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Chapter 13 – Magnetic Effects of Electric Current

Magnetic Effects of Electric Current

NCERT IN-TEXT QUESTIONS SOLVED
1.  Why does a compass needle get deflected when brought near a bar magnet?
Ans. A compass needle is a small bar magnet with north and south pole. The compass needle deflects due to repulsion/attraction with the bar magnet.
2.  Draw magnetic field lines around a bar magnet.
Ans.
3.  List the properties of magnetic lines of force.
Ans. Properties of magnetic field lines of force:
      (i) Magnetic field lines travel from N-pole of magnet towards 5-pole, outside the magnet and from S-pole to N-pole inside the magnet.
      (ii) The strength of magnet/magnetic field is given by how close the field lines are if the field lines are closer magnet is stronger.
      (iii) No two magnetic field lines ever intersect each other.
4.  Why don’t two magnetic lines of force intersect each other?
Ans. The magnetic field lines never intersect each other as an intersection of the field lines means the magnetic field at that point has two directions, which is not possible because the resultant force on a pole (north/south) at any point can only be in one direction.
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.
Ans.
6.  The magnetic field in a given region is uniform. Draw a diagram to represent it.
Ans.
           The magnetic field in a given region is uniform can be represented by showing parallel lines at equidistance.
7.  Choose the correct option.
           The magnetic Meld 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.
Ans. (d) is the same at all points.
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
Ans. Velocity and momentum
9.  In activity 13.7 (on page 234) how do we think 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.
Ans. (i) If current in rod AB is increased its displacement will increase.
           (ii) If a stronger horse-shoe magnet is used then the displacement of rod AB will increase.
           (iii) If the length of the rod AB is increased force acting on it will increase and the displacement of the rod also increases.
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
           (c) downward
(d) upward
           Ans. (c) downward
11.  State Fleming’s left hand rule.
Ans. According to this rule, stretch the thumb, fore-finger and middle finger of left hand such that they are mutually perpendicular. If the middle finger points in the direction of the magnetic field, the fore-finger points in the direction of flow of current, then the thumb points in the direction of motion i.e., force acting on the conductor.
12.  What is the principle of an electric motor?
Ans. The principle of an electric motor is based on Fleming’s left hand rule. i.e., a current carrying conductor experiences a force when placed in a magnetic field.
13.  What is the role of the split ring in on electric motor?
Ans. In an electric motor split ring acts as a commutator. Split ring is connected to the ends of the coil and the direction of current flowing in motor coil reverses after every half turn. Which results in the continuous rotation of the coil and the ends.
14.  Explain different ways to induce current in a coil.
Ans. We can induce current in a coil
           (i) by moving a coil in a magnetic field or by changing the magnetic field around it.
           (ii) By increasing the number of coils the induced current value can be increased.
15.  State the principle of an electric generator:
Ans. In an electric generator mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
           Fleming’s right hand rule is used where motion and magnetic field is given and the current is induced.
16.  Name some sources of direct current.
Ans. Dynamo and battery cells.
17.  Which sources produce alternating current?
Ans. Dynamo or generators with split ring type commutator.
18.  Choose the correct option.
           A rectangular coil of copper wire is rotated in magnetic field. The direction of the induced current changes ones in each
           (a) two revolutions                      (b) one revolution
           (c) half revolution                        (d) one-fourth revolution
Ans. (c) half revolution
19.  Name two safety measures commonly used in electric circuits and appliances.
Ans. (i) Use earthing wiring for metallic body electrical appliances. (ii) Use miniature circuit breakers, (MCB) i.e., fuse
20.  An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V that has a current rating of 5 A. What result do you expect? Explain.
Ans. P = 2 kW ? 2000 W
           V = 220 V as , P = VI
           I =5A 2000 W = 220 × 5 2000 W ? 1100
           Here power and value of VI is not same that means the current required for the oven is more than 5 A that the circuit can supply.
           Hence due to overloading, the circuit can break or catch fire.
20.  What precautions should be taken to avoid the overloading of domestic electric circuits?
Ans. To avoid overloading following precautions should be taken:
           (i) Two separate circuits should be used one of 5 A current and other 15 A.
           (ii) For both 5 A and 15 A circuits, fuse should be installed.
           (iii) Parallel circuits should be used.
           (iv) Never use too many electrical appliances at one point.
QUESTIONS FROM NCERT TEXTBOOK
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.
Ans. (d) The field consists of concentric circles centred on the wire.
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.
Ans. (d) The process of rotating a coil of an electric motor.
3.  The device used for producing electric current is called a
       (a) generator           (h) galvanometer
       (c) ammeter:           (d) motor.
Ans. (a) generator
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 split rings while the DC generator has a commutator
Ans. (d) AC generator has split rings while the DC generator has a commutator.
5.  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.
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 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) False                      (b) True
           (c) True                        (d) False.
7.  List two methods of producing magnetic fields.
Ans. Magnetic field can be produced by any of the following methods:
       (i) Any magnet-bar magnet, horse-shoe magnet or round magnet can be used.
       (ii) A wire carrying current produces a field around it.
       (iii) A loop or solenoid carrying current.
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 liar magnet? Explain.
Ans. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid. One end of the solenoid behaves as a magnetic north pole, while the other end behaves as the south pole. The field lines inside the solenoid are in the form of parallel straight lines. By taking a bar-magnet with known north poles near one end of the solenoid and if it shows repulsion then that end of solenoid is north pole and the other end is south pole. The property of magnet i.e. like poles repel and unlike poles attract is used for the determination of poles of solenoid.
9.  When is the force experienced by a current carrying conductor placed in a magnetic field largest?
Ans. According to Flemings left hand rule the force experienced by a current carrying conductor placed in a magnetic field is largest when they both are perpendicular to each other.
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?
Ans. According to Fleming’s left hand rule, the direction of magnetic field is downwards.
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?
Ans.
       Principle: A current carrying conductor when placed at right angle to a magnetic field, experiences a force due to a which we get motion. The direction of the force is given by Fleming’s left hand rule.
       Working: Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through Y. The current flows from arm A to B arm and then C to D, the direction of flow of current in both arms is opposite. As per Fleming’s left hand rule, the force acting on arm AB pushes it down while the force acting on CD pushes it upwards. Thus, the coil and the axle rotate anticlockwise. Due to action of split ring commutator at half rotation, split rings P and Q change their contacts with brushes. Now P makes contact with Y and Q with X. As a result, current begins to flow in coil along DCBA, As a result now arm AB is being pushed upward and arm CD downward by the magnetic force. So coil rotate half a turn more in the same direction. This reversing of current direction is repeated at each half rotation and so the coil continues to rotate in the same direction. The split ring helps in changing the direction of the current.
12.  Name some devices in which electric motors are used.
Ans. Electric motor is used in all such devices where electric energy is used, converted into mechanical energy to get the motion of machine. E.g. it is used in electric fans, mixer grinders, coolers, A.C., washing machines, computers etc.
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?
Ans. (i) If bar magnet is pushed into the coil of insulated copper wire, galvanometer shows the deflection as current is induced in the coil.
       (ii) When the bar magnet is withdrawn from the coil the galvanometer shows deflection again but now to the opposite side.
       (iii) When the bar magnet is held stationary inside the coil, the galvanometer does not show any deflection no induced current.
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.
Ans. The current if changed in coil A. Some current will be induced in the coil B.
       Reason: When the current in coil A is changed, the magnetic field around it also changes. As the coil B is placed very close to the coil A, the magnetic field lines around B coil also changes due to this as current is induced in it.
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.
Ans. (i) To know the direction of magnetic field produced around a straight conductor-carrying current. Rule used is-Right hand thumb rule’.
       (ii) Fleming’s left hand rule is used to find the direction of force experienced by a current carrying straight conductor, when placed in a magnetic field, which is perpendicular to it.
       (iii) Fleming’s right hand rule is used to determine the direction of current induced in a coil due to its rotation in a magnetic field.
16.  Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Ans.
       Principle: An electric generator works on the principle of electromagnetic induction. When a coil is rotated between the magnet or when the magnet is rotated in and out of the coil the current is induced in the coil and the direction of current is given by Fleming’s right hand rule.
       Working: As shown in the figure, when the axle attached to the two rings is rotated such that arm AB moves up and arm CD moves down in the magnetic field produced by the permanent magnet i.e. ABCD rotates clockwise. By Fleming’s right hand rule, the induced currents are set up in these arms and flows in the direction ABCD, which flows from B2 to B.
       After half a rotation, arm CD starts moving up and AB moves down. As the directions of the induced currents in both the arms change current is induced in direction DCBA, which further flows from Bl to B2 externally. After every half rotation the polarity of the current in arms changes. Such current which changes direction after equal intervals of time is called an alternating current (AC).
       Brushes: are used to transmit current induced externally from coil ABCD to the external circuit.
17.  When does an electric short circuit occur?
Ans. When the insulation of a wire gets damaged and this naked wire comes in contact with other such wire i.e. live wire and neutral wire comes in contact, the current flowing in the circuit rises and short circuiting occurs.
18.  What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans. The earth wire is connected as a safety measure with all electrical appliances that have metallic body e.g., microwave, electric press, toaster, geyser, cooler, AC, etc. The earth wire provides a low resistance conducting path for electric current. If there is any leakage of current then the user would not get any current because the current flows down into the earth and keeps the potential of the appliance and earth same.
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