Chapter 12: Magnetic Effects of Electric Current

Multiple Choice Questions

1. Choose the incorrect statement from the following regarding magnetic lines of field

(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points

(b) Magnetic field lines are closed curves

(c) If magnetic field lines are parallel and equidistant, they represent zero field strength

(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines

Answer: (c) If magnetic field lines are parallel and equidistant, they represent zero field strength.

[ Parallel and equidistant magnetic field lines represent a uniform magnetic field, not zero field strength, so the statement is incorrect.]

2. If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

  

(a) concentric circles

(b) elliptical in shape

(c) straight lines parallel to each other

(d) concentric circles near the point O but of elliptical shapes as we go away from it

Answer: (a) concentric circles

[ The magnetic field lines over the horizontal plane ABCD would form concentric circles. This occurs when a current-carrying conductor is in the shape of a loop, creating a circular magnetic field pattern.]

3. A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of paper and on the axis of the coil) is anti clockwise and clockwise respectively. The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to

  

(a) A        (b) B

(c) A if the current is small, and B if the current is large

(d) B if the current is small and A if the current is large

Answer:  (a) A

[ The current flows anticlockwise from A and clockwise from B, meaning the magnetic field lines emerge from the face near A (acting as N-pole) and enter near B. Thus, N-pole is close to A.]

4. For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is

(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid

(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil

(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet

(d) The N- and S-poles exchange position when the direction of current through the solenoid is reversed

Answer: (c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet.

[ The magnetic field pattern around a solenoid is similar to that around a bar magnet, not different. Both have distinct north and south poles with similar field lines.]

5. A uniform magnetic field exists in the plane of paper pointing from left to right as shown in Figure 13.3. In the field an electron and a proton move as shown. The electron and the proton
experience

          
(a) forces both pointing into the plane of paper
(b) forces both pointing out of the plane of paper
(c) forces pointing into the plane of paper and out of the plane of paper, respectively
(d) force pointing opposite and along the direction of the uniform magnetic field respectively

Answer: (a) forces both pointing into the plane of paper.

[ Using Fleming’s left-hand rule, the proton moving upward in a magnetic field towards right experiences force into the plane of paper. Since electron has negative charge, its force direction also becomes into the plane of paper.]

6. Commercial electric motors do not use

(a) an electromagnet to rotate the armature

(b) effectively large number of turns of conducting wire in the current carrying coil

(c) a permanent magnet to rotate the armature

(d) a soft iron core on which the coil is wound

Answer: (c) a permanent magnet to rotate the armature.

[ Commercial motors use electromagnets (not permanent magnets) to produce a strong magnetic field, allowing better control and higher efficiency.]

7. In the arrangement shown in Figure 13.4 there are two coils wound on a non-conducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then

      

(a) the deflection in the galvanometer remains zero throughout

(b) there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed

 (c) there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction

(d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions .

Answer: (d) There are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions.

[ When the key is inserted or removed, induced currents are produced in the coils, resulting in momentary galvanometer deflections in opposite directions.]

8. Choose the incorrect statement

(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current

(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors

(c) The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically

(d) In India, the AC changes direction after every second .

Answer:  (d) In India, the AC changes direction after every second .

[ In India, AC frequency is 50 Hz, meaning it completes 50 cycles per second. One cycle includes two direction changes, so direction changes 100 times per second (every 1/100 second), not every 1/50 second.]

9. A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in Figure 13.5. The direction of magnetic field at a point will be North to South

            

(a) directly above the wire

(b) directly below the wire

(c) at a point located in the plane of the paper, on the north side of the wire

(d) at a point located in the plane of the paper, on the south side of the wire

Answer: (b) Directly below the wire.

[ Using the right-hand thumb rule (thumb along current east to west), the magnetic field circles the wire. Below the wire, field direction is north to south.]

10. The strength of magnetic field inside a long current carrying straight solenoid is

(a) more at the ends than at the centre

(b) minimum in the middle

(c) same at all points

(d) found to increase from one end to the other

Answer: (c) Same at all points.

[  In a long current-carrying straight solenoid, the magnetic field strength is uniform inside and the same at all points.]

11. To convert an AC generator into DC generator

(a) split-ring type commutator must be used

(b) slip rings and brushes must be used

(c) a stronger magnetic field has to be used

(d) a rectangular wire loop has to be used

Answer: (a) split-ring type commutator must be used

[ A split-ring commutator converts the alternating current produced in the coil into direct current by reversing the connections every half rotation, giving unidirectional current.]

12. The most important safety method used for protecting home appliances from short circuiting or overloading is

(a) earthing                    (b) use of fuse

(c) use of stabilizers       (d) use of electric meter

Answer: (b) Use of fuse.

[ A fuse melts and breaks the circuit when excessive current flows due to short circuit or overloading, preventing damage to appliances and reducing fire risk.]

Short Answer Questions :

13. A magnetic compass needle is placed in the plane of paper near point A as shown in Figure 13.6. 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: The straight conductor should be placed in the plane of the paper, in the same plane as the compass needle, passing through A.

The deflection is maximum when the conductor is placed perpendicular to the plane of the paper through A because the magnetic field produced is perpendicular to the plane of the needle and exerts maximum force on it.

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.

Answer:  A permanent magnet can be obtained from a current-carrying solenoid when:

(a) A steel rod is placed inside the solenoid because steel retains magnetism for a long time.

(b) A strong direct current is passed through the solenoid for some time.

(c) The solenoid has a large number of turns of insulated copper wire.

When current flows through the solenoid, the steel rod gets magnetised. After switching off the current, the steel rod retains its magnetism and behaves like a permanent magnet.

Photo

15. AB is a current carrying conductor in the plane of the paper as shown in Figure 13.7. What are the directions of magnetic fields produced by it at points P and Q? Given , where will the strength of the magnetic field be larger ?

         

Answer : The magnetic field produced by the current-carrying conductor AB at points P and Q can be determined using the right-hand thumb rule.

At point P, the magnetic field will be directed into the plane of the paper.

At point Q, the magnetic field will be directed out of the plane of the paper.

Since  ​, the magnetic field strength will be larger at point Q because the magnetic field strength decreases with increasing distance from the conductor.

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: When the current in the wire is increased, the deflection of the magnetic compass also increases. This happens because the magnetic field produced around a current-carrying wire becomes stronger when the current increases. A stronger magnetic field exerts a greater force on the compass needle, causing a larger deflection.

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: (i) A thin beam of alpha particles produces a magnetic field around it because alpha particles are positively charged and moving charges constitute electric current. Hence, a magnetic field is produced.

(ii) A thin beam of neutrons does not produce a magnetic field because neutrons have no electric charge. Since there is no flow of charge, no electric current is produced and therefore no magnetic field is formed.

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:  In the right-hand thumb rule, the direction of the thumb indicates the direction of current flowing through the conductor. The curled fingers show the direction of the magnetic field around the conductor.

This rule is different from Fleming’s left-hand rule because:

(a) The right-hand thumb rule is used to find the direction of the magnetic field produced by a current-carrying conductor.

(b) Fleming’s left-hand rule is used to find the direction of force or motion experienced by a current-carrying conductor placed in a magnetic field.

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: The magnetic field lines near the centre of a current-carrying circular loop are close to each other, showing that the magnetic field is strong there. As Meena moves away from the centre of the loop, the field lines diverge because the strength of the magnetic field decreases with distance from the loop. Thus, the spreading of magnetic field lines indicates that the magnetic field becomes weaker away from the centre of the circular loop.

20. What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?

Answer: The divergence of magnetic field lines near the ends of a current-carrying solenoid indicates that the magnetic field becomes weaker at the ends. The field lines spread out and are less crowded there, showing a decrease in magnetic field strength compared to the centre of the solenoid.

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 : not in Syllabus

22. What is the role of the two conducting stationary brushes in a simple electric motor?

Answer : not in Syllabus .

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: The difference between a direct current and an alternating current is :

Direct Current (DC)	Alternating Current (AC)
It flows only in one direction.	It changes direction periodically.
Its magnitude remains constant.	Its magnitude changes continuously with time.
It is obtained from cells and batteries.	It is supplied to homes and industries.

In India, AC has a frequency of 50 Hz . Since one cycle involves the current changing direction two times, the AC changes direction 100 times per second.

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: A fuse protects an electrical appliance and the circuit from damage due to excessive current or short circuit. When the current exceeds the safe limit, the fuse wire melts and breaks the circuit.

A fuse with a larger rating should not be used because it may not melt at the required current, causing overheating and possible damage to the appliance or fire hazard.

Long Answer Questions

25. Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.

Answer: A magnetic compass needle normally points in the North–South direction due to the Earth’s magnetic field. When a bar magnet or a current-carrying loop is brought near it, the magnetic field produced by them interacts with the compass needle. As a result, the needle gets deflected from its original direction.

Salient features of magnetic field lines:

(a) Magnetic field lines emerge from the North pole and enter the South pole outside the magnet.

(b) Inside the magnet, they move from South pole to North pole, forming closed curves.

(c) The tangent at any point on a field line gives the direction of the magnetic field.

(d) Magnetic field lines are closer where the field is stronger.

(e) No two magnetic field lines intersect each other.

26. With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find direction of magnetic field associated with a current carrying conductor ?

Answer:    Photo

The magnetic field lines around a current-carrying straight conductor are concentric circles centred on the wire. The direction of the field lines depends on the direction of current.

Right-hand thumb rule: According to the right-hand thumb rule, if we hold the straight conductor in the right hand such that the thumb points in the direction of current, then the curled fingers give the direction of the magnetic field lines around the conductor.

27. Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has n turns the field produced at any point is n times as large as that produced by a single turn ?

Answer: Magnetic Field due to a Current through a Circular Loop :

The magnetic field lines around a current-carrying circular loop are circular near the wire. At the centre of the loop, the field lines become almost straight and parallel, showing that the magnetic field is strong and uniform there. The magnetic field is maximum at the centre of the loop.

If a current-carrying coil has nnn turns, the magnetic field produced at any point becomes nnn times that produced by a single turn because the magnetic field due to each turn acts in the same direction. Hence, the individual magnetic fields add together and strengthen the total magnetic field.

28. Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?

Answer :  Activity to Show Force on a Current-Carrying Conductor in a Magnetic Field:

Setup: Take a straight, horizontal current-carrying conductor (like a copper wire) and suspend it between the poles of a strong magnet, with the conductor perpendicular to the magnetic field.

Observation: Connect the conductor to a power source so that current flows through it. When the current is switched on, the conductor experiences a force and moves either up or down, perpendicular to both the magnetic field and the length of the conductor.

Explanation: This demonstrates that a current-carrying conductor experiences a force when placed in a magnetic field, and this force acts perpendicular to the length of the conductor and the direction of the magnetic field.

Fleming’s Left-Hand Rule:

Fleming’s left-hand rule helps to determine the direction of the force acting on the current-carrying conductor. According to the rule:

Thumb: Represents the direction of the Force (motion of the conductor).

Forefinger: Points in the direction of the magnetic Field.

Middle finger: Points in the direction of the Current.

When the fingers are arranged as per Fleming's left-hand rule, the thumb will indicate the direction of the force exerted on the conductor, helping to predict the movement of the conductor in the magnetic field.

29. Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are diffferent from commercial motors?

Answer : not in Syllabus

30. Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.

Answer : not in Syllabus

31. Describe the working of an AC generator with the help of a labeled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?

32. Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt out fuse should be replaced by another fuse of identical rating?

Answer :  Photo

A fuse is crucial for electrical safety as it protects circuits from overcurrent. By melting and breaking the circuit when the current exceeds a specified limit, a fuse prevents overheating, potential fires, and damage to electrical components. It serves as a simple yet effective safety device to safeguard both people and equipment.

 

A burnt-out fuse should be replaced with another of identical rating to ensure proper protection. Using a fuse with the same rating ensures it will blow at the same safe current level, preventing excessive current flow that could damage appliances or cause electrical fires.