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NCERT Exemplar Class 10 Science Chapter 13 Magnetic Effects of Electric Current Solutions

NCERT Exemplar Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current covers all the important questions and answers as well as advanced level questions. It helps in learning about the magnetic field and field lines, Magnetic field due to a current-carrying conductor, right-hand thumb rule, circular loop, solenoid and Force on a current-carrying conductor in a magnetic field.

The NCERT Exemplar solutions for class 10 science is very important for board exams. NCERT Exemplar Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current is provided by our experts. They prepared the best solutions which help the students in understanding the solutions in an easy way. This chapters also covers the other topics like electric motor, electromagnetic induction, Fleming's right-hand rule, electric generator, electric power and domestic electric circuits.


Chapter Name

Chapter 13 Magnetic Effects of Electric Current

Book Title

NCERT Exemplar for Class 10 Science

Related Study

  • NCERT Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • Revision Notes for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • MCQ for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • Important Questions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current

Topics Covered

  • MCQ
  • Short Answers Questions
  • Long Answers Questions

NCERT Exemplar Solutions for Chapter 13 Magnetic Effects of Electric Current Class 10 Science

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

Solution

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

Parallel lines of magnetic field represent uniform magnetic field.


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

Solution

(a) concentric circles

If key is taken out then current will stop and no magnetic field exists due to the conductor. Therefore, at the point O, there will be earth’s magnetic field and they will be straight lines parallel to each other.


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

Solution

(a) A

The direction of magnetic field line is from south to north pole. As the field lines are pointing form B to A, point A is indicating the north pole.


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

Solution

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

A solenoid behaves like a bar magnet, because the pattern of the magnetic field lines associated with a solenoid are similar to the pattern of the magnetic field lines around a bar magnet.


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.

Solution

(a) forces both pointing into the plane of paper 

The direction of movement of electron is opposite to the direction of flow of electric current. This makes the current flow upwards. If the index finger shows the direction of magnetic field, ring finger shows the direction of current then the direction of thumb is in to 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

Solution

(c) a permanent magnet to rotate the armature

In electric motors electromagnet is used instead of permanent magnet.


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.

Solution

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

When key is plugged in, the galvanometer shows deflection in one direction and the direction of deflection reverses if the key is unplugged.


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 1/50 second

Solution

(d) In India, the AC changes direction after every 1/50 second

AC frequency in India is 50 Hz. As the direction changes twice in each cycle so that change of direction takes place after every 1/00 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

Solution

(b) directly below the wire

Using the right hand thumb rule, it can be found that the direction of magnetic field is from North to South below the wire.


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

Solution

(c) same at all points

Magnetic field lines are straight inside the solenoid. This indicates strong magnetic field. Therefore, inside the solenoid the magnetic field is uniform 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

Solution

(a) split-ring type commutator must be used

After each half turn, the split-ring type commutator reverse the direction of current. This maintains a DC 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

Solution

(b) use of fuse

Fuse has a thin wire made of tin and lead in the ratio of 75: 25%. The fuse wire melts and breaks the circuit when current exceeds specified limit thereby protecting home appliances.


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?

Solution

The current carrying conductor is placed in the plane of the paper itself. This is because the axis of the compass is vertical and the field due to the conductor is also vertical. The deflection is maximum when the conductor passes through A in a plane perpendicular to the plane of paper. This is because the field is maximum in the plane of the paper.


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.

Solution

Conditions in which a permanent electromagnet is obtained if a current carrying solenoid is used are:

  1. Circuit should be closed.
  2. (A core of soft iron should be used.


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 r1 > r2, where will the strength of the magnetic field be larger?

Solution

At P into the plane of the paper and out of it at Q. The strength of the magnetic field is larger at Q as it is located closer.


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.

Solution 

The deflection of the magnetic compass increases because the strength of magnetic field is directly proportional to the magnitude of current passing through the straight conductor.


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.

Solution

(i) As alpha particles are positively charged, they constitutes current in the direction of motion.

(ii) Neutrons do not conduct electricity as the have no charge on them.


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?

Solution

The thumb indicates the direction of current in the straight conductor held by curled fingers, whereas the Fleming’s left-hand rule gives the direction of force experienced by current carrying conductor placed in an external 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.

Solution

As the distance increases, the strength of the magnetic field falls. This is indicated by the decrease in degree of closeness of the lines of field.


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

Solution

The divergence of magnetic field lines indicates the decrease in strength of magnetic field near and beyond the ends 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?

Solution

Electric fans, mixers, washing machines, computer drives, etc. are devices that convert electrical energy into mechanical energy.

Generators convert mechanical energy into electrical energy.


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

Solution

The brushes are connected to the battery and hence they draw current and supply it to the armature of the coil.


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?

Solution

Direct current always flows in one direction whereas the alternating current reverses its direction periodically. The frequency of AC in India is 50 Hz and in each cycle it alters direction twice. Therefore AC changes direction 2×50 = 100 times in one 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?

Solution

Fuse is used for protecting appliances during short-circuiting or overloading. The fuse is rated for a certain maximum current and the fuse wire melts when the current flowing through it exceeds the rated value. The appliance may get damaged, if a fuse is replaced by one with larger ratings because the fuse wire will not melt. Therefore, the practice of using fuse of improper rating should always be avoided.


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.

Solution

Current carrying loops act as bar magnets which have associated magnetic fields. These field lines change the existing earth’s magnetic field and cause a deflection.

Some salient features of magnetic lines of field are as follows:

  • Magnetic field has both direction and magnitude.
  • Magnetic field lines emerge from the north pole and enter the south pole.
  • The strength of a magnetic field is represented diagrammatically by the degree of closeness of the field lines.
  • At a single point, two values of net field cannot as the field lines cannot cross each other.
  • If the field lines are parallel and equi-spaced in some region, this means that the field is uniform.


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?

Solution

Right hand thumb rule states that if a current carrying straight conductor is held in the right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the magnetic field line.


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?

Solution

Even in current carrying loop, the right hand thumb rule is obeyed. This shows that the magnetic field lines are present around the conducting wire. But the circular shape of conductor means that field lines at different points of the loop appear to be making ring around the periphery of the loop. This looks like small ring looping around big ring.

If the number of turns of the coils increases, the magnetic field increases. Therefore the strength of the magnetic field increases with number of turns in a coil.


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?

Solution

Activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field:

  • Take a small aluminium rod AB (of about 5 cm).
  • Suspend the rod horizontally from a stand, as shown in Fig.
  • Place a strong horse-shoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. 
  • Connect the aluminium rod in series with a battery, a key and a rheostat.
  • Now, allow a current to pass through the aluminium rod from end B to end A.

Observation:

It is observed that the rod is displaced towards the left. On reversing the direction of current flowing through the rod, the direction of displacement of the rod also changes.

According to Fleming’s left hand rule, stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.


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?

Solution

An electric motor consists of a coil ABCD placed between the two poles of a permanent magnet. The coil is placed in such a way that the direction of current is perpendicular to the direction of magnetic field.

  • Arms of the coil are attached to a split ring. Arm AB is attached to the half P and arm CD is attached to the half Q.
  • The split rings are touching two static brushes X and Y which provide electric supply to the split rings. The split rings are insulated from inside. An axle is passing through the split rings.
  • A battery supplies electric current to the coil. The flow of current is from A to B and from C to D.

Working of electric motor:

When current flows from A to B, then the arm AB moves down according to the Flemings left hand rule. On the contrary, the arm CD of the coil moves up when current passes from C to D.

P of split ring touches brush Y when the coil completes half turn and the opposite happens with Q of split ring. Therefore, the direction of current gets reversed. That is the current now flows D to C and from B to A. This pushes the arm CD downwards and AB upwards. As a result the coil turns continuously.

Simple electric motor

Commercial electric motor

Permanent magnet is used

Electromagnet megnet is used

Turn number in coil is less

Turn number in coil is large

Iron core is not used

Soft Iron core is used


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.

Solution

The phenomenon of generation of an electric current in a closed circuit by changing the magnetic field is known as electromagnetic induction. The current produced is called the induced current and the potential difference set up is called the induced emf.
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:
  • Take two different coils of copper wire having large number of turns (say 50 and 100 turns respectively). 
  • Insert them over a non-conducting cylindrical roll, as shown.
  • Connect the coil-1having larger number of turns, in series with a battery and a plug key. 
  • Also connect the other coil-2 with a galvanometer as shown.
  • Plug in the key.
Observation:
  • When the key is plugged, the needle of the galvanometer shows deflection, indicating a momentary current in coil-2.
  • When the key is unplugged, the needle momentarily moves, but now to the opposite side. 
  • This shows that there is current in coil-2 whenever there is change in magnetic field.

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

Solution

A conductor coil ABCD is placed in a magnetic field. The direction of magnetic flux will be form N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes resting on the slip rings.
Consider the case 1 from above figure. The coil is rotating clockwise, and thus the direction of induced current is along A-B-C-D according to the Fleming’s right hand rule. After half time period, the coil is rotating clockwise. Therefore, the direction of the induced current will be along D-C-B-A. This shows that the direction of the current changes after half of the time period. That means the induced current is an alternating current.
A split ring type commutator must be used in place of slip ring type commutator to get a direct current.

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?

Solution

Fuse is important for the following reasons:
  • It is a safety device used in household wiring and in electrical appliance.
  • It prevents the damage to the circuit and appliance due to overload.
  • It prevents accidental fire which may happen because of short circuit.
Fuse of a particular rating is used with a particular appliance. A fuse with lower rating will need to be changed frequently, hence it would result in lot of inconvenience. A fuse with higher rating will not melt if the current exceeds the safety limit, hence it would not serve the purpose. Therefore, a burnt out fuse should be replaced by another fuse of identical rating.

NCERT Exemplar Chapter 13 Magnetic Effects of Electric Current Class 10 Science Solutions

NCERT Exemplar Class 10 Science Chapter 13 Magnetic Effects of Electric Current Solutions

NCERT Exemplar Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current covers all the important questions and answers as well as advanced level questions. It helps in learning about the magnetic field and field lines, Magnetic field due to a current-carrying conductor, right-hand thumb rule, circular loop, solenoid and Force on a current-carrying conductor in a magnetic field.

The NCERT Exemplar solutions for class 10 science is very important for board exams. NCERT Exemplar Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current is provided by our experts. They prepared the best solutions which help the students in understanding the solutions in an easy way. This chapters also covers the other topics like electric motor, electromagnetic induction, Fleming's right-hand rule, electric generator, electric power and domestic electric circuits.


Chapter Name

Chapter 13 Magnetic Effects of Electric Current

Book Title

NCERT Exemplar for Class 10 Science

Related Study

  • NCERT Solutions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • Revision Notes for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • MCQ for Class 10 Science Chapter 13 Magnetic Effects of Electric Current
  • Important Questions for Class 10 Science Chapter 13 Magnetic Effects of Electric Current

Topics Covered

  • MCQ
  • Short Answers Questions
  • Long Answers Questions

NCERT Exemplar Solutions for Chapter 13 Magnetic Effects of Electric Current Class 10 Science

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

Solution

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

Parallel lines of magnetic field represent uniform magnetic field.


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

Solution

(a) concentric circles

If key is taken out then current will stop and no magnetic field exists due to the conductor. Therefore, at the point O, there will be earth’s magnetic field and they will be straight lines parallel to each other.


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

Solution

(a) A

The direction of magnetic field line is from south to north pole. As the field lines are pointing form B to A, point A is indicating the north pole.


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

Solution

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

A solenoid behaves like a bar magnet, because the pattern of the magnetic field lines associated with a solenoid are similar to the pattern of the magnetic field lines around a bar magnet.


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.

Solution

(a) forces both pointing into the plane of paper 

The direction of movement of electron is opposite to the direction of flow of electric current. This makes the current flow upwards. If the index finger shows the direction of magnetic field, ring finger shows the direction of current then the direction of thumb is in to 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

Solution

(c) a permanent magnet to rotate the armature

In electric motors electromagnet is used instead of permanent magnet.


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.

Solution

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

When key is plugged in, the galvanometer shows deflection in one direction and the direction of deflection reverses if the key is unplugged.


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 1/50 second

Solution

(d) In India, the AC changes direction after every 1/50 second

AC frequency in India is 50 Hz. As the direction changes twice in each cycle so that change of direction takes place after every 1/00 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

Solution

(b) directly below the wire

Using the right hand thumb rule, it can be found that the direction of magnetic field is from North to South below the wire.


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

Solution

(c) same at all points

Magnetic field lines are straight inside the solenoid. This indicates strong magnetic field. Therefore, inside the solenoid the magnetic field is uniform 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

Solution

(a) split-ring type commutator must be used

After each half turn, the split-ring type commutator reverse the direction of current. This maintains a DC 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

Solution

(b) use of fuse

Fuse has a thin wire made of tin and lead in the ratio of 75: 25%. The fuse wire melts and breaks the circuit when current exceeds specified limit thereby protecting home appliances.


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?

Solution

The current carrying conductor is placed in the plane of the paper itself. This is because the axis of the compass is vertical and the field due to the conductor is also vertical. The deflection is maximum when the conductor passes through A in a plane perpendicular to the plane of paper. This is because the field is maximum in the plane of the paper.


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.

Solution

Conditions in which a permanent electromagnet is obtained if a current carrying solenoid is used are:

  1. Circuit should be closed.
  2. (A core of soft iron should be used.


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 r1 > r2, where will the strength of the magnetic field be larger?

Solution

At P into the plane of the paper and out of it at Q. The strength of the magnetic field is larger at Q as it is located closer.


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.

Solution 

The deflection of the magnetic compass increases because the strength of magnetic field is directly proportional to the magnitude of current passing through the straight conductor.


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.

Solution

(i) As alpha particles are positively charged, they constitutes current in the direction of motion.

(ii) Neutrons do not conduct electricity as the have no charge on them.


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?

Solution

The thumb indicates the direction of current in the straight conductor held by curled fingers, whereas the Fleming’s left-hand rule gives the direction of force experienced by current carrying conductor placed in an external 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.

Solution

As the distance increases, the strength of the magnetic field falls. This is indicated by the decrease in degree of closeness of the lines of field.


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

Solution

The divergence of magnetic field lines indicates the decrease in strength of magnetic field near and beyond the ends 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?

Solution

Electric fans, mixers, washing machines, computer drives, etc. are devices that convert electrical energy into mechanical energy.

Generators convert mechanical energy into electrical energy.


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

Solution

The brushes are connected to the battery and hence they draw current and supply it to the armature of the coil.


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?

Solution

Direct current always flows in one direction whereas the alternating current reverses its direction periodically. The frequency of AC in India is 50 Hz and in each cycle it alters direction twice. Therefore AC changes direction 2×50 = 100 times in one 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?

Solution

Fuse is used for protecting appliances during short-circuiting or overloading. The fuse is rated for a certain maximum current and the fuse wire melts when the current flowing through it exceeds the rated value. The appliance may get damaged, if a fuse is replaced by one with larger ratings because the fuse wire will not melt. Therefore, the practice of using fuse of improper rating should always be avoided.


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.

Solution

Current carrying loops act as bar magnets which have associated magnetic fields. These field lines change the existing earth’s magnetic field and cause a deflection.

Some salient features of magnetic lines of field are as follows:

  • Magnetic field has both direction and magnitude.
  • Magnetic field lines emerge from the north pole and enter the south pole.
  • The strength of a magnetic field is represented diagrammatically by the degree of closeness of the field lines.
  • At a single point, two values of net field cannot as the field lines cannot cross each other.
  • If the field lines are parallel and equi-spaced in some region, this means that the field is uniform.


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?

Solution

Right hand thumb rule states that if a current carrying straight conductor is held in the right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the magnetic field line.


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?

Solution

Even in current carrying loop, the right hand thumb rule is obeyed. This shows that the magnetic field lines are present around the conducting wire. But the circular shape of conductor means that field lines at different points of the loop appear to be making ring around the periphery of the loop. This looks like small ring looping around big ring.

If the number of turns of the coils increases, the magnetic field increases. Therefore the strength of the magnetic field increases with number of turns in a coil.


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?

Solution

Activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field:

  • Take a small aluminium rod AB (of about 5 cm).
  • Suspend the rod horizontally from a stand, as shown in Fig.
  • Place a strong horse-shoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. 
  • Connect the aluminium rod in series with a battery, a key and a rheostat.
  • Now, allow a current to pass through the aluminium rod from end B to end A.

Observation:

It is observed that the rod is displaced towards the left. On reversing the direction of current flowing through the rod, the direction of displacement of the rod also changes.

According to Fleming’s left hand rule, stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.


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?

Solution

An electric motor consists of a coil ABCD placed between the two poles of a permanent magnet. The coil is placed in such a way that the direction of current is perpendicular to the direction of magnetic field.

  • Arms of the coil are attached to a split ring. Arm AB is attached to the half P and arm CD is attached to the half Q.
  • The split rings are touching two static brushes X and Y which provide electric supply to the split rings. The split rings are insulated from inside. An axle is passing through the split rings.
  • A battery supplies electric current to the coil. The flow of current is from A to B and from C to D.

Working of electric motor:

When current flows from A to B, then the arm AB moves down according to the Flemings left hand rule. On the contrary, the arm CD of the coil moves up when current passes from C to D.

P of split ring touches brush Y when the coil completes half turn and the opposite happens with Q of split ring. Therefore, the direction of current gets reversed. That is the current now flows D to C and from B to A. This pushes the arm CD downwards and AB upwards. As a result the coil turns continuously.

Simple electric motor

Commercial electric motor

Permanent magnet is used

Electromagnet megnet is used

Turn number in coil is less

Turn number in coil is large

Iron core is not used

Soft Iron core is used


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.

Solution

The phenomenon of generation of an electric current in a closed circuit by changing the magnetic field is known as electromagnetic induction. The current produced is called the induced current and the potential difference set up is called the induced emf.
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:
  • Take two different coils of copper wire having large number of turns (say 50 and 100 turns respectively). 
  • Insert them over a non-conducting cylindrical roll, as shown.
  • Connect the coil-1having larger number of turns, in series with a battery and a plug key. 
  • Also connect the other coil-2 with a galvanometer as shown.
  • Plug in the key.
Observation:
  • When the key is plugged, the needle of the galvanometer shows deflection, indicating a momentary current in coil-2.
  • When the key is unplugged, the needle momentarily moves, but now to the opposite side. 
  • This shows that there is current in coil-2 whenever there is change in magnetic field.

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

Solution

A conductor coil ABCD is placed in a magnetic field. The direction of magnetic flux will be form N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes resting on the slip rings.
Consider the case 1 from above figure. The coil is rotating clockwise, and thus the direction of induced current is along A-B-C-D according to the Fleming’s right hand rule. After half time period, the coil is rotating clockwise. Therefore, the direction of the induced current will be along D-C-B-A. This shows that the direction of the current changes after half of the time period. That means the induced current is an alternating current.
A split ring type commutator must be used in place of slip ring type commutator to get a direct current.

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?

Solution

Fuse is important for the following reasons:
  • It is a safety device used in household wiring and in electrical appliance.
  • It prevents the damage to the circuit and appliance due to overload.
  • It prevents accidental fire which may happen because of short circuit.
Fuse of a particular rating is used with a particular appliance. A fuse with lower rating will need to be changed frequently, hence it would result in lot of inconvenience. A fuse with higher rating will not melt if the current exceeds the safety limit, hence it would not serve the purpose. Therefore, a burnt out fuse should be replaced by another fuse of identical rating.

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