Chapter 13. Magnetic Effects of Electric Current
Text-book Questions:
Page 224:
Q1. Why does a compass needle get deflected when brought near a bar magnet?
Ans: As compass needle is also act like a bar magnet. When a compass needle is placed near a bar magnet due to repulsive force between unlike poles and attraction between two unlike poles take place. The compass needle gets deflected.
Page 228:
Q1. Draw magnetic field lines around a bar magnet.
Ans: The magnetic field lines are as shown in the figure.
Q2. List the properties of magnetic lines of force.
Ans: The properties of magnetic lines of force are as follows:
(i) The magentic field lines emerge from north pole and merge at the south pole.
(ii) Inside the magnet, the direction of field lines is from its south pole to its north pole.
(iii) The magnetic field lines are closed curves.
(iv) The magnetic field is stronger, where the field lines are crowded.
(v) Two field lines do not intersect/cross each other.
Q3. Why don’t two magnetic lines of force intersect each other?
Ans: This is so, magnetic field lines do not intersect each other. If they did, it would mean that at the point of intersection, the compass needle would point
towards two directions, which is not possible.
Page 229:
Q1. 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: According to right-hand rule, we observe that inside the loop, the magnetic field lines are directed perpendicular to the plane of paper in the inward direction. Outside the loop magnetic field lines are directed out of the plane paper.
Q2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Ans: When the magnetic field in a region is uniform, then the lines are parallel to each other, at the same distance and of equal length.
uniform magnetic field lines 
Q3. Choose the correct option.
The magnetic field 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.
Page 231:
Q1. 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: (c) Velocity and (d) momentum
Q2. In Activity 13.7, 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) O increasing the current in rod AB its displacement will increase.
(ii) If stronger horse-shoe magnet is used then the displacement of rod AB will increase.
(iii) If length of the rod is increased, force acting on it will increase and hence, displacement of the rod increases.
Q3. 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: According to the right hand thumb rule the direction of magnetic field will be vetically (d) upward.
Page 233:
Q1. State Fleming’s left-hand rule.
Ans: Fleming’s left-hand rule:
According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. 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.
Q2. What is the principle of an electric motor?
Ans: An electric motor is a rotating device that converts electrical energy to
mechanical energy. It is based on the principle in which a current carrying conductor experiences a force when placed in a magnetic field. If the direction of the magnetic field and that of the direction of current which pass through the magnetic field are mutually perpendicular then the direction of the force is given by Fleming’s left-hand rule.
Q3. What is the role of the split ring in an electric motor?
Ans: The split ring in a electric motor works as a commutator. It reverses the flowing of current in moter after a half rotation. Which gives rise to a contunuous roation of the coil and the axle.
Page 236:
Q1. Explain different ways to induce current in a coil.
Ans:
(a) Moving the coil in a magnetic field.
(b) By changing the magnetic field lines around the coil.
(c) It is convenient in most situations to move the coil in a magnetic field.
(d) The motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced electric current in the circuit.
Page 237:
Q1. State the principle of an electric generator.
Ans: The principle of electric generator is based on electromagnetic induction. When a rectangular coil is rotated in a uniform magnetic field, it produces induced electricity at the ends of the coil. This is the principle of the electric generator.
Q2. Name some sources of direct current.
Ans: Cell, battery and D.C generator or dynamo etc. are the sources of direct current.
Q3. Which sources produce alternating current?
Ans: A.C. Generators and Inverters etc. which generate alternating current.
Q4. Choose the correct option.
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 |
Ans: (c) half revolution
Page 238:
Q1. Name two safety measures commonly used in electric circuits and appliances.
Ans: he names of two safety measures commonly used in electric circuits and appliances are -
(i) use of fuse and
(ii) Use of ground contact wire.
Q2. 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: Power of electric tandoor (P) = 2 kW = 2000 W
Thus, the current drawn by the electric tandoor (I) = P/V
= 2000/220 = 9 A (approx.)
Whereas the current rating of the electric circuit is 5 A and the tandoor draws current at the rate of 9 A, so this circuit will break due to overheating or overloading.
Q3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Ans: The following precautions should be taken to avoid overloading in domestic electrical circuits.
(i) Not more than one device should be connected to a single socket.
(ii) Always connect the electrical equipment in the circuit in parallel.
(iii) Suitable fuse should be used in the circuit.
(iv) Multiple electrical appliances should not be used at the same time to avoid overloading.
