CBSE Class 10 Science Magnetic effects of electric current Sure Shot Questions Set 01

This Class 10 Science assignment on Magnetic Effects of Electric Current covers very short answer, short answer and long answer questions on Oersted's experiment, magnetic field lines, solenoids, electromagnets, Fleming's left hand rule, electric motors, electromagnetic induction, AC and DC generators, and household electrical safety. Each question is answered with a clear, exam-ready solution and a Teacher's Note.

ASSIGNMENT QUESTIONS FOR PRACTICE
MAGNETIC EFFECTS OF ELECTRIC CURRENT

VERY SHORT ANSWER TYPE QUESTIONS (1 MARK)

1. Name the scientist who discovered the magnetic effect of current. [1 Mark]

Answer: Hans Christian Oersted.

Teacher's Note:
a) Oersted noticed in 1820 that a compass needle moved when a current flowed in a nearby wire.

2. Does a current flowing in a wire always give rise to a magnetic field around it? [1 Mark]

Answer: Yes, every current carrying conductor produces a magnetic field around it.

Teacher's Note:
a) The field exists whether or not a compass is nearby; the compass only shows it.

3. State any two properties of magnetic field lines. [1 Mark]

Answer: Magnetic field lines never intersect each other, and they are closer together where the field is stronger.

Teacher's Note:
a) Field lines are closed curves that emerge from the north pole and enter the south pole outside the magnet.

4. Why does a compass needle get deflected when brought near a bar magnet? [1 Mark]

Answer: The compass needle is a small magnet, so it experiences a force from the bar magnet's field and turns to align with it.

Teacher's Note:
a) A compass needle always lines up along the magnetic field at its position.

5. Name the effect of current on which an electromagnet works. [1 Mark]

Answer: Magnetic effect of current.

Teacher's Note:
a) An electromagnet is magnetised only while current flows through its coil.

6. What name is given to the combination of a solenoid and a soft iron core? [1 Mark]

Answer: Electromagnet.

Teacher's Note:
a) The soft iron core becomes strongly magnetised when current flows through the solenoid.

7. Can steel be used for making electromagnets? [1 Mark]

Answer: No, steel is not used for electromagnets because it stays magnetised after the current is switched off.

Teacher's Note:
a) Soft iron loses its magnetism quickly, which is why it is preferred for electromagnets.

8. Name the scientist who discovered that a current carrying conductor when placed in a magnetic field experiences a mechanical force. [1 Mark]

Answer: Michael Faraday.

Teacher's Note:
a) Faraday's 1821 experiment showed a wire carrying current rotating around a magnet, which is the motor effect.

9. When is the maximum force exerted on a current carrying conductor while it is kept in a magnetic field? [1 Mark]

Answer: The force is maximum when the conductor is held at right angles (90°) to the magnetic field.

Teacher's Note:
a) The force is given by F = BIl sin θ, which is largest when sin θ = 1.

10. Does a current carrying conductor experience some force when kept parallel to the magnetic field? [1 Mark]

Answer: No, the force is zero when the conductor is parallel to the magnetic field.

Teacher's Note:
a) Since sin 0° = 0, the magnetic force on a conductor placed parallel to the field is zero.

11. Which rule is employed to find the direction of force on a current carrying conductor when kept in a magnetic field? State the rule and explain it by a diagram. [1 Mark]

Answer: Fleming's left hand rule. Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular; the forefinger shows the direction of the field (north to south), the middle finger shows the direction of the current, and the thumb shows the direction of the force.
[Figure: Fleming's left hand rule diagram with three mutually perpendicular fingers labelled Thumb (Force), Forefinger (Field, N to S) and Middle finger (Current); diagram to be added]

Teacher's Note:
a) Use the left hand for motors and the right hand for generators; this is a frequent exam confusion.

12. Name the transformation of energy involved in the electric motor. [1 Mark]

Answer: Electrical energy is converted into mechanical energy (kinetic energy).

Teacher's Note:
a) The motor's rotation is the mechanical output of the electrical input.

13. What is the function of commutator rings in the electric motor? [1 Mark]

Answer: The commutator reverses the direction of current in the coil every half rotation, so the coil keeps rotating in the same direction.

Teacher's Note:
a) Without the commutator, the coil would reverse direction at the vertical position and stop.

14. What is the function of carbon brushes in the electric motor? [1 Mark]

Answer: Carbon brushes conduct current from the battery to the commutator while the coil rotates.

Teacher's Note:
a) Carbon is used because it is a good conductor and wears less than metal.

15. Name one application of electromagnetic induction. [1 Mark]

Answer: Electric generator (dynamo).

Teacher's Note:
a) A transformer also works on electromagnetic induction.

16. What is the other name of electric generator? [1 Mark]

Answer: Dynamo.

Teacher's Note:
a) A dynamo converts mechanical energy into electrical energy.

17. Name the transformation of energy in an electric generator. [1 Mark]

Answer: Mechanical energy is converted into electrical energy.

Teacher's Note:
a) The turning of the coil by an external source provides the mechanical energy.

18. What is a turbine used for? [1 Mark]

Answer: A turbine is used to convert the kinetic energy of flowing water, steam or wind into mechanical energy that rotates the generator.

Teacher's Note:
a) In a power plant, the turbine shaft is directly connected to the generator rotor.

19. Name the different types of electric power plants for generating electricity on large scale. [1 Mark]

Answer: Thermal power plants, hydroelectric power plants and nuclear (atomic) power plants.

Teacher's Note:
a) Each type uses a different energy source to turn the turbine.

20. Name the fuel used in a thermal power plant. [1 Mark]

Answer: Coal.

Teacher's Note:
a) Some thermal plants burn oil or natural gas, but coal is the main fuel.

21. Name the fuel used in an atomic power plant. [1 Mark]

Answer: Uranium.

Teacher's Note:
a) Atomic power plants use nuclear fission of uranium to produce heat.

22. What do you understand by magnetic field? [1 Mark]

Answer: A magnetic field is the region around a magnet or a current carrying conductor in which a magnetic force can be detected.

Teacher's Note:
a) A magnetic field is shown by field lines, which give its direction and strength.

23. What do you mean by electromagnetism? [1 Mark]

Answer: Electromagnetism is the branch of physics that studies the relation between electricity and magnetism.

Teacher's Note:
a) Oersted's discovery showed that an electric current produces a magnetic field.

24. What is meant by magnetic effect of current? [1 Mark]

Answer: The magnetic effect of current is the production of a magnetic field around a current carrying conductor.

Teacher's Note:
a) The strength of this field increases with the current.

25. Can you observe the magnetic field? [1 Mark]

Answer: No, a magnetic field cannot be seen directly.

Teacher's Note:
a) Its presence is shown by a compass needle or by iron filings arranged along the field lines.

26. What do you mean by a magnetic line of force? [1 Mark]

Answer: A magnetic line of force is an imaginary curve along which a north pole would move in a magnetic field.

Teacher's Note:
a) The direction of the line at any point gives the direction of the magnetic field there.

27. Is a magnetic line of force always a straight line? [1 Mark]

Answer: No, magnetic lines of force are often curved, as around a bar magnet or a circular coil.

Teacher's Note:
a) Only the field inside a long solenoid and between parallel plates of opposite poles is nearly straight.

28. What do you conclude from Oersted's experiment? [1 Mark]

Answer: An electric current produces a magnetic field around the conductor, so electricity and magnetism are related.

Teacher's Note:
a) The deflection of the compass needle reversed when the direction of current was reversed.

29. Can you magnetic line of force ever intersect each other? [1 Mark]

Answer: No, magnetic lines of force never intersect each other.

Teacher's Note:
a) If they intersected, a compass placed at the point would point in two directions at once, which is impossible.

30. What kind of magnetic field is produced by a straight current carrying conductor? [1 Mark]

Answer: Concentric circular magnetic field lines, with the centres lying on the wire.

Teacher's Note:
a) The field gets weaker as the distance from the wire increases.

31. What kind of magnetic field is produced by a current carrying circular field? [1 Mark]

Answer: The field lines near the wire are circular, and at the centre of the loop they appear as nearly straight lines, similar to a bar magnet's field.

Teacher's Note:
a) The field is strongest at the centre of the loop and weakens away from it.

32. What do you mean by a solenoid? [1 Mark]

Answer: A solenoid is a coil of many circular turns of insulated copper wire wound closely in the form of a cylinder.

Teacher's Note:
a) A solenoid behaves like a bar magnet when current flows through it.

33. State the clock rule for a current carrying solenoid. [1 Mark]

Answer: If the current flows clockwise at one end of the solenoid, that end is a south pole; if it flows anticlockwise, that end is a north pole.

Teacher's Note:
a) The clock rule is used to find the polarity of a solenoid when the direction of current is known.

34. How does a current carrying solenoid behave? [1 Mark]

Answer: A current carrying solenoid behaves like a bar magnet, with one end acting as a north pole and the other as a south pole.

Teacher's Note:
a) Reversing the current reverses the polarity of the solenoid.

35. What is the nature of magnetic field produced by a current carrying solenoid? [1 Mark]

Answer: Inside the solenoid the field is nearly uniform, and outside it resembles the field of a bar magnet.

Teacher's Note:
a) The field lines inside the solenoid are parallel and equally spaced.

36. What is the magnitude and direction of the magnetic field inside a current carrying solenoid? [1 Mark]

Answer: The magnitude is B = μ₀nI, where n is the number of turns per unit length and I is the current; the field is uniform and directed along the axis from the south pole to the north pole inside the solenoid.

Teacher's Note:
a) The field inside depends only on the turns per unit length and the current, not on the length of the solenoid.

37. Name the effect of current upon which electromagnets are based? [1 Mark]

Answer: Magnetic effect of current.

Teacher's Note:
a) The electromagnet works only when current flows through its coil.

38. Are electromagnets permanent magnets? [1 Mark]

Answer: No, electromagnets are temporary magnets that lose their magnetism when the current is switched off.

Teacher's Note:
a) This makes electromagnets useful in switches, relays and electric bells.

39. Name the material used for making the core of an electromagnet. [1 Mark]

Answer: Soft iron.

Teacher's Note:
a) Soft iron is easily magnetised and easily demagnetised.

40. Can we use steel, instead of soft iron, for making the core of an electromagnet? Why? [1 Mark]

Answer: No, steel cannot be used because it retains its magnetism for a long time after the current is switched off.

Teacher's Note:
a) Soft iron is preferred because it loses magnetism quickly when the current stops.

41. Can we change the polarity of a permanent magnet? [1 Mark]

Answer: Yes, but only by re-magnetising it in the reverse direction, for example by stroking it with a strong magnet of the opposite pole.

Teacher's Note:
a) Reversing the direction of current does not affect a permanent magnet.

42. Can we change the polarity of an electromagnet? [1 Mark]

Answer: Yes, the polarity is changed by reversing the direction of current through the solenoid.

Teacher's Note:
a) Reversing the battery connections is the simplest way to reverse the polarity.

43. Is the strength of an electromagnet always constant? [1 Mark]

Answer: No, its strength changes with the current, the number of turns and the core material.

Teacher's Note:
a) Increasing the current makes the electromagnet stronger, up to the point where the core saturates.

44. Name the rule applied to know direction of the force acting on a current carrying conductor when placed in a magnetic field. [1 Mark]

Answer: Fleming's left hand rule.

Teacher's Note:
a) Remember that the left hand is used for motors, and the right hand for generators.

45. Name the transformations of energies takes place in an electric motor. [1 Mark]

Answer: Electrical energy is converted into mechanical energy.

Teacher's Note:
a) Some energy is also lost as heat in the coil and the brushes.

46. Name the two kinds of motors. [1 Mark]

Answer: DC motor and AC motor.

Teacher's Note:
a) A DC motor runs on direct current, while an AC motor runs on alternating current.

47. Which kind of motor is used in a fan? [1 Mark]

Answer: AC motor (single phase induction motor).

Teacher's Note:
a) Fans run on the household alternating current supply.

48. Which kind of motor used in a battery-operated toy? [1 Mark]

Answer: DC motor.

Teacher's Note:
a) Batteries supply direct current, so a DC motor is used in battery operated toys.

49. Name the experiment which formed the basis of an electric motor. [1 Mark]

Answer: The experiment in which a current carrying conductor placed in a magnetic field experiences a force, which shows the motor effect.

Teacher's Note:
a) A straight wire placed between the poles of a magnet moves when current passes through it.

50. What forms the commutator of an electric motor? [1 Mark]

Answer: A split ring, which is a metal ring cut into two halves insulated from each other.

Teacher's Note:
a) The two halves of the split ring change contact with the brushes every half rotation.

51. State quantitatively, the effect of inserting an iron core into a current carrying solenoid. [1 Mark]

Answer: The magnetic field increases by a factor equal to the relative permeability μr of the iron, B = μ₀μrnI, which is several hundred times for soft iron.

Teacher's Note:
a) The iron core concentrates the magnetic field lines, which makes the magnetic field much stronger.

52. Name the types of electromagnets commonly used. [1 Mark]

Answer: Straight (solenoid) electromagnets and horseshoe electromagnets.

Teacher's Note:
a) A horseshoe electromagnet is bent so that both poles are close together, giving a strong field at the ends.

53. What happens to the strength of an electromagnet when the magnitude of current decreases? [1 Mark]

Answer: The strength of the electromagnet decreases.

Teacher's Note:
a) The magnetic field strength of a solenoid is directly proportional to the current.

54. What will you prefer, soft iron or steel to make an electromagnet? [1 Mark]

Answer: Soft iron.

Teacher's Note:
a) Soft iron gives a strong magnetic effect and loses magnetism quickly when the current stops.

55. Can we produce electricity from magnetism? [1 Mark]

Answer: Yes, electricity is produced from magnetism by electromagnetic induction.

Teacher's Note:
a) A generator uses a moving magnet or moving coil to produce electricity.

56. Name the phenomenon in which an electric current could be produced in a circuit by changing the magnetic field. [1 Mark]

Answer: Electromagnetic induction.

Teacher's Note:
a) The induced current appears only while the magnetic field is changing.

57. What do you mean by electromagnetic induction? [1 Mark]

Answer: Electromagnetic induction is the production of an induced emf and current in a coil when the magnetic field linked with it changes.

Teacher's Note:
a) The change in magnetic field can be caused by moving a magnet or a coil.

58. What is e.m.i. or E.M.I.? [1 Mark]

Answer: E.M.I. is the short form of electromagnetic induction.

Teacher's Note:
a) The terms electromagnetic induction and E.M.I. refer to the same phenomenon.

59. What do you understand by electric motor effect? [1 Mark]

Answer: The electric motor effect is the force experienced by a current carrying conductor placed in a magnetic field, which makes it move.

Teacher's Note:
a) This force is the basis of all electric motors.

60. What is the cause of electromagnetic induction? [1 Mark]

Answer: The cause is a change in the magnetic field linked with a coil, produced by relative motion between the magnet and the coil.

Teacher's Note:
a) Electromagnetic induction does not occur when a steady magnetic field is linked with the coil.

61. Does the AC generator have any slip ring? [1 Mark]

Answer: Yes, an AC generator has two slip rings.

Teacher's Note:
a) Slip rings give continuous contact with the coil, so the output alternates in direction.

62. Does the DC generator have two slip rings? [1 Mark]

Answer: No, a DC generator has a split ring commutator, not slip rings.

Teacher's Note:
a) The split ring commutator reverses the connections every half rotation to give a one direction output.

63. What is the frequency DC? [1 Mark]

Answer: The frequency of direct current is zero.

Teacher's Note:
a) Direct current flows in one direction at a constant value, so it does not change periodically.

64. Name the fuel used by nuclear power station. [1 Mark]

Answer: Uranium.

Teacher's Note:
a) Nuclear fission of uranium releases heat, which produces steam to drive the turbine.

65. Name the device which converts electrical energy into kinetic energy. [1 Mark]

Answer: Electric motor.

Teacher's Note:
a) An electric motor produces rotation, which is kinetic energy, from the electrical supply.

66. What is the SI unit of induced emf? [1 Mark]

Answer: Volt (V).

Teacher's Note:
a) Induced emf is measured in volts, the same unit as any other emf.

67. State two factors on which the strength of induced current depends. [1 Mark]

Answer: The speed of relative motion between the magnet and the coil (rate of change of magnetic field) and the number of turns in the coil.

Teacher's Note:
a) Faster motion gives a larger induced current.

68. What is the SI unit of induced current? [1 Mark]

Answer: Ampere (A).

Teacher's Note:
a) Induced current is measured in amperes, the same unit as any electric current.

69. What is electromagnetic induction? [1 Mark]

Answer: Electromagnetic induction is the production of an induced emf or current in a coil when the magnetic field linked with it changes.

Teacher's Note:
a) Faraday and Henry discovered this phenomenon independently.

70. What do you mean by a solenoid? [1 Mark]

Answer: A solenoid is a coil of many circular turns of insulated copper wire wound closely in the form of a cylinder.

Teacher's Note:
a) A solenoid carrying current behaves like a bar magnet.

SHORT ANSWER TYPE - I QUESTIONS (2 MARKS)

1. Draw a labeled diagram of an electric motor. [2 Marks]

Answer:
1. [Figure: Labelled diagram of a DC electric motor showing the rectangular armature coil ABCD between the poles N and S of a magnet, the split ring commutator, the two carbon brushes connected to the battery, and the axle; diagram to be added]
2. The coil is labelled ABCD, the poles are labelled N and S, and the brushes are shown touching the split rings.

Teacher's Note:
a) Label the commutator, the brushes and the coil clearly, since examiners check these parts.
b) Common mistake: drawing slip rings instead of split rings for a DC motor.

2. State and explain Fleming's right hand rule for the direction of induced current. [2 Marks]

Answer:
1. Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular to each other.
2. The thumb shows the direction of motion of the conductor, the forefinger shows the direction of the magnetic field (north to south), and the middle finger shows the direction of the induced current.

Teacher's Note:
a) This rule is used for generators, while the left hand rule is used for motors.
b) Remember that the middle finger always gives the direction of the induced current.

3. What do you mean by DC? Show by a diagram. [2 Marks]

Answer:
1. Direct current (DC) is the current that flows in one fixed direction at a constant magnitude.
2. [Figure: Graph of current against time showing a straight horizontal line above the time axis; diagram to be added]

Teacher's Note:
a) A battery supplies direct current, so the flow of charge never reverses.
b) A DC source such as a cell gives a steady current, and its graph is a straight line parallel to the time axis.

4. What do you mean by AC? Show by a diagram. [2 Marks]

Answer:
1. Alternating current (AC) is the current that changes its direction periodically, usually many times per second.
2. [Figure: Graph of current against time showing a sinusoidal wave that crosses the time axis regularly; diagram to be added]

Teacher's Note:
a) The domestic supply in India alternates at 50 cycles per second, or 50 Hz.
b) AC is produced by an AC generator and can be stepped up or down by transformers.

5. Draw a labeled to show the magnetic field pattern due to a straight wire carrying current. [2 Marks]

Answer:
1. The magnetic field lines are concentric circles with their centres on the wire.
2. [Figure: Concentric circles drawn around a straight vertical wire carrying current, with arrows on the circles showing the direction of the magnetic field; diagram to be added]

Teacher's Note:
a) The circles get farther apart as the distance from the wire increases, showing that the field weakens.
b) A straight wire has no end, so the field lines form closed circles around it.

6. With the help of a diagram, indicate the direction of magnetic field produced by a current carrying conductor. Name the rule employed and state it. [2 Marks]

Answer:
1. The rule employed is Maxwell's right hand thumb rule.
2. Hold the straight conductor with the right hand so that the thumb points along the direction of current; the curled fingers give the direction of the magnetic field lines around the conductor.
3. [Figure: Straight conductor with the right hand thumb along the current and curled fingers showing the field direction; diagram to be added]

Teacher's Note:
a) Point the thumb along conventional current, from positive to negative.
b) Common mistake: using the thumb to show the field direction instead of the curled fingers.

7. With the help of a diagram, indicate the direction of magnetic field produced due to a circular wire carrying current. [2 Marks]

Answer:
1. Use the right hand thumb rule: curl the fingers of the right hand in the direction of the current in the loop.
2. The thumb then points along the axis of the loop, which gives the direction of the magnetic field inside the loop; [Figure: Circular loop carrying current with arrows showing the field lines inside the loop pointing along the axis; diagram to be added]

Teacher's Note:
a) The field lines inside the loop are straight and parallel near the centre.
b) The field lines inside the loop are the most useful part for finding the direction.

8. Indicate the direction of the magnetic field produced in a solenoid when some current is passed through it. [2 Marks]

Answer:
1. Use the right hand grip rule: curl the fingers in the direction of current around the solenoid.
2. The outstretched thumb points towards the north pole of the solenoid, and the field inside the solenoid runs from the south pole to the north pole.
3. [Figure: Solenoid with current arrows around the coil and field lines running along the axis from south to north inside; diagram to be added]

Teacher's Note:
a) The field inside the solenoid is uniform, which is why the grip rule is useful.
b) Reversing the current reverses the direction of the field, and the grip rule shows this.

9. How can we increase the strength of magnetic field produced by a circular coil carrying conductor? [2 Marks]

Answer:
1. Increase the current flowing through the coil.
2. Increase the number of turns in the coil, or use a coil of smaller radius, since B = \( \frac{\mu_0 N I}{2r} \) at the centre of the coil.

Teacher's Note:
a) Doubling the number of turns doubles the field at the centre, while doubling the radius halves it.
b) Inserting an iron core also increases the field strength.

10. What are the factors on which the strength of magnetic field produced by a current carrying solenoid depends? [2 Marks]

Answer:
1. The strength depends on the number of turns per unit length of the solenoid, n.
2. It also depends on the current I flowing through the solenoid and on the nature of the core material.

Teacher's Note:
a) The length of the solenoid does not directly change the field inside, provided the number of turns per unit length is fixed.
b) A soft iron core gives a much stronger field than an air core.

11. List the factors affecting the strength of an electromagnet. [2 Marks]

Answer:
1. The number of turns in the coil: more turns give a stronger electromagnet.
2. The strength of the current: a larger current gives a stronger electromagnet; the core material is also important, with soft iron giving the strongest effect.

Teacher's Note:
a) Using a soft iron core instead of an air core increases the field many times.
b) The shape of the core also affects the strength of the electromagnet.

12. Show that magnetic lines of force due to a bar magnet. [2 Marks]

Answer:
1. The printed question is incomplete, so it is taken to mean the pattern of field lines of a bar magnet.
2. The field lines emerge from the north pole, curve around outside the magnet and enter the south pole; inside the magnet they run from south to north. [Figure: Bar magnet with curved field lines from north pole to south pole, crowded near the poles; diagram to be added]

Teacher's Note:
a) The field lines are closer together near the poles, where the field is strongest.
b) Field lines never cross each other.

SHORT ANSWER TYPE - II QUESTIONS (3 MARKS)

1. Briefly describe Oersted's experiment to demonstrate the magnetic effect of current. [3 Marks]

Answer:
1. Place a straight copper wire parallel to and above a compass needle, with the needle lying in the north-south direction.
2. When a current is passed through the wire, the compass needle deflects; reversing the current reverses the deflection, and increasing the current increases the deflection.
3. This shows that a current carrying conductor produces a magnetic field around it.

Teacher's Note:
a) Oersted's experiment was the first to link electricity with magnetism.
b) Check that the wire is parallel to the needle, otherwise the effect is weak.
c) The compass needle deflects only when a current flows through the wire.

2. What are magnetic field lines? Give their important properties. [3 Marks]

Answer:
1. Magnetic field lines are imaginary curves that show the direction and strength of a magnetic field.
2. They emerge from the north pole and enter the south pole outside a magnet, and they form closed curves.
3. They never intersect each other, and they are closer together where the field is stronger.

Teacher's Note:
a) The tangent to a field line at any point gives the direction of the field there.
b) Common mistake: thinking that field lines are real physical lines of force.
c) Field lines are a convenient way to represent a field, not a physical object.

3. How will your experimentally show the magnetic field produced by a straight current carrying conductor? Also state Maxwell's right hand grip rule. [3 Marks]

Answer:
1. Pass a straight thick copper wire vertically through the centre of a cardboard sheet and sprinkle iron filings on the sheet.
2. Pass a current through the wire and gently tap the cardboard; the iron filings arrange in concentric circles around the wire.
3. Maxwell's right hand grip rule: hold the wire in the right hand with the thumb pointing along the current; the curled fingers show the direction of the magnetic field.

Teacher's Note:
a) Keep the current steady and tap gently so that the filings settle into clear circles.
b) Common mistake: holding the cardboard at an angle, which distorts the pattern.
c) A compass placed at different points on the cardboard shows the same direction as the filings.

4. What kind of magnetic field is produced by a current carrying circular coil? Show it with the help of a labeled diagram. [3 Marks]

Answer:
1. A current carrying circular coil produces magnetic field lines that are circular near each part of the wire.
2. At the centre of the coil the field lines are nearly straight and perpendicular to the plane of the coil.
3. [Figure: Circular coil carrying current with labelled field lines, crowded near the wire and straight at the centre, resembling a bar magnet; diagram to be added]

Teacher's Note:
a) Field lines inside the loop all point in the same direction, which makes the field strong at the centre.
b) The field pattern outside the loop resembles that of a bar magnet.
c) The current direction determines the direction of the field inside the loop, which is found with the right hand rule.

5. What do you mean by a solenoid? With the help of a labeled, show the magnetic field due to a current carrying solenoid. [3 Marks]

Answer:
1. A solenoid is a long coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.
2. Inside a long solenoid the magnetic field is uniform and parallel to the axis; outside it resembles the field of a bar magnet.
3. [Figure: Labelled solenoid with field lines inside running parallel to the axis and curved lines outside from north end to south end; diagram to be added]

Teacher's Note:
a) The field inside a solenoid is strongest and most uniform.
b) Each turn of the coil adds to the field along the axis.
c) Use the right hand grip rule to find the north pole of the solenoid quickly.

6. What do you mean by an electromagnet? With the help of diagram show the two types of electromagnets. Give two uses of electromagnets. [3 Marks]

Answer:
1. An electromagnet is a temporary magnet made by passing current through a solenoid wound around a soft iron core.
2. The two types are the straight (solenoid) electromagnet and the horseshoe electromagnet. [Figure: Diagrams of a straight electromagnet and a horseshoe electromagnet with coils and soft iron cores; diagram to be added]
3. Two uses are in electric bells and in cranes for lifting heavy iron and steel loads.

Teacher's Note:
a) Electromagnets are used in relays, circuit breakers and MRI machines as well.
b) The magnetism disappears when the current is switched off, which is what makes them useful.
c) A horseshoe electromagnet gives a strong, concentrated field between its two close poles.

7. How does AC differ from DC? What are the advantages and disadvantages of AC over DC? [3 Marks]

Answer:
1. AC changes its direction periodically, while DC flows in one fixed direction.
2. Advantages of AC: it can be transmitted over long distances at high voltage with less energy loss, and its voltage can be easily stepped up or down with transformers.
3. Disadvantage of AC: it cannot be stored in a battery, and at the same voltage it can be more dangerous to touch than DC.

Teacher's Note:
a) Mains electricity in India is alternating current with a frequency of 50 Hz.
b) Transmission losses are reduced because the high voltage gives a low current for the same power.
c) AC is named after its alternating direction, and DC after its steady, direct flow.

8. What is the basic difference between an AC generator and a DC generator? [3 Marks]

Answer:
1. An AC generator uses two slip rings, while a DC generator uses a split ring commutator.
2. Because of this, the AC generator gives an output current that alternates in direction, while the DC generator gives a current that flows in one direction.

Teacher's Note:
a) The commutator reverses the connection to the external circuit every half rotation.
b) Both generators work on the principle of electromagnetic induction.
c) Both machines work on the same principle, but the output connection differs.

9. Briefly explain the phenomenon of earthing using examples. [3 Marks]

Answer:
1. Earthing connects the metal body of an electrical appliance to the earth through a thick copper wire, which provides a low resistance path to the earth.
2. If the live wire touches the metal body, the leakage current flows into the earth instead of the person touching the appliance; for example, the metal bodies of electric irons, refrigerators and washing machines are earthed.

Teacher's Note:
a) Earthing protects people from electric shock and prevents damage to appliances.
b) A fuse also protects the circuit, but it does not protect a person from shock.
c) A fuse protects the wiring from overcurrent, while the earth wire protects the user from shock.

10. Describe the salient features of tree system of wiring. [3 Marks]

Answer:
1. In the tree system, the main wire from the meter branches out into separate circuits for each room or group of appliances.
2. Each branch has its own fuse or circuit breaker, so a fault in one branch does not affect the others.
3. The system uses less wire than a ring system and is cheaper to install.

Teacher's Note:
a) Each appliance is connected in parallel within its branch, so each receives the full supply voltage.
b) The tree system is the most common wiring system used in Indian homes.
c) The tree system is the simplest wiring layout to understand for a single house.

LONG ANSWER TYPE QUESTIONS (5 MARKS)

1. Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept. [5 Marks]

Answer:
1. In the absence of a nearby magnet, the compass needle aligns with the Earth's magnetic field, which points roughly north to south.
2. When a bar magnet or a current carrying loop is brought near, it produces its own magnetic field, and the needle turns to align with the resultant of the two fields.
3. Salient features of magnetic field lines: they are closed curves that run from the north pole to the south pole outside a magnet.
4. They never intersect each other, and they are closer together where the field is stronger.
5. The tangent to a field line at any point gives the direction of the magnetic field at that point.

Teacher's Note:
a) The needle deflects because it is a small magnet that aligns with the field where it is placed.
b) Common mistake: thinking that the needle is attracted to the magnet; it turns to align with the field.
c) Field lines inside a bar magnet run from its south pole to its north pole.
d) A stronger field is shown by lines that are packed more closely together.

2. With the help of a labeled circuit diagram, illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find the direction of magnetic field associated with a current carrying conductor? [5 Marks]

Answer:
1. Connect a straight long copper wire in a circuit with a battery, a plug key, a rheostat and an ammeter in series.
2. [Figure: Labelled circuit diagram with battery, plug key, rheostat and ammeter in series with a straight long wire passing through a cardboard, showing concentric circular field lines around the wire; diagram to be added]
3. The field lines are concentric circles with their centres on the wire, and they lie in planes perpendicular to the wire.
4. The right hand thumb rule: hold the wire in the right hand with the thumb pointing along the direction of the current.
5. The curled fingers of the right hand then point in the direction of the magnetic field lines around the wire.

Teacher's Note:
a) The thumb rule works for the direction of the field only, not for the force on the wire.
b) Common mistake: pointing the thumb against the current, which reverses the field direction.
c) Reversing the current reverses the direction of the field lines.
d) The field at a given distance from the wire is the same all around it, which gives circular lines.

3. Explain with the help of a labeled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has 'n' turn, the field produced at any point is n times as large as that produced by a single turn? [5 Marks]

Answer:
1. A current through a circular loop produces circular field lines around each part of the wire.
2. At the centre of the loop the field lines are straight and perpendicular to the plane of the loop.
3. [Figure: Labelled circular loop carrying current with field lines crowded near the wire and straight at the centre; diagram to be added]
4. Each turn of a coil produces a field of the same direction at a given point, because the current flows the same way in every turn.
5. So the fields from all n turns add up, and the total field at any point is n times the field produced by a single turn.

Teacher's Note:
a) Make sure the field from each turn points in the same direction before adding them.
b) The field at the centre is given by B = \( \frac{\mu_0 N I}{2r} \), which shows the proportionality to N, the number of turns.
c) Common mistake: forgetting that the field is proportional to the current as well as the number of turns.
d) Increasing the number of turns is an easy way to strengthen the field at the centre.

4. (a) State the factors on which the strength of an electromagnet depends. (b) How does an electromagnet differ from a bar magnet or permanent magnet? [5 Marks]

Answer:
1. (a) The strength of an electromagnet depends on the number of turns in the coil, the magnitude of the current, the nature of the core material and the shape of the core.
2. (b) An electromagnet is a temporary magnet, and it loses its magnetism when the current is switched off.
3. A bar magnet is a permanent magnet, which keeps its magnetism for a long time without any current.
4. The strength of an electromagnet can be controlled by changing the current, while the strength of a bar magnet is fixed.

Teacher's Note:
a) A soft iron core is used in an electromagnet because it loses magnetism quickly.
b) Electromagnets can be switched on and off, which makes them useful in relays and electric bells.
c) Permanent magnets are usually made of steel, which retains magnetism for a long time.
d) A larger current makes the electromagnet stronger, up to the point where the core becomes saturated.

5. How will you experimentally show that a current carrying conductor experiences a force when kept in a magnetic field? [5 Marks]

Answer:
1. Place a horseshoe magnet on a table so that its poles are on either side of a light aluminium rod suspended horizontally between them.
2. Connect the rod in a circuit with a battery, a plug key and a rheostat, and pass current through it; the rod moves, showing that a force acts on it.
3. Reverse the direction of current; the rod moves in the opposite direction, which shows that the force depends on the direction of current.
4. Reverse the magnetic poles; the rod again moves in the opposite direction, showing that the force also depends on the direction of the field.
5. The direction of the force is given by Fleming's left hand rule, with the forefinger along the field, the middle finger along the current and the thumb along the force.

Teacher's Note:
a) Use a light rod so that the movement is clearly visible.
b) Common mistake: placing the magnet with its poles in the same direction as the current, which gives no motion.
c) The force is maximum when the rod is perpendicular to the field.
d) Keep the rod horizontal so that the force acts in a horizontal plane.

6. What is the principle of an electric motor? Briefly explain the construction and working of an electric motor using a labeled diagram. State the factors on which the strength of a motor depends. [5 Marks]

Answer:
1. The principle of an electric motor is that a current carrying coil placed in a magnetic field experiences a torque, which makes it rotate.
2. Construction: a rectangular armature coil is placed between the poles of a strong magnet and is connected to a split ring commutator, which touches two carbon brushes connected to the battery.
3. Working: when current flows, the two sides of the coil experience forces in opposite directions, which turn the coil.
4. The split ring commutator reverses the direction of current in the coil every half rotation, so the coil keeps rotating in the same direction.
5. [Figure: Labelled diagram of a DC motor with armature, magnet poles N and S, split ring commutator, carbon brushes and battery; diagram to be added]
6. The strength of the motor depends on the strength of the magnetic field, the current, the number of turns in the coil and the area of the coil.

Teacher's Note:
a) The commutator is the key part that keeps the coil turning in one direction.
b) Common mistake: using slip rings instead of a split ring commutator in a DC motor.
c) Fleming's left hand rule gives the direction of the force on each side of the coil.
d) Adding more turns or a stronger magnet increases the turning effect.

7. What is meant by electromagnetic inductions? How will you demonstrate this phenomenon with the help of an experiment? State the factors on which the strength the induced current depends. [5 Marks]

Answer:
1. Electromagnetic induction is the phenomenon in which an emf and current are induced in a coil when the magnetic field linked with it changes.
2. Connect a coil of many turns to a galvanometer, and move a bar magnet quickly into the coil; the galvanometer needle deflects, showing an induced current.
3. Moving the magnet out of the coil gives a deflection in the opposite direction, and keeping the magnet still gives no deflection.
4. The induced current depends on the speed at which the magnet is moved, which is the rate of change of magnetic field.
5. The induced current also depends on the number of turns in the coil and on the strength of the magnet.

Teacher's Note:
a) Faster movement of the magnet gives a larger deflection, which is a simple check of the rate factor.
b) Common mistake: holding the magnet still inside the coil, which gives no induced current.
c) The direction of induced current is given by Fleming's right hand rule.
d) A galvanometer shows both the presence and the direction of the induced current.

8. Briefly describe the principle, construction and working of an AC generator or dynamo. [5 Marks]

Answer:
1. The principle is electromagnetic induction: an emf is induced in a coil when it rotates in a magnetic field, because the magnetic flux linked with it changes.
2. Construction: an armature coil of many turns is mounted on a shaft, placed between the poles of a strong magnet, and connected to two slip rings that touch carbon brushes.
3. Working: when the coil is rotated by an external source, the flux linked with it changes continuously, so the induced emf and current reverse direction every half rotation.
4. The current flows to the external circuit through the slip rings and brushes, giving an alternating output.
5. [Figure: Labelled AC generator with armature coil, field magnets N and S, slip rings and brushes; diagram to be added]

Teacher's Note:
a) The frequency of the output depends on the speed of rotation of the coil.
b) Common mistake: using a split ring commutator in an AC generator, which would give direct current.
c) The output of a dynamo is maximum when the coil is parallel to the field.
d) An AC generator delivers its output through slip rings, not through a commutator.

9. (a) How are electrical installations carried out in a house? (b) What is the main function of electric fuse? Briefly explain it. [5 Marks]

Answer:
1. (a) Electricity from the mains enters the house through the main supply line, passes through an energy meter and a main switch, and reaches a distribution board.
2. From the distribution board separate circuits run to rooms, with live, neutral and earth wires; each circuit has its own fuse or circuit breaker, and each socket is connected in parallel.
3. (b) A fuse is a safety device that protects the circuit from excessive current; it melts when the current exceeds its rated value and breaks the circuit.
4. A fuse is made of tin-lead alloy with a low melting point and is always connected in series with the live wire.

Teacher's Note:
a) Fuses protect the wires from overheating and reduce the risk of fire.
b) A fuse must be replaced with one of the same rating, never with a wire of higher rating.
c) The earth wire is connected to the metal body of appliances for additional safety.
d) The earth wire protects the user, while the neutral wire completes the circuit.

10. What safety measures do you employ in the use of electricity? [5 Marks]

Answer:
1. Use insulated wires and plugs, and never touch bare wires or live terminals.
2. Do not overload a socket by connecting too many appliances, and use fuses or circuit breaker of the correct rating.
3. Connect metal bodies of appliances to earth, and use three pin plugs for such appliances.
4. Never touch switches or appliances with wet hands, and switch off the main supply before any repair work.
5. Use only standard, ISI marked appliances and get faulty wiring checked by a qualified electrician.

Teacher's Note:
a) Water is a good conductor when it contains dissolved salts, so wet hands increase the risk of shock.
b) Common mistake: using a higher rated fuse to avoid frequent blowing, which removes the protection.
c) Rubber soled footwear gives some protection, but it is no substitute for switching off the supply.
d) Replace damaged cords and plugs at once rather than repairing them with insulating tape.

effects of electric current Sure Shot Questions A.