NCERT Solution Exercise
Q1. Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
Ans: (d) The field consists of concentric circles centred on the wire.
Q2. The phenomenon of electromagnetic induction is
(a) the process of charging a body.
(b) the process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.
Ans: (c) producing induced current in a coil due to relative motion between a magnet and the coil.
Q3. The device used for producing electric current is called a
(a) generator.
(b) galvanometer.
(c) ammeter.
(d) motor.
Ans: (a) generator.
Q4. The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Ans: (d) AC generator has slip rings while the DC generator has a commutator.
Q5. 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.
Q6. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
Ans: (a) Electric motor converts mechanical energy into electrical energy. (Wrong)
(b) Electric generator works on the principle of electromagnetic induction. (Correct)
(c) The magnetic field at the center of a long circular current carrying coil is parallel straight field lines. (Correct)
(d) The green insulating wire is usually an insulating wire. (Wrong)
Q7. List three sources of magnetic fields.
Ans: (a) A natural magnet has a magnetic field around it.
(b) A current carrying straight conductor has a magnetic field around it.
(c) A current carrying solenoid has a magnetic field around it.
Q8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.
Ans: A cylinder-shaped coil with several turns of insulating copper wire wrapped around it is called a solenoid. One end of the current-carrying solenoid acts as the South Pole and the other end as the North Pole. The magnetic field lines inside the solenoid are parallel to each other. This means that the electric field at the center of the solenoid is maximum and is the same everywhere.
Yes, the north pole and south pole of the solenoid can be identified with a compass. If the north pole of the compass needle is attracted towards the solenoid. So this end is the South Pole. Similarly identify the South Pole.
Q9. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Ans:
Q10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Ans: The magnetic field will be from the direction perpendicular to the plane in which the flow and force of electrons are perpendicular to each other.
Q11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor? Q12. Name some devices in which electric motors are used.
Ans:
Q12. Name some devices in which electric motors are used.
Ans: Electric motors are used in coolers, fans, air conditioners, pumps etc.
Q13. 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) withdrawn from inside the coil.
(iii) held stationary inside the coil?
Ans: (i) In this case induced current is generated in the coil.
(a) If the north pole is pushed into the coil, then the direction of current in the coil is counter-clockwise.
(b) If the south pole is pushed in the coil, then the direction of current in the coil is clockwise.
(ii) If you take out the South Pole magnet in the coil, then the current in the coil will be in anticlockwise direction and if you take out the North Pole, then the current in the coil will be generated in the clockwise direction.
(iii) In this case no current is generated in the coil.
Q14. Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Ans: Yes, induced current will be generated.
Due to change of current in coil A, current in B is induced due to change in the number of magnetic field lines passing through A.
Q15. State the rule to determine the direction of a
(i) magnetic field produced around a straight conductor-carrying current. (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
(iii) current induced in a coil due to its rotation in a magnetic field.
Ans: (i) The direction of the magnetic field produced around a current carrying straight conductor is determined by Maxwell's right-hand rule.
Maxwell's right-hand rule:- If the right hand is holding a current-carrying conductor in such a way that the thumb points in the direction of the current flowing through the conductor, then the fingers holding the conductor are in the direction of the magnetic field.
(ii) The direction of force on a current carrying conductor in a magnetic field is determined by Fleming's left hand law.
Fleming left hand rule: - If the first three fingers of the left hand are placed perpendicular to each other in such a way that the index finger is in the direction of the magnetic field and the middle finger is in the direction of the current, then the direction of the thumb shows the direction of the force acting on the conductor. |
(iii) Fleming's right hand law is used to find the direction of the induced current produced in a conductor moving in a magnetic field.
Fleming's right hand rule: - According to this rule, if the first three fingers of the right hand are kept perpendicular to each other in such a way that the index finger shows the direction of magnetic field and the thumb shows the direction of motion in the conductor, then the amount of induced current in the conductor will be Direction is indicated by Madhyama.
Q16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Ans:
Q17. When does an electric short circuit occur?
Ans: When the neutral wire and neutral wire come in contact with each other in a domestic electric circuit, the value of current in the circuit becomes very high. This phenomenon is called short circuit.
Q18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans: When the neutral wire and neutral wire come in contact with each other in a domestic electric circuit, the value of current in the circuit becomes very high. This phenomenon is called short circuit.
