Magnetic Effect of Electric Current
NCERT Exemplar Problems
Short Answer Questions
Question 13: A magnetic compass needle is placed in the plane of paper near point A as shown in given figure. In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?
Answer: We know that when the magnetic field and the direction of current are perpendicular to each other, the deflection is maximum. But when they are in the same plane, no deflection takes place. So, the current carrying conductor needs to be placed in the same plane as the magnetic compass to attain no deflection.
Question 14: Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.
We know that for making an electromagnet we need to place a soft iron core inside a solenoid. Soft iron is a ferro-magnetic material and hence it augments the magnetic property of solenoid by thousands of times. Thus, for making a permanent electromagnet, we need to place a soft iron core inside a solenoid.
Question: 15: AB is a current carrying conductor in the plane of the paper as shown in given figure. What are the directions of magnetic fields produced by it at points P and Q? Given r1 > r2, where will the strength of the magnetic field be larger?
Answer: Applying the right hand thumb rule, the direction of magnetic field would be anti-clockwise around the direction of current. So, the magnetic field would be to point P and towards the plane of paper. At point Q, the direction of magnetic current would be from the conductor and away from the plane of paper. Since magnetic field is stronger near the conductor and weaker as we move away from the conductor, so the magnetic field would be stronger near point Q than near point P.
Question 16: A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason.
Answer: The strength of magnetic field varies directly as the magnitude of the electric current. So, in case of an increase in current the magnetic field would increase.
Question 17: It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.
Answer: It is a fact that in case of movement of a charged particle, magnetic field is created around the path on which the charged particle moves. Since alpha particles are positively charged so a magnetic field would be created around its path. But, as neutrons carry no charge so no magnetic field would be created around its path.
Question 18: What does the direction of thumb indicate in the right-hand thumb rule. In what way this rule is different from Fleming’s left-hand rule?
Answer: As per right hand thumb rule, the thumb shows the direction of electric current. The right hand thumb rule explains the magnetic field created due to a current carrying conductor. On the other hand, Fleming’s left hand rule explains effect of magnetic field on a current carrying conductor.
Question 19: Meena draws magnetic field lines of field close to the axis of a current carrying circular loop. As she moves away from the centre of the circular loop she observes that the lines keep on diverging. How will you explain her observation?
Answer: We know that the magnetic field is stronger near the current carrying conductor and tends to weaken as we move away from the conductor. In case of a current carrying circular loop, the magnetic field is stronger near the periphery but weaker near the centre of the loop. Due to this, the magnetic field lines appear as straight lines near the centre. As we move towards the periphery of the circular loop, the magnetic field lines appear to be diverging so that they can be circular around the wire of the loop.
Question 20: What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?
Answer: We know that magnetic field lines make loop around a magnet. The solenoid behaves like a magnet and due to this, the magnetic field lines diverge. Due to this, the magnetic field is strongest near the ends of the solenoid and the ends become the poles of the magnet thus formed.
Question 21: Name four appliances wherein an electric motor, a rotating device that converts electrical energy to mechanical energy, is used as an important component. In what respect motors are different from generators?
Answer: Table fan, CD player, vacuum cleaner and hand blender are four examples of appliances in which an electric motor is used as an important component. Electric motors convert electrical energy into mechanical energy, while generators do the opposite. Electric motor works on the concept of Fleming’s Left Hand Rule; while generator works on the concept of Fleming’s Right Hand Rule.
Question 22: What is the role of the two conducting stationary brushes in a simple electric motor?
Answer: They keep the two poles of the armature in contact with the power supply.
Question 23: What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second?
Answer: In case of AC, the direction of current keeps on changing at frequent intervals, while the direction of current always remains the same in case of DC. The AC in India changes its direction at the rate of 100 times in a second.
Question 24: What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating?
Answer: We know that an electric fuse is a simple device which breaks the current in case of an overload. A fuse; used in series with an electrical appliance; helps in protecting that appliance from potential damage which may happen due to overload. A fuse wire works because of its lower melting point which is possible because of its respective rating. If a fuse with larger rating is used with an appliance, the fuse wire shall not melt and hence would fail to serve the required purpose. Due to this, a fuse with defined rating should not be replaced by one with a larger rating.