Class 10 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.

[ This is incorrect because parallel and equidistant field lines indicate uniform field strength, not zero field strength.]

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 N-pole of the resultant magnet is on the face close to (a) A.

This is because the direction of the magnetic field lines follows the right-hand thumb rule, and the N-pole is determined by the direction of current flow.]

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.

[ This is incorrect. The magnetic field pattern inside a solenoid is similar to that around a bar magnet, with field lines running from the north pole to the south pole inside the solenoid.]

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 :  (a) an electromagnet to rotate the armature.

[ Commercial electric motors typically use a permanent magnet or a combination of permanent magnets and soft iron cores, but they do not rely on an electromagnet to rotate the armature. ]

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 : (d) force pointing opposite and along the direction of the uniform magnetic field.

[ The force experienced by a charged particle moving in a magnetic field is perpendicular to both the velocity of the particle and the direction of the magnetic field.]

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 150 second .

Answer : (d) In India, the AC changes direction after every 150 seconds.

[ This is incorrect. In India, as in many other places, the standard frequency of AC power is 50 hertz (Hz), which means it changes direction (alternates) 50 times per second, not every 150 seconds.]

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.

[ The direction of the magnetic field due to a current in a wire is concentric circles around the wire. Directly below the wire, the magnetic field points from 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.

[ To convert an AC generator into a DC generator, a split-ring commutator is employed. The commutator helps in changing the direction of the current in the coil, converting the alternating current to direct 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.

[ The most important safety method for protecting home appliances from short-circuiting or overloading is the use of a fuse. A fuse interrupts the circuit if there is excessive current, preventing damage to the appliances and the wiring. ]

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 current-carrying conductor should be placed in the plane of the paper, perpendicular to the compass needle. This orientation ensures that the magnetic field produced by the current does not affect the compass.

The deflection is maximum when the conductor is perpendicular to the plane of the magnetic needle because this orientation maximizes the magnetic field interaction with the needle.

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 electromagnet is obtained using a current-carrying solenoid when the core of the solenoid is made of a ferromagnetic material, such as iron, and the current is passed through the solenoid for a sufficiently long time. The magnetic domains in the ferromagnetic core align in the direction of the magnetic field created by the solenoid, and when the current is switched off, the core retains this alignment, creating a permanent magnet.

  

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 : The deflection of the magnetic compass will increase if the current in the wire is increased. This is because the magnetic field produced by a current-carrying wire is directly proportional to the magnitude of the current. As the current increases, the strength of the magnetic field around the wire becomes stronger, causing a greater interaction with the compass needle, which leads to 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 :  Yes, a magnetic field is produced around a thin beam of moving alpha particles but not around neutrons:

Alpha Particles: These are positively charged particles (consisting of 2 protons and 2 neutrons). When alpha particles move, they constitute an electric current, which generates a magnetic field around the beam, similar to a current-carrying conductor.

Neutrons: Neutrons are neutral particles with no electric charge. Since a magnetic field is produced by moving charges, a beam of neutrons does not generate a magnetic field, as there is no net charge to create such a field.

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 the current in a straight conductor, while the curled fingers show the direction of the magnetic field around the conductor.

Fleming’s left-hand rule, on the other hand, is used to determine the direction of force on a current-carrying conductor 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 : Meena's observation that magnetic field lines diverge as she moves away from the center of a current-carrying circular loop can be explained by the fact that:

(i) Near the Center: The magnetic field lines are nearly concentric circles around the loop and appear denser.

(ii) Farther Away: The field lines spread out and become less dense because the magnetic field strength decreases with distance from the loop. This spreading of lines indicates a reduction in field intensity as one moves away from the 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 straight solenoid indicates that the magnetic field is weaker in these regions compared to the central region of the solenoid. This occurs because the field lines spread out as they exit the solenoid, leading to a decrease in field strength near the ends.

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 :  Not in Syllabus .

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 : The role of a fuse in series with an electrical appliance is to protect the appliance and the circuit from excessive current, which could cause overheating or damage. The fuse is designed to "blow" or melt if the current exceeds a certain safe level, breaking the circuit and stopping the flow of electricity.

A fuse with a defined rating should not be replaced by one with a larger rating because the larger fuse would allow more current to flow before blowing, potentially causing damage to the appliance or creating a fire hazard by allowing the wires to overheat.

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, which points North and South in the absence of a nearby magnet, gets deflected when a bar magnet or a current-carrying loop is brought near it because the magnetic field produced by these objects interacts with the Earth's magnetic field. The compass needle aligns itself with the resultant magnetic field, causing deflection.

Salient features of the magnetic field lines concept:

(i) Magnetic field lines emerge from the North pole and enter the South pole of a magnet, indicating the direction of the magnetic field.

(ii) Magnetic field lines form closed loops, continuing inside the magnet from the South pole back to the North pole.

(iii) Magnetic field lines never intersect, as each point in space has a unique magnetic field direction.

(iv) The density of the field lines indicates the strength of the magnetic field; closer lines represent a stronger field.

(v) Magnetic materials can distort the field lines, concentrating them within the material.

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 : In a current-carrying straight long conducting wire, the magnetic field lines form concentric circles around the wire. The direction of these field lines can be determined using the right-hand thumb rule: if you grip the wire with your right hand so that your thumb points in the direction of the current, your fingers will curl in the direction of the magnetic field lines. The strength of the magnetic field decreases as the distance from the wire increases.

   

The right-hand thumb rule helps determine the direction of the magnetic field around a current-carrying conductor. According to this rule, if you point your right thumb in the direction of the electric current, the curl of your fingers around the conductor shows the direction of the magnetic field lines encircling the conductor. This rule is especially useful for visualizing the circular magnetic field pattern.

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 : When a current flows through a circular loop, the magnetic field lines form concentric circles around each segment of the loop. At the center of the loop, these circular lines merge, appearing as straight lines. The field is strongest at the center and weakens as we move away. In a coil with multiple turns, the magnetic field is enhanced, as each turn's field adds up in the same direction, amplifying the total magnetic field.

  

When a current-carrying coil has  turns, the magnetic field produced at any point is nnn times as large as that produced by a single turn because each turn of the coil contributes to the magnetic field at that point. The magnetic fields from all the individual turns add up in the same direction, reinforcing each other, thereby resulting in a total magnetic field that is  times stronger than the field produced by a single turn.

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?

Answer : Not in Syllabus .

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 : The schematic diagram of common domestic circuits :

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.