Q. The strength of magnetic field inside a long current carrying straight solenoid is:

  • (a) more at the ends than at the centre.
  • (b) minimum in the middle.
  • (c) uniform at all points.
  • (d) found to increase from one end to the other.
  • Ans. (c) uniform at all points.
 Explanation :

Inside the solenoid magnetic field lines are straight. This indicates strong magnetic field. Hence, magnetic field is uniform at all points inside the solenoid.

Q. The direction of force acting on a current carrying conductor placed in a magnetic field can be obtained by:

(a) Fleming’s left hand rule.

(b) Fleming’s right hand rule.

(c) Clock face rule.

(d) Ampere’s swimming rule.

Ans. (a) Fleming’s left hand rule.

 Explanation :

The direction of force acting on a current carrying conductor placed in a magnetic field can be obtained by Fleming’s left hand rule. According to this rule, when a current-carrying conductor is placed in an external magnetic field, the conductor experiences a force perpendicular to both the field and to the direction of the current flow.

Q. The device used for producing electric current is called a :

(a) Generator

(b) Galvanometer

(c) Ammeter

(d) Motor

Ans. (a) Generator

 Explanation :

A.C. generator is used to convert mechanical energy to electrical energy.

Q. The essential difference between an A.C. generator and a D.C. generator is that:

(a) A.C. generator has an electromagnet while a D.C. generator has permanent magnet.

(b) D.C. generator will generate a higher voltage.

(c) A.C. generator will generate a higher voltage.

(d) A.C. generator has slip rings while the D.C. generator has a commutator.

Ans. (d) A.C. generator has slip rings while the D.C. generator has a commutator.

 Explanation :

Most essential difference between A.C. generator and D.C. generator is using slip ring in A.C. generator and commutator in D.C. generator.

Q. At the time of short circuit, the current in the circuit:

  • (a) Reduces substantially
  • (b) Does not change
  • (c) Increases heavily
  • (d) Vary continuously

Ans. (c) Increases heavily

 Explanation :

A short circuit occurs when circuit current rises rapidly and the electric circuit drown an heavy amount of current from the supply.

Q. State Fleming's Left Hand Rule.

Ans. According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular to each other. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Q. What is the principle of an electric motor?

Ans. Electric motor works on the principle that ‘when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously. Thus, when the coil rotates, the shaft attached to it also rotates converting the electrical energy supplied to the motor to the mechanical energy of rotation.

Q. What is the role of a split ring in an electric motor?

  • Ans. In an electric motor, after every half rotation the direction of coil gets reversed due to change in orientation of the magnetic field. To ensure a continuous rotation; a split ring is attached to the coil so that the polarity of the coil changes after every half rotation. This changes the direction of current and thus the armature keeps on rotating continuously.

Q. Why does a compass needle get deflected when brought near a bar magnet?

Ans. When a compass needle is brought near a bar magnet, the compass needle experiences a deflection due to the interaction of magnetic fields of the compass needle and the bar magnet.

Q. When does an electric short circuit occur?

Ans. When live and neutral wires touch each other, the resistance suddenly decreases and current increases. This leads to excessive heating of wire which manifests in the form of sparks. This is called short circuit.

Q. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

  • Ans. The earth wire transfers any leakage of electric current to the earth. The leaked current can otherwise reach the metallic body of an appliance and can lead to electric shock. Earth wire prevents from electric shock by safely transferring the leaked current to the earth.

Q. Answer the following questions:

(i) Name and state the rule to determine the polarity of the two faces of a current carrying circular loop.

(ii) State Fleming’s Left Hand Rule.

  • Ans. (i) Clock face rule is used to determine the polarity of the two faces of a current carrying circular loop. According to this rule, “If the current around the face of circular wire flows in the clockwise direction, then that face of the circular wire will be South pole (S-Pole) and if the current around the face of circular wire flows in the anticlockwise direction, then that face of the circular wire will be North pole (N-Pole).”
  • (ii) According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular to each other. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.
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Q. Answer the following questions:

(i) What is the principle of an electric motor?

(ii) What is the role of a split ring in an electric motor?

(iii) Define magnetism.

  • Ans. (i) Electric motor works on the principle that ‘when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously. Thus, when the coil rotates, the shaft attached to it also rotates converting the electrical energy supplied to the motor to the mechanical energy of rotation.
  • (ii) In an electric motor, after every half rotation the direction of coil gets reversed due to change in orientation of the magnetic field. To ensure a continuous rotation; a split ring is attached to the coil so that the polarity of the coil changes after every half rotation. This changes the direction of current and thus the armature keeps on rotating continuously
  • (iii) The property by virtue of which a magnet attracts certain metals such as iron, cobalt, nickel etc., is termed as magnetism.

Q. Two coils A and B are placed close to each other. If the current in coil A is changed, will some current be induced in the coil B ? Given reason.

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  • Ans. When we switch on current in coil A, it becomes an electromagnet and produces a magnetic field around coil B. So, an induced cur rent flows in coil B for a moment. When the current in coil A becomes steady, its magnetic field also becomes steady and the current in coil B stops. When we switch off the current in coil A, then the magnetic field in coil B stops quickly and in this case an induced current flows in coil B in the opposite direction.

Q. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

  • Ans. The force experienced by a current-carrying conductor placed in a magnetic field is largest when the current-carrying conductor is placed perpendicular to the direction of magnetic field.

Q. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clock-wise. Apply the right hand rule to find out the direction of the magnetic field inside and outside the loop.

  • Ans. Since, the current is flowing clockwise through a circular loop. The direction of magnetic field around the conductor can be found by using the right hand thumb rule. As the figure shows, the magnetic field would be towards the plane of the paper when it is inside the loop. On the other hand, the magnetic field would be away from the paper when it is outside the loop.

Q. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is:

  • (i) pushed into the coil ?
  • (ii) held stationary inside the coil ?
  • (iii) withdrawn from the coil ?
  • Ans. (i) When a bar magnet is pushed into the coil, a momentary deflection is observed in the galvanometer.
  • (ii) When the bar magnet is held stationary inside the coil, there is no deflection in the galvanometer.
  • (iii) When the bar magnet is withdrawn from the coil, the deflection in the galvanometer is in the opposite direction.

Q. Name some devices in which electric motors are used.

  • Ans. Electric fan, mixer grinder, tape recorder, CD player, hard disk drive, washing machine, cooler, toy car, vacuum cleaner, etc., are some devices in which electric motor is used.

Q. Explain different ways to induce current in a coil.

  • Ans. The different ways to induce current in a coil are :
  • 1. By moving the coil in a magnetic field.
  • 2. By changing the magnetic field around the coil.