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Magnetic Effect of Electric Current : NCERT Intext Questions

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Page 224

Q.1     Why does a compass needle get deflected when brought near a bar magnet?
Sol.      When a compass needle is brought near a bar magnet, the compass needle experiences a deflection. This happens because of interaction of magnetic fields of the compass needle and the bar magnet.

Page 228

Q.1     Draw magnetic field lines around a bar magnet.

Sol.     Magnetic field line

Q.2     List the properties of magnetic lines of force.
Sol.      Properties of magnetic field lines:

            (a) Magnetic field lines follow the direction from the north pole to the south pole.
            (b) Magnetic field lines always show concentric pattern.
            (c) Magnetic field lines do not cross one another.
            (d) Closer the field lines; stronger is the magnetic field and vice-versa is also true.
            (e) Magnetic field lines are closer near the poles; which shows greater strength of magnetic field near the poles.

Q.3     Why don’t two magnetic lines of force intersect each other?
Sol.      The direction of magnetic field lines is always from the north pole to the south pole. If the field lines would cross each other then the direction of field line would change at the point of crossing which is practically impossible. Hence, two magnetic field lines never intersect each other.

Page 229

Q.1      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.
Sol.      As the given figure shows; current is flowing clockwise through a circular loop. The direction of magnetic field around the conductor can be known 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. 2     The magnetic field in a given region is uniform. Draw a diagram to represent it.



Page 230

Q.1     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.
Sol.     (d) Is the same at all points.

Page 231

Q.1     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), i.e. velocity and mass of proton change when it enters a magnetic field. When a proton enters a magnetic field it starts moving on a circular path. Because of its movement along a circular path it attains angular momentum. We know that momentum is a product of mass and velocity.

Page 232

Q.2      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?
Sol.       In this case, the displacement would vary directly as the strength of current, strength of magnetic field and length of the conductor. Due to this, the displacement of conductor would be increased in all the three cases.

Q.3     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
Sol.     This question can be solved by using Fleming’s Left Hand Rule. We know that the direction of current is opposite to the direction of electron’s movement and hence it would be same as the direction of proton’s movement. So, the direction of current is towards west. As per Fleming’s Left Hand Rule; the middle finger shows the direction of current, the forefinger shows the direction of magnetic field and the thumb shows the direction of motion. Here, the deflection is towards north, i.e. in north westerly direction and hence, the direction of magnetic field would be towards north, i.e. upward.

Page 233

Q.1     State Fleming’s Left Hand Rule.
Sol.      Fleming’s Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions; then the index finger and middle finger of a stretched left hand show the direction of magnetic field and direction of electric current respectively and the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes.

Q.2     What is the principle of an electric motor?
Sol.     Principle of Electric Motor: The electric motor works on the principle of Fleming’s Left Hand Rule. When a rectangular coil is placed within a magnetic field and current is passed through the coil, there is deflection in the coil. The deflection changes into rotation of coil because of split ring commutator in the motor.

Q.3     What is the role of a split ring in an electric motor?
Sol.     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.

Page 236

Q.1     Explain different ways to induce current in a coil.
Sol.     For electromagnetic induction; the coil and the magnet should be in relative motion. This can be ensured by any of the following two ways:
           (a) The coil should be moved within a magnetic field.
           (b) The magnet should be moved and coil can be kept static.

Page 237

Q.1     State the principle of an electric generator.
Sol.     Electric Generator works on the principle of electromagnetic induction which obeys Fleming’s Right Hand Rule. When coil is moved inside a magnetic field a current is induced in the coil. The electric current is thus ‘generated’ by electric generator.

Q.2     Name some sources of direct current.
Sol.     Electrochemical cell, DC generator, photovoltaic cell, etc.

Q.3     Which sources produce alternating current?
Sol.     AC generator 

Q.4     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
Sol.     (c) Half revolution

Page 238

Q.1     Name two safety measures commonly used in electric circuits and appliances.
Sol.      Earth wire and electric fuse

Q.2     An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What results do you expect? Explain.
Sol.     The current drawn by the electric oven can be calculated using following formula:
           P = VI\,  Or , I = {P \over V}
           We have; P = 2kW = 2000 W and  V = 220 V
           So, I = {{2000w} \over {220V}} = 9.09A

           Here, the oven is drawing 9.09 ampere of current from a 5 ampere source.
           This means there would be an overload on the circuit. This can result in accidental fire in the circuit.

Q.3     What precaution should be taken to avoid the overloading of domestic electric circuits?
Sol.     Precautions to avoid overloading of domestic electric circuit:

           (a) Avoid using too many appliances in a single line, i.e. from a single output.
           (b) Appliances should always be checked for potential faults and should be repaired in time.
           (c) Fuse or MCB of proper rating should be used.


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