NCERT INTEXT QUESTIONS
Question 1. Why does a compass needle get deflected when brought near a bar magnet?
Sol. Compass needle is a tiny magnet, so due to force of repulsion or attraction between the poles of a magnet, there is a deflection in the compass needle.
Note: Like poles will repel while unlike poles of magnet attract each other.
Question 2. Draw magnetic field lines around a bar magnet.
Question 3. List the properties of magnetic lines of force.
Sol. Following are the given properties of magnetic lines of force:
(i) Magnetic field lines emerge from north-pole and enter south-pole.
(ii) The magnetic field strength is represented diagrammatically by the degree of closeness of the field lines.
(iii) Field lines cannot cross each other as two values of net field at a single point cannot exist. Only one value, a unique net value, can exist.
(iv) If in a given region, lines of field are shown to be parallel and equispaced, the field is understood to be uniform.
Question 4. Why don’t two magnetic lines of force intersect each other?
Sol. Because we cannot have two directions of the magnetic field at the same point.
Note: The direction of magnetic field is taken as the direction in which the north pole of a compass needle moves in it.
Question 5. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Question 6. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Sol. Magnetic field lines are parallel straight lines for a uniform field. The lines are closer for a strong field and for away for a weak one.
Question 7. Choose the correct option?
The magnetic field inside a long straight solenoid-carrying current
(a) is zero.
(b) decrease as we move towards its end.
(c) increases as we move towards its end.
(d) is the same at all points.
Sol. (d) The magnetic field inside a current- carrying solenoid is constant in magnitude and direction, and acts along the axis of the solenoid.
Question 8. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer).
(a) mass (b) speed (c) velocity (d) momentum
Sol. (c) and (d) Each moving charged particle in a magnetic field experiences a force. The direction of force experienced by a positive charge (i.e., a proton) is given by Fleming’s left hand rule. The force acting on the proton would change both velocity and momentum.
Question 9. How the displacement of rod AB will be affected if (i) current in rod AB is increased, (ii) a stronger horse-shoe magnet is used, and (iii) length of the rod AB is increased?
Sol. We know that when a current carrying conductor is kept in a magnetic field, the force acting on it is given by
(i) F oc I (I increases then force increases, hence, displacement is more)
(ii) F oc B (B increases then force increases, hence, displacement is more)
(iii) F oc L (L increases then force increases, hence, displacement is more)
Note: You can find here the direction of this force using Flemings Left hand rule.
Question 10. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a)towards south (b)towards east
Sol. (d) A positively charged particle projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is upward. Here the direction of positively charged alpha particle is towards west this implies the direction of current is also towards west and the direction of magnetic force is towards north. Hence, according to Fleming’s left hand rule, the direction of magnetic field will be upwards.
Question 11. State Fleming’s left hand rule.
Sol. Fleming’s left hand rule: According to this rule, stretch thumb, fore finger and central finger of left hand in such a way that these fingers are perpendicular to each other. If fore finger is placed in the direction of magnetic field and middle finger is in direction of current, then thumb will point towards the direction of force.
Question 12. What is the principle of an electric motor?
Sol. Electric motor is a device that converts electricity into mechanical energy. It works on the principle that when an insulated conductor coil is placed in a magnetic field and the current is passed through it, a force acts on the coil which rotates continuously.
Question 13. What is the role of the split ring in an electric motor?
Sol. The split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the arms of the coil. At this stage, the commutator reverses the direction of current.
Question 14. Explain the different ways to induce current in a coil.
Sol. The different ways to induce current in a coil are as follows:
(a) Either coil or a magnet should be in motion relative to each other.
(b) If there is a relative motion between a current carrying coil and a coil without current.
Question 15. State the principle of an electric generator.
Sol. An electric generator works on the principle of Faraday’s law of electromagnetic induction which states that whenever a current carrying conductor is placed in a magnetic field, then a flux is induces in the circuit due to which a current start to flow in the rotor also.
Question 16. Name some sources of direct current.
Sol. The source of direct current are:
(i) Electrochemical cells
(ii) Lead accumulator
(iii) D.C. generator
Question 17. Which sources produce alternating current?
Sol. A.C. generator, hydropower stations, etc., produce alternating current.
Question 18. Choose the correct options.
A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each
(a) two revolutions (b) one revolution
(c) half revolution (d) one-fourth revolution
Sol. (c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced current in the coil changes in every half rotation.
Question 19. Name two safety measures commonly used in electric circuits and appliances.
Sol. The two safety measures commonly used in electric circuits and appliances are as follows:
(i) A fuse of proper rating should be used to avoid damages due to short circuiting and overloading.
(ii) Earthing should be provided to avoid shocks when the live wire touches the body of the appliances.
Question 20. An electric oven of2 kW power rating is operated in a domestic circuit (220 V) that has a current rating of 5A. What result do you expect ? Explain.
Question 21. What precaution should be taken to avoid the overloading of domestic electric circuits?
Sol. We can avoid overloading by not connecting too many appliances to a single socket.
Question 1. 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 centered on the wire.
Sol. (d) The field consists of concentric circles centered on the wire according to Maxwell’s right hand thumb rule.
Note:This field decreases as we move away from the wire and at a point it increases if the current in the wire is increased.
Question 2. 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.
Sol. (c) Electro magnetic induction is the production of an electromotive force across an electrical conductor in a changing magnetic field. It assist in producing induced current in a coil due to relative motion between a magnet & the coil.
Note: The current in the coil stops the moment the motion of the magnet stops
Question 3. The device used for producing electric current is called a
Sol. (a) An electric generator converts mechanical energy into electricity and produces electric current. The electric generator works on the principle of “Faraday’s law” of electromagnetic induction.
Question 4. 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.
Sol. (d) AC generator has slip rings while the DC generator has a commutator.
Note: Slip ring ensures that the current produced by the AC generator flows in both direction.
Question 5. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) very continuously.
Sol. (c) At the time of short circuiting, the live wire and the neutral wire come into direct contact. As a result, the current in the circuit abruptly increases.
Question 6. 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 center 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.
Sol. (a) False. As an electric motor converts mechanical energy into electrical energy.
(d) False. The live wire is brown in colour and brings in the current.
Note: The electrical motor converts electrical energy into mechanical energy. This motor is used in various devices such as fans, washing machines etc.
Question 7. List three sources of magnetic fields.
Sol. The three sources of magnetic fields are:
(i) A natural magnet
(ii) A current carrying conductor
(iii) A solenoid
Question 8. 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.
Sol. A coil of many circular turns of insulated copper wire in the shape of a cylinder is called a solenoid. One end of a solenoid behaves like north pole and the other end as south pole. The field lines inside the solenoid are in the form of parallel lines. So, the field is uniform inside the solenoid.
Yes, if we bring the north pole of a bar magnet near one end of a current carrying solenoid and if it is attracted, then this end behaves as south pole & the other end is north pole.
Question 9. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Sol. We know that the force experienced by a current carrying conductor in a magnetic field is
F= BIL sin 0.
So, if 0 (i.e., angle between the direction of magnetic field and current carrying conductor) is maximum i.e., 90°, then the force is maximum.
Question 10. 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?
Sol. The magnetic field is perpendicular to the plane containing the beam of electrons and the force acting on it.
Question 11. .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?
Principle of electric motor: A current-carrying conductor, when placed in a magnetic field, experiences a force. If the direction of the field and that of the current are mutually perpendicular then force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. Due to this force the conductor begins to move.
Working of electric motor: Current in the coilABCD enters from the battery through conducting brush X and flows back to the battery through brush Y. The current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction ofcurrent through arm AB. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus, the coil and the axle 0, mounted free to turn about an axis, rotate anti-clockwise. At halfrotation, Q makes contact with the brush X and P with brush Y Therefore the current in the coil gets reversed and flows along the path DCBA. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a tum more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.
Function of split ring: It consists of a cylindrical metal ring split into two
halves S1 and S2. The two ends Aand D of the armature coil are connected to the split rings Sl and S2 respectively. As the coil rotates, the split rings also rotate about the same axis of rotation. The function of the split ring commutator is to reverse the direction of current in the coil after every half rotation.
Question 12. Name some devices in which electric motors are used.
Sol. The devices in which electric motors are used are as follows:
(a) Fan (b) Motor cycle
(c) Cooler (d) Air conditioner
Question 13. 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?
Sol. (i) When a bar magnet with its north pole facing the coil is moved towards the coil, the galvanometer shows a deflection.
(ii) When a bar magnet is moved away from the coil galvanometer again shows deflection but now in opposite direction.
(iii) When the bar magnet is stationary near the coil, no deflection in the galvanometer is observed.
Question 14. Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Sol. Yes, we know that larger the current, stronger will be the magnetic field. As there is change in current in coil A, there is a change in the number of magnetic field lines passing through coil Band hence, there is an induced current in coil B.
Note: The direction of induced current is given by Flemings right-hand rule.
Question 15. 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, and (iii) current induced in a coil due to its rotation in a magnetic field.
Sol. (i) Maxwell’s right hand thumb rule
(ii) Fleming’s left hand rule
(iii) Fleming’s right hand rule
Question 16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Sol. It is a device which converts mechanical energy into electrical energy.
Principle: It works on the principle of the electromagnetic induction, i.e., when there is a relative change in magnetic field linked with a coil, there is an induced current in the coil.
Working of an electric generator (AC dynamo)
(i) As the coil rotates, the two rings also rotate with the coil. The brushes P and Q always keep contact with the rotating ring.
(ii) When the coil starts rotating with the arm AB moving up and CD moving down (clockwise) cutting the magnetic lines, then according to Fleming’s right hand rule, a current is induced in these arms in the direction ABCD.
After a half rotation, arm CD moves down and AD moves up, hence the direction of current in each segment changes, giving rise to net induced current in the direction DCBA. Thus, after every rotation polrity of the current in the respective arm changes, thereby generating an alternating current.
Function of brushes: The function of brushes is to make contact with the rotating rings and through them to supply current to the coil.
Question 17. When does an electric short circuit occur?
Sol. Electric short circuit occurs when:
(i) a current of value more than its rating passes through a wire.
(ii) the live wire touches with the earth or neutral-wire.
(iii) the insulation of the wires is weak.
(iv) over load is provided in the circuit.
Question 18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Sol. The earth wire is a wire which connects a given appliance to the earth. It is used as a safety measure especially for electric appliances having a metallic body. Whenever the live wire touches the body of the appliance, the current passes to the earth and the user doesn’t suffer a severe electric shock.
Note: This earth wire provides a low resistance conducting path for electric current
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