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

NCERT Exemplar Class 10 Science Solutions Chapter 13-MCQ

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
Magnetic field lines are imaginary lines, which are used to represent magnetic field strength and its direction in the space.
The closeness of magnetic field lines represents the magnetic field strength.
The direction of magnetic field strength at any point is along direction of magnetic field line.
At any point direction can be deduced by using magnetic needle, the North Pole of the magnetic needle will be in the direction of magnetic field line.
These magnetic field lines forms close loop.
If magnetic field lines are drawn equidistant and parallel to each other it represents uniform magnetic field.
Hence the correct option of this question his option C

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: C
If the key is open there will be no current in the wire and it will not create any magnetic field.
In that case, the magnetic field present will be because of earth only.
Therefore magnetic field lines will be straight lines parallel to each other.
Hence the correct option is option C.

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 anticlockwise 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
The magnetic field lines forms close loop, but if we see any magnet then magnetic field lines seems to originate from North Pole.
Within the magnet the magnetic field lines will be from South pole to North Pole.
As in this question, the magnetic field lines point from B to A. It means for outside world it is coming out from A.
Hence, point A will be North Pole and point B will be South Pole.
The nature of face whether it will be South Pole or fool North Pole depends on the sense of current in the loop. This nature
does not depend on the magnitude of current.
Hence the correct option of this question is option 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 magnetize 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
A solenoid is a similar arrangement as a bar magnet and it is known as electromagnet.
Its magnetic field line will be similar to any naturally found bar magnet.
Inside the long solenoid magnetic field is approximately uniform at every point.
Therefore magnetic field lines inside the solenoid are drawn in form of parallel straight lines.
The two ends of the solenoid can be North or South Pole depending on sense of current or direction of current.
Hence the correct option of this question is option C

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
We know that direction of current will be in the direction of positive charge flow.
As the electron is coming downward, we can assume the corresponding current will be moving upward.
Similarly the motion of proton in upward direction will cause corresponding current in upward direction.
Hence both the particles will have correspond current in same direction which is upward.
By using Flemings left hand rule we can say that force on proton as well as electron will be in the direction into the plane of paper.
Therefore correct option of this question is option A

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
Commercial electric motors use a solenoid to rotate the armature.
This solenoid is also known as electromagnet.
This is made by whirling large number of wire turns on a soft iron core to create strong magnetic field.
Hence, the correct option of this question is option C

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
Changing magnetic field can cause current in any loop.
We know magnetic field can be created by current carrying wire.
Therefore, we can see that changing current in one Loop can cause current in another loop.
This effect is known as mutual electromagnetic inductive effect.
As the key is inserted, the current become non-0 from zero which will cause current in second loop.
When key is removed later the current become zero from non-0,which will again cause current in second loop.
The direction of current in second loop will be oppose it in both cases as suggested by Flemings right hand rule
Hence the correct answer of this question is option D

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/50 second

Answer: D
Fleming’s right hand rule is used to find out direction of induced current.
Fleming’s left hand rule is used to find out force on a current carrying wire in magnetic field.
Right hand thumb rule is used to find the direction of magnetic field created by a current carrying wire.
Direct current source or DC source will cause current in same direction all the time.
Alternating-current source or AC source will cause the periodic change in direction of current.
The alternating-current coming to our houses in India has a frequency of 50 Hz.
In one cycle direction of current changes twice to become same.
We can say that the direction of current changes after every half cycle.
Therefore, time to change the direction will be half of the time period that would be 1/100 seconds.
Hence the correct option of this question is option D

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
We can use right hand thumb rule to find out magnetic field due to a current carrying wire.
The magnetic field will be forming close loop and if we see it from East the loop will be clockwise.
At a point above the wire, the direction of magnetic field will be towards North from south and at a point below the wire, the magnetic field will be towards south from North.
At a point located towards north, the magnetic field will be into the paper. At a point located towards south the magnetic field will be out of the paper. Hence the correct option is option B

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
For a long current carrying straight solenoid, the magnetic field is approximately uniform at every point inside the solenoid.
So if we draw the magnetic field lines that would be parallel straight line with equal gaps.
Hence the correct answer of this question is option C

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
To get a direct current from an AC source or converting AC source into DC source, we have to use a split ring type commutator.
The split ring commutator is used in electric motor also.
This commutator guarantees unidirectional current in the circuit.
Hence option A is the correct option of this question

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
Fuse wire is a short length thin wire generally made of tin and lead in the ratio of 3:1.
If the supply become high-voltage which exceeds the current then after reaching at a
specified value of current, the fuse wire melts and breaks.
As the fuse wire melts, the circuit of the whole household breaks and thus household appliances get protected from damage.
Hence the correct answer of this question is option B.

NCERT Exemplar Class 10 Science Solutions Chapter 13-Short Answer

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:

By using right hand thumb rule, we can find out the direction of magnetic field due to a current carrying wire. These field lines form circle in plane perpendicular to the wire.
In the given case; If we keep the wire in the plane of paper, it will cause magnetic field perpendicular to the plane.
The field will be parallel to the vertical axis of compass needle. As a result there will be no change in the deflection of compass needle.
The deflection is maximum when the wire through the point A is perpendicular to the plane of paper. In that case, field due to it is parallel to the plane of 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.

Answer:

Permanent electromagnet as a special case of solenoid.
This solenoid is also known as electromagnet.
This is made by whirling large number of wire turns on a soft iron core to create strong magnetic field
In this case the current carrying long solenoid will have soft iron core within its self.
Therefore we can say, “To obtain permanent electromagnet, we need Soft iron core and current in tightly wound solenoid.”
Soft iron core attracts magnetic field lines, hence it causes closeness in the lines with in the solenoid.


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.

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?

Answer:

By using right hand thumb rule, we can find out the direction of magnetic field due to a current carrying wire.
In the given diagram, the magnetic field lines will be close circular loop.
By keeping thumb along the current carrying wire, the direction can be deduced and it will be anticlockwise, if we see it from above.
Therefore, magnetic field at point B will be into the plane of paper and at point Q, it would be outside the plane of paper.
As we go away from the wire, the magnetic field strength decreases in magnitude. Hence magnetic field strength will be
stronger at point B and weaker at point Q relatively.

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:

If the current is increased in the conductor the deflection of the compass needle increases. This happens because the strength of the magnetic field varies directly as the magnitude of the electric current or the current passing through the wire.

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 current-carrying wire can be treated as the motion of positive charge particles along the wire in the direction of current.
Therefore we can say that any moving charged particles or beam can be considered as current-carrying wire.
Alpha particle is a positively charged particle and have charge equal to 2 protons.
Its beam can be considered as a current-carrying wire. Therefore, it will cause a magnetic field around it.
(ii) Neutron is uncharged neutral particle, hence the magnetic field will not be created because of the beam of neutron.

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:

Right hand thumb rule is used to find out magnetic field due to a current carrying wire in the vicinity of the wire.
The Thumb must be along the current carrying wire and fingers are curled in the direction of magnetic field produced by this wire.
While Fleming’s left hand rule is used to find out force on a current carrying wire, present in 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 closeness of magnetic field represents the magnetic field strength.
As we go away from the current carrying wire, the strength of magnetic field decreases.
This decrement of magnetic field can be shown as decrease in closeness of the line of magnetic fields.

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

Answer:

For practical length solenoid, the magnetic field strength is weaker at the ends and stronger within the solenoid.
Hence if we draw magnetic field lines, the degree of closeness will be maximum within the solenoid and it decreases as we go towards the end.
This can be shown as the divergence of magnetic field lines near the ends of a current-carrying 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:

Electric motor is a device which converts electrical energy into mechanical energy.
The current in the armature experiences force due to magnetic field and causes armature to rotate.
The electric motor is used in all such devices, in which we have to rotate any part of the device such as washing machine, electric fans, mixers and computer drives.
The electric generator uses the mechanical power to create electrical energy. Hence generators can be assumed as exactly opposite devices of electric motor

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

Answer:

We need two conducting stationary Brushes to create contact between two parts of split rings and the electric source (Battery).
These Brushes work as connecting wires.
Current in the coil enters from the source battery through one conducting brush and flows back to the battery through another brush.

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 direct current, the direction of current remain same at all the time. The source of direct current is also known as DC source. Example: battery
In alternating current source, the direction of current changes periodically.
In one cycle direction of current changes twice to get the same direction.
The time period of this cycle is different for different countries as per their convention and guidelines.
The frequency of AC in India is 50 Hz and in each cycle, it alters direction twice. Therefore, AC changes direction 2 X 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?

Answer:

Fuse wire is a short length thin wire generally made of tin and lead in the ratio of 3:1.
If the supply become high-voltage which exceeds the current then after reaching at a specified value of current, the fuse wire melts and breaks.
As the fuse wire melts, the circuit of the whole household breaks and thus
household appliances get protected from damage.
Fuse wire comes with a current rating and they melt for any current more than that.
If we use high current rating fuse, it will allow large current and home appliances can get damaged by them.

NCERT Exemplar Class 10 Science Solutions Chapter 13-Long Answer

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:

Due to churning of earth core metals, earth creates a magnetic field around itself.
This effect is called dynamo effect.
Due to magnetic field of earth, any magnetic compass get deflected and try to align itself pointing North and South.
When a bar magnet or current carrying wire comes close to the magnetic needle, the net
magnetic field strength which is the combination of magnetic field due to earth and the current carrying wire changes. This new magnetic field deflects the magnetic needle in different direction, which will be along the resultant magnetic field lines.
In the absence of a bar magnet or current carrying wire, the magnetic field strength due to earth can be considered as uniform at a particular place the magnetic field lines will be straight and parallel to each other. In presence of bar magnet or current carrying wire the magnetic field line get distorted.
Magnetic Field Line (Features):
Magnetic field lines are imaginary lines, which are used to represent magnetic field strength and its direction in the space.
The closeness of magnetic field lines represents the magnetic field strength.
The direction of magnetic field strength at any point is along direction of magnetic field line.
At any point direction can be deduced by using magnetic needle, the North Pole of the magnetic needle will be in the direction of magnetic field line.
These magnetic field lines forms close loop.
If magnetic field lines are drawn equidistant and parallel to each other it represents uniform magnetic field.

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:

Labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire:

Concentric magnetic field lines will be produced when current passes through the conductor. As we move away from the wire, the strength of magnetic field keeps on decreasing.
We can use right hand thumb rule to find the direction of magnetic field.
It states that if a current carrying conductor is held by our right hand (keeping the thumb straight) and if the direction of electric current is in the direction of thumb, then the direction of magnetic field will be given by the direction of wrapping of other fingers. This is shown as follows:

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:

The magnetic field pattern due to a circular coil is as shown in the figure below:

The concentric circles represent the magnetic field around the circular coil.
For every point on the circular loop, as we move away from the wire, the concentric circles around it becomes larger and larger. At the center of the loop, the field appears as a straight line.
The magnetic field produced by current-carrying circular wire at a given point is directly proportional to the current flowing through the wire and number of turns of the wire.
If a current carrying coil has n turns, the current in each circular turns has the same direction. So, the field due to each turn adds up and the field produced at any point is n times as large as that 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:

Here we have a small aluminium rod AB. It is suspend horizontally from a stand, using two connecting wires.
A strong horse-shoe magnet is placed in such a way such that the rod lies between the two poles with the magnetic field directed upwards.
The North Pole of the magnet lies vertically below and South Pole lies vertically above the aluminium rod.
The rod in connected in series with a battery, a key and a rheostat.
The arrangement looks as:

Now if we pass a current through the aluminium rod from end B to end A, we observe that the rod is displaced towards left.
If we reverse the direction of current, we observe that the direction of its displacement is now towards right.This is explained using Fleming’s left hand rule. It states that:
Stretch your thumb, forefinger and central finger of left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central finger points 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 different from commercial motors?

Answer:

Circuit diagram of a simple electric motor is as follows:


AC motor is a device which converts electrical energy into mechanical energy.
It can be seen as reverse mechanism of AC generator.
With the help of and electrical source like battery the current in any armature is
controlled. This armature is exposed to strong magnetic field between permanent magnetic poles.
In magnetic field, the wires of armature experiences a force which causes them to rotate.
With the help of split ring commutator, the direction of current changes in half period. Thus force on armature keeps rotating it in the same sense.
The commercial motor is more powerful than regular electric motor.
To make it more powerful, commercial motor uses electromagnets in place of permanent magnets.
These electromagnets are way stronger than permanent magnets because of large number of winding and soft iron core.

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:

When magnetic field passing through any coil changes it causes an electromagnetic force in the coil.
This electromagnetic force produces current in the coil. This phenomenon is called electromagnetic induction.
Whether the magnetic field through coil is increasing or decreasing, it will cause induced current in the coil.
The sense of current will be different for decreasing or increasing magnetic field.
In the given diagram, if we switch on the circuit of left coil, the current in left coil will increase.

We know that current carrying wire produces magnetic field, therefore magnetic field due to left coil will increase and cause induced current in the right coil.
Similarly if we open the switch of the left circuit, the magnetic field decreases because of it.
This change will again cause the induced current in the right coil.
But this time direction of current will be reversed.
This experiment clearly demonstrates that whether magnetic field decreases or increases through any coil, it will cause induced current.

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?

Answer:

AC generator works on the principle of electromagnetic induction.
With the help of a motor, armature is rotated between two fixed permanent magnetic poles.
As the armature experiences the changing magnetic field due to its rotation, an electromotive force is generated.
The polarity of this electromotive force changes periodically depending on the angular speed of armature.
In this way the generator creates an alternating current source.
Therefore we can say AC generator is a device which converts mechanical energy into electrical energy.
If we want to use this AC generator as the DC source we have to use half split rings.
This split ring commutator changes the direction of current in half period, thus output current will have same direction
all the time

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:

Solution:
Schematic diagram showing common domestic circuits:

Supply from the power distribution system first goes through fuse wire then it goes to the meter which calculates electricity usage.
After passing through the meter, the wires go to distribution box which has fuses four different circuits corresponding to different loads in households.
Fuse wire is a short length of thin wire generally made of tin and lead in the ratio of 3:1.
If the supply become high-voltage which exceeds the current then after reaching at a specified value of current, the fuse wire melts and breaks.
As the fuse wire melts, the circuit of the whole household breaks and thus household appliances get protected from damage.
Fuse wire comes with a current rating and they melt for any current more than that.
If we use high current rating fuse, it will allow large current and home appliances can get damaged by them.