The magnetic field inside the circular loop is directed into the plane of the table.
The magnetic field outside the loop is directed out of the plane of the table.
6. The magnetic field in a given region is uniform. Draw a diagram to represent it.
A uniform magnetic field is represented by straight, parallel and equally spaced magnetic field lines. These lines indicate that the magnetic field has the same magnitude and direction at every point in that region.
7. Choose the correct option. The magnetic field inside a long straight solenoid-carrying current:
(a) Is zero.
(b) Decreases as we move towards its end.
(c) Increases as we move towards its end.
(d) Is the same at all points.
Answer: (d) Is the same at all points.
8. Which of the following statements is correct regarding the magnetic field produced by a current-carrying conductor?
The magnetic field produced by a current-carrying conductor depends on the magnitude of current flowing through it and the distance from the conductor.
9. What is the function of a fuse in an electric circuit?
A fuse is a safety device used to protect an electric circuit from excessive current. When the current exceeds the safe limit, the fuse wire melts and breaks the circuit, preventing damage to electrical appliances.
10. What is the direction of the magnetic field produced by a current-carrying straight conductor?
The direction of the magnetic field is determined by the right-hand thumb rule. If the thumb of the right hand points in the direction of current, the curled fingers indicate the direction of the magnetic field.
11. What is a solenoid?
A solenoid is a long cylindrical coil of insulated copper wire consisting of many closely wound circular turns. When electric current passes through it, it produces a magnetic field similar to that of a bar magnet.
12. What happens to the magnetic field inside a solenoid when the current flowing through it is increased?
The strength of the magnetic field inside a solenoid increases when the current flowing through it is increased.
13. What is an electromagnet?
An electromagnet is a temporary magnet produced by passing electric current through a coil of insulated wire wound around a soft iron core. It behaves like a magnet as long as current flows through the coil.
14. What is the principle of an electric motor?
An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force. This force produces rotational motion in the coil.
15. What is electromagnetic induction?
Electromagnetic induction is the phenomenon of producing an electric current in a conductor by changing the magnetic field around it.
16. What is the function of a split ring in an electric motor?
The split ring acts as a commutator. It reverses the direction of current in the coil after every half rotation, allowing the coil to continue rotating in the same direction.
17. What is the function of brushes in an electric motor?
Brushes are used to maintain electrical contact between the rotating split rings and the external circuit. They allow current to flow into the rotating coil.
18. State Fleming's left-hand rule.
According to Fleming's left-hand rule, stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular to each other.
Forefinger indicates the direction of the magnetic field.
Middle finger indicates the direction of current.
Thumb indicates the direction of force or motion of the conductor.
19. What is the difference between an electric motor and an electric generator?
An electric motor converts electrical energy into mechanical energy, whereas an electric generator converts mechanical energy into electrical energy.
20. State Fleming's right-hand rule.
According to Fleming's right-hand rule, stretch the thumb, forefinger and middle finger of the right hand so that they are mutually perpendicular to each other.
Forefinger indicates the direction of the magnetic field.
Thumb indicates the direction of motion of the conductor.
Middle finger indicates the direction of induced current.
EXERCISE QUESTIONS AND ANSWERS
1. Which of the following correctly describes the magnetic field near a long straight current-carrying wire?
(a) The magnetic field lines are straight and parallel.
(b) The magnetic field lines are circular around the wire.
(c) The magnetic field lines are directed away from the wire.
(d) The magnetic field lines are directed towards the wire.
Answer: (b) The magnetic field lines are circular around the wire.
2. The phenomenon of electromagnetic induction is:
(a) The process of producing magnetic field due to current.
(b) The process of producing current due to changing magnetic field.
(c) The process of producing heat due to current.
(d) The process of producing light due to current.
Answer: (b) The process of producing current due to changing magnetic field.
3. The device used for producing electric current is called:
(a) Electric motor.
(b) Electric generator.
(c) Ammeter.
(d) Voltmeter.
Answer: (b) Electric generator.
4. The essential difference between an AC generator and a DC generator is that:
(a) AC generator produces higher voltage.
(b) DC generator produces higher current.
(c) AC generator uses slip rings while DC generator uses a split-ring commutator.
(d) AC generator uses a split-ring commutator while DC generator uses slip rings.
Answer: (c) AC generator uses slip rings while DC generator uses a split-ring commutator.
5. At the time of short circuit, the current in the circuit:
(a) Reduces substantially.
(b) Does not change.
(c) Increases heavily.
(d) Varies continuously.
Answer: (c) Increases heavily.
6. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
Answer: False. An electric motor converts electrical energy into mechanical energy.
(b) An electric generator works on the principle of electromagnetic induction.
Answer: True.
(c) The magnetic field at the centre of a circular current-carrying coil is perpendicular to its plane.
Answer: True.
(d) A fuse is connected in parallel with an electrical appliance.
Answer: False. A fuse is connected in series with the electrical circuit.
7. Why does a compass needle get deflected when brought near a current-carrying wire?
A current-carrying wire produces a magnetic field around it. When a compass needle is brought near the wire, this magnetic field interacts with the magnetic field of the compass needle, causing it to deflect.
8. What is the direction of the magnetic field at the centre of a circular current-carrying coil?
The direction of the magnetic field at the centre of a circular current-carrying coil is perpendicular to the plane of the coil. It can be determined using the right-hand thumb rule.
9. What is the effect of increasing the number of turns in a current-carrying circular coil?
The strength of the magnetic field produced by a circular coil increases when the number of turns is increased, provided the current remains constant.
10. What is the role of the soft iron core in an electromagnet?
The soft iron core increases the strength of the magnetic field produced by the coil. It becomes strongly magnetised when current flows and loses most of its magnetism when the current is switched off.
11. Why is an electromagnet called a temporary magnet?
An electromagnet is called a temporary magnet because it behaves like a magnet only when electric current flows through its coil. When the current is switched off, it loses most of its magnetism.
12. Explain the working principle of an electric motor.
An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force.
When current passes through a rectangular coil placed between the poles of a magnet, opposite forces act on its two sides. These forces produce a turning effect, causing the coil to rotate.
The split-ring commutator reverses the direction of current after every half rotation, allowing continuous rotation in the same direction.
13. What is the difference between AC and DC?
Alternating Current (AC):
The direction of current changes periodically.
Its magnitude may also change with time.
It is used in household electricity supply.
Direct Current (DC):
Current flows in one direction only.
Its direction does not change with time.
It is supplied by cells and batteries.
14. Why is AC preferred over DC for transmitting electricity over long distances?
AC is preferred because its voltage can be easily increased or decreased using transformers. High-voltage transmission reduces current and therefore reduces power loss due to heating in transmission wires.
15. What is a short circuit?
A short circuit occurs when the live wire and neutral wire come into direct contact, creating a low-resistance path. This causes a sudden increase in current and may damage electrical appliances or cause a fire.
16. What is overloading in an electric circuit?
Overloading occurs when too many electrical appliances are connected to a circuit or when appliances draw more current than the circuit can safely carry.
It may cause excessive heating of wires and can lead to fire hazards.
17. What are the advantages of using a fuse?
The advantages of using a fuse are:
It protects electrical appliances from excessive current.
It prevents damage due to short circuits.
It breaks the circuit when the current exceeds the safe limit.
It helps prevent electrical fires.
18. What is the difference between an electromagnet and a permanent magnet?
Electromagnet:
It produces magnetism when electric current flows through it.
Its magnetic strength can be changed by changing the current.
Its polarity can be reversed by reversing the current.
Soft iron is commonly used as its core.
Permanent Magnet:
It produces a magnetic field without requiring electric current.
Its magnetic strength is generally fixed.
Its polarity remains unchanged under normal conditions.
Steel and other magnetic materials are used to make permanent magnets.
19. What is the function of an electric generator?
An electric generator converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction.
20. Explain the working principle of an electric generator.
An electric generator works on the principle of electromagnetic induction.
When a coil is rotated in a magnetic field, the magnetic field linked with the coil changes continuously. This induces an electric current in the coil.
The direction of induced current can be determined using Fleming's right-hand rule.
21. What is the difference between an AC generator and a DC generator?
AC Generator:
It produces alternating current.
It uses two slip rings.
The direction of current changes periodically.
DC Generator:
It produces direct current.
It uses a split-ring commutator.
The current flows in one direction in the external circuit.
FEW IMPORTANT ADDITIONAL QUESTIONS AND ANSWERS
1. What is magnetism?
Magnetism is a physical phenomenon by which certain materials attract or repel other magnetic materials. It is associated with magnetic fields produced by magnets and moving electric charges.
2. What are magnetic poles?
The two regions of a magnet where its magnetic effect is strongest are called magnetic poles. They are known as the North pole and South pole.
3. State the properties of magnets.
The properties of magnets are:
A magnet attracts magnetic materials such as iron, nickel and cobalt.
Every magnet has two poles: North and South.
Like poles repel each other.
Unlike poles attract each other.
Magnetic poles always exist in pairs.
A freely suspended magnet aligns itself approximately in the North-South direction.
4. What is a magnetic field?
The region surrounding a magnet or a current-carrying conductor in which its magnetic force can be experienced is called a magnetic field.
5. What is the SI unit of magnetic field?
The SI unit of magnetic field is tesla (T).
6. What is the right-hand thumb rule?
According to the right-hand thumb rule, if we hold a straight current-carrying conductor in our right hand such that the thumb points in the direction of current, the curled fingers indicate the direction of the magnetic field lines around the conductor.
7. What factors affect the magnetic field produced by a straight current-carrying conductor?
The magnetic field produced by a straight current-carrying conductor depends on:
The magnitude of current flowing through the conductor.
The distance from the conductor.
The direction of current.
The magnetic field increases with an increase in current and decreases with an increase in distance from the conductor.
8. What happens to the magnetic field when the direction of current is reversed?
When the direction of current is reversed, the direction of the magnetic field around the conductor also gets reversed.
9. What is the magnetic field due to a circular current-carrying loop?
A circular current-carrying loop produces a magnetic field around it. The magnetic field lines near the wire are circular, while at the centre of the loop they become nearly straight and perpendicular to the plane of the coil.
10. What is a solenoid and how does it behave like a bar magnet?
A solenoid is a long cylindrical coil made up of many closely wound turns of insulated copper wire.
When electric current passes through it, a magnetic field is produced. One end behaves like a North pole and the other end behaves like a South pole. Its magnetic field pattern is similar to that of a bar magnet.
11. How can the strength of the magnetic field inside a solenoid be increased?
The strength of the magnetic field inside a solenoid can be increased by:
Increasing the current flowing through it.
Increasing the number of turns per unit length.
Inserting a soft iron core inside the solenoid.
12. What is the magnetic effect of electric current?
The phenomenon in which an electric current flowing through a conductor produces a magnetic field around it is called the magnetic effect of electric current.
13. What is the force on a current-carrying conductor placed in a magnetic field?
A current-carrying conductor placed in a magnetic field experiences a force due to the interaction between the magnetic field produced by the current and the external magnetic field.
The direction of force depends on the direction of current and the magnetic field.
14. What happens when the direction of current and magnetic field are parallel?
When the direction of current and magnetic field are parallel or antiparallel, the force acting on the current-carrying conductor is zero.
15. What is Fleming's left-hand rule used for?
Fleming's left-hand rule is used to determine the direction of force or motion experienced by a current-carrying conductor placed in a magnetic field.
16. What is the role of a split-ring commutator in an electric motor?
The split-ring commutator reverses the direction of current in the rotating coil after every half rotation. This helps the coil rotate continuously in the same direction.
17. What is the role of a battery in an electric motor?
A battery supplies electrical energy to the motor and provides current to the rotating coil, allowing it to experience a force in the magnetic field.
18. What is the principle of electromagnetic induction?
Whenever the magnetic field linked with a conductor or coil changes, an electric current is induced in it. This phenomenon is called electromagnetic induction.
19. How can induced current be produced in a coil?
Induced current can be produced by:
Moving a magnet towards or away from a coil.
Moving a coil towards or away from a magnet.
Changing the magnetic field around the coil.
20. What factors affect the magnitude of induced current?
The magnitude of induced current depends on:
The rate of change of magnetic field linked with the coil.
The number of turns in the coil.
The resistance of the coil and circuit.
21. What is Fleming's right-hand rule used for?
Fleming's right-hand rule is used to determine the direction of induced current in a conductor moving in a magnetic field.
22. What is the difference between an electric motor and an electric generator?
Electric Motor:
Converts electrical energy into mechanical energy.
Works on the magnetic effect of electric current.
Uses Fleming's left-hand rule.
Used in fans, mixers, pumps and washing machines.
Electric Generator:
Converts mechanical energy into electrical energy.
Works on electromagnetic induction.
Uses Fleming's right-hand rule.
Used in power stations to generate electricity.
23. What is the frequency of alternating current supplied in India?
The frequency of alternating current supplied in India is 50 Hz.
This means the direction of current changes 100 times per second, completing 50 full cycles in one second.
24. What is the time period of alternating current in India?
The frequency of AC supply in India is 50 Hz.
Time period:
T = 1/f
T = 1/50
T = 0.02 seconds.
Answer: 0.02 seconds.
25. What are the two wires used in domestic electric circuits?
The two main wires used in domestic electric circuits are:
Live wire: It carries current from the electrical supply to the appliance.
Neutral wire: It provides the return path for current.
An earth wire is also used as a safety measure to protect users from electric shocks.
26. What is the function of an earth wire?
An earth wire is a safety wire connected to the metallic body of an electrical appliance. It provides a low-resistance path for leakage current to flow into the earth, helping prevent electric shocks.
27. Why is the earth wire connected to the metallic body of an appliance?
The earth wire is connected to the metallic body of an appliance to prevent electric shock. If the live wire accidentally touches the metallic body, the leakage current flows through the earth wire and helps operate the protective device.
28. Why is a fuse connected in series?
A fuse is connected in series so that the entire current flowing through the circuit passes through it. When excessive current flows, the fuse melts and breaks the circuit.
29. What is the difference between a fuse and an MCB?
Fuse:
It contains a wire that melts when excessive current flows.
It needs replacement after it operates.
It protects circuits from excessive current.
MCB:
MCB stands for Miniature Circuit Breaker.
It automatically switches off the circuit when excessive current flows.
It can be reset and used again after the fault is corrected.
30. Why should we not touch electrical appliances with wet hands?
Water containing dissolved salts conducts electricity. Wet hands can provide an easier path for electric current to pass through the body, increasing the risk of electric shock.
IMPORTANT FORMULAE AND RULES
Magnetic field around a straight conductor:
B ∝ I
Magnetic field due to a straight conductor:
B ∝ 1/r
Magnetic field at the centre of a circular coil:
B ∝ NI/R
Magnetic field inside a solenoid:
B ∝ nI
Force on a current-carrying conductor:
F = BIL sin θ
Fleming's Left-Hand Rule:
Used to determine the direction of force or motion.
Fleming's Right-Hand Rule:
Used to determine the direction of induced current.
Time period of AC:
T = 1/f
Frequency:
f = 1/T
Frequency of AC supply in India:
f = 50 Hz
Time period of AC supply in India:
T = 0.02 seconds
Note: B represents magnetic field, I represents current, r represents distance from the conductor, N represents the number of turns, R represents the radius of a circular coil, n represents turns per unit length, L represents the length of conductor in the magnetic field, θ represents the angle between current and magnetic field, f represents frequency and T represents time period.
IMPORTANT TERMS TO REMEMBER
Magnetic Field – Region where magnetic force can be experienced.
Magnetic Field Lines – Imaginary lines representing the magnetic field.
Magnetic Poles – North pole and South pole of a magnet.
Right-Hand Thumb Rule – Rule used to find the direction of the magnetic field around a current-carrying conductor.
Solenoid – A cylindrical coil of insulated copper wire.
Electromagnet – A temporary magnet produced by electric current.
Electric Motor – A device that converts electrical energy into mechanical energy.
Electromagnetic Induction – Production of electric current due to a changing magnetic field.
Electric Generator – A device that converts mechanical energy into electrical energy.
Fleming's Left-Hand Rule – Rule used to determine the direction of force.
Fleming's Right-Hand Rule – Rule used to determine the direction of induced current.
Alternating Current (AC) – Current that changes its direction periodically.
Direct Current (DC) – Current that flows in one direction.
Short Circuit – A condition in which a low-resistance path causes a sudden increase in current.
Overloading – A condition in which excessive current flows through an electrical circuit.
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