CLASS 10 Science CHAPTER 10 Light- Reflection And Refraction (NCERT Solution)

CLASS 10 Science 
CHAPTER 10
Light- Reflection And Refraction (NCERT Solution)

Question 1: Define the principal focus of a concave mirror.
Answer: Light rays that are parallel to the principal axis of a concave mirror converge at a
specific point on its principal axis after reflecting from the mirror. This point is known as the
principal focus of the concave mirror.

Question 2: The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer: Radius of curvature, R = 20 cm
Radius of curvature of a spherical mirror = 2 × Focal length ( f )
R = 2f
f = R / 2     = 20 /2  = 10
Hence, the focal length of the given spherical mirror is 10 cm.

Question 3: Name the mirror that can give an erect and enlarged image of an object.
Answer: When an object is placed between the pole and the principal focus of a concave mirror,
the image formed is virtual, erect, and enlarged.

Question 4: Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer: Convex mirrors give a virtual, erect, and diminished image of the objects placed in front
of them. They are preferred as a rear-view mirror in vehicles because they give a wider field of
view, which allows the driver to see most of the traffic behind him.

Question 5: Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer: Radius of curvature, R = 32 cm
Radius of curvature = 2 × Focal length ( f )
f = R/ 2   = 32/2 = 16 cm
Hence, the focal length of the given convex mirror is 16 cm.

Question 6: A concave mirror produces three times magnified (enlarged) real image of object
placed at 10 cm in front of it. Where is the image located?
Answer: Magnification produced by a spherical mirror is given by the relation,
Let the height of the object, ho = h
Then, height of the image, hi = −3h (Image formed is real)
Object distance, u = −10 cm
v = 3 × (−10) = −30 cm
Here, the negative sign indicates that an inverted image is formed at a distance of 30 cm in front of the given concave mirror.

Question 7: A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer: The light ray bends towards the normal.
When a ray of light travels from an optically rarer medium to an optically denser medium, it gets
bent towards the normal. Since water is optically denser than air, a ray of light travelling from air
into the water will bend towards the normal.

Question 8: Light enters from air to glass having refractive index 1.50. What is the speed of light
in the glass ? The speed of light in vacuum is 3 × 108 m s−1.
Answer: Refractive index of a medium nm is given by,
Speed of light in vacuum, c = 3 × 108 m s−1
Refractive index of glass, ng = 1.50
Speed of light in the glass,

Question 9: Find out, from Table, the medium having highest optical density. Also find the
medium with lowest optical density.
Material medium         Refractive index   Material medium        Refractive medium index
Air                                    1.0003                  Canada Balsam            1.53
Ice                                    1.31                               -                             -
Water                               1.33                       Rock salt                      1.54
Alcohol                           1.36                                -                              -
Kerosene                         1.44                     Carbon disulphide          1.63
Fused                                                              Dense                  
quartz                             1.46                       flint glass                       1.65                           
Turpentine oil                1.47                        Ruby                              1.71
Benzene                         1.50                        Crown  glass                   1.52        
Sapphire                        1.77                         Diamond                       2.42
Answer: Highest optical density = Diamond
Lowest optical density = Air
Optical density of a medium is directly related with the refractive index of that medium. A medium which has the highest refractive index will have the highest optical density and vice- versa.
It can be observed from table 10.3 that diamond and air respectively have the highest and lowest  refractive index. Therefore, diamond has the highest optical density and air has the lowest optical density.

Question 10: You are given kerosene, turpentine and water. In which of these does the light travel
fastest? Use the information given in Table.
Material medium         Refractive index                             Material medium         Refractive index
Air                               1.0003                                             Canada Balsam           1.53
Ice                                1.3                                                              -                           -
Water                            1.33                                                Rock salt                      1.54
Alcohol                        1.36                                                            -                          -
Kerosene                      1.44                                              Carbon disulphide           1.63
Fused quartz                 1.46                                                 Dense
flint glass                      1.65                                                 Turpentine oil               1.47
Ruby                             1.71                                               Benzene
Crown                           1.50                                                  Sapphire                        1.77
glass                                        1.52                                                            Diamond 2.42
Answer: Speed of light in a medium is given by the relation for refractive index (nm). The relation is given as 


It can be inferred from the relation that light will travel the slowest in the material which has the highest refractive index and travel the fastest in the material which has the lowest refractive index .It can be observed from table 10.3 that the refractive indices of kerosene, turpentine, and water are
1.44, 1.47, and 1.33 respectively. Therefore, light travels the fastest in water.

Question 11: The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer: Refractive index of a medium nm is related to the speed of light in that medium v by the
relation: 


.
Where, c is the speed of light in vacuum/air
The refractive index of diamond is 2.42. This suggests that the speed of light in diamond will
reduce by a factor 2.42 compared to its speed in air.

Question 12: Define 1 dioptre of power of a lens.
Answer: Power of lens is defined as the reciprocal of its focal length. If P is the power of a lens of
focal length F in metres, then
    P = 1/f ( in meters )
The S.I. unit of power of a lens is Dioptre. It is denoted by D.
1 dioptre is defined as the power of a lens of focal length 1 metre.
1 D = 1 m−1

Question 13: A convex lens forms a real and inverted image of a needle at a distance of 50 cm
from it. Where is the needle placed in front of the convex lens if the image is equal to the size of
the object? Also, find the power of the lens.
Answer: When an object is placed at the centre of curvature, 2F1, of a convex lens, its image is
formed at the centre of curvature, 2F2, on the other side of the lens. The image formed is inverted
and of the same size as the object, as shown in the given figure.


It is given that the image of the needle is formed at a distance of 50 cm from the convex lens.
Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object distance, u = −50 cm
Image distance, v = 50 cm
Focal length = f
According to the lens formula,
Hence, the power of the given lens is +4 D.

Question 14: Find the power of a concave lens of focal length 2 m.
Answer: Focal length of concave lens, f = 2 m
Power of a lens, P = 1/ f (in meters)  = 1/ (-50)  =  -0.5 D
Here, negative sign arises due to the divergent nature of concave lens.
Hence, the power of the given concave lens is −0.5 D.

Question 15: Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay
Answer: (d) A lens allows light to pass through it. Since clay does not show such property, it cannot be used to make a lens.

Question 16: The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus
Answer: 

Question 17: Where should an object be placed in front of a convex lens to get a real image of the
size of the object?www.ncrtsolutions.in
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.
Answer: (b) When an object is placed at the centre of curvature in front of a convex lens, its image is formed at the centre of curvature on the other side of the lens. The image formed is real, inverted, and of the same size as the object.

Question 18: A spherical mirror and a thin spherical lens have each a focal length of −15 cm. The
mirror and the lens are likely to be
(a) both concave
(b) both convex
(c) the mirror is concave and the lens is convex
(d) the mirror is convex, but the lens is concave
Answer: (a) By convention, the focal length of a concave mirror and a concave lens are taken as
negative. Hence, both the spherical mirror and the thin spherical lens are concave in nature.

Question 19: No matter how far you stand from a mirror, your image appears erect. The mirror is
likely to be
(a) plane
(b) concave
(c) convex
(d) either plane or convex
Answer: (d) A convex mirror always gives a virtual and erect image of smaller size of the object
placed in front of it. Similarly, a plane mirror will always give a virtual and erect image of same
size as that of the object placed in front of it. Therefore, the given mirror could be either plane or
convex.

Question 20: Which of the following lenses would you prefer to use while reading small letters
found in a dictionary?
(a) A convex lens of focal length 50 cm
(b) A concave lens of focal length 50 cm
(c) A convex lens of focal length 5 cm
(d) A concave lens of focal length 5 cm
Answer: (c) A convex lens gives a magnified image of an object when it is placed between the radius of curvature and focal length. Also, magnification is more for convex lenses having shorter focal length. Therefore, for reading small letters, a convex lens of focal length 5 cm should be used.

Question 21: We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Answer: Range of object distance = 0 cm to15 cm
A concave mirror gives an erect image when an object is placed between its pole (P) and the principal focus (F). Hence, to obtain an erect image of an object from a concave mirror of focal length 15 cm, the
object must be placed anywhere between the pole and the focus. The image formed will be virtual,
erect, and magnified in nature, as shown in the given figure.


Question 19: Name the type of mirror used in the following situations.
(a) Headlights of a car
(b) Side/rear-view mirror of a vehicle
(c) Solar furnace
Support your answer with reason.
Answer: (a) Concave (b) Convex (c) Concave
(a) Concave mirror is used in the headlights of a car. This is because concave mirrors can produce
powerful parallel beam of light when the light source is placed at their principal focus.
(b) Convex mirror is used in side/rear view mirror of a vehicle. Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of it. Because of this, they have a wide field of view. It enables the driver to see most of the traffic behind him/her.
(c) Concave mirrors are convergent mirrors. That is why they are used to construct solar furnaces.
Concave mirrors converge the light incident on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.

Question 20: One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Answer: The convex lens will form complete image of an object, even if its one half is covered
with black paper. It can be understood by the following two cases.
Case I When the upper half of the lens is covered
In this case, a ray of light coming from the object will be refracted by the lower half of the lens.
These rays meet at the other side of the lens to form the image of the given object, as shown in the
following figure.


Case II When the lower half of the lens is covered
In this case, a ray of light coming from the object is refracted by the upper half of the lens. These
rays meet at the other side of the lens to form the image of the given object, as shown in the
following figure.



Question 21: An object 5 cm in length is held 25 cm away from a converging lens of focal length
10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Answer: Object distance, u = −25 cm
Object height, ho = 5 cm
Focal length, f = +10 cm
According to the lens formula,


The negative value of image height indicates that the image formed is inverted. The position, size, and nature of image are shown in the following ray diagram.



Question 22: A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far
is the object placed from the lens? Draw the ray diagram.
Answer: Focal length of concave lens (OF1), f = −15 cm
Image distance, v = −10 cm
According to the lens formula,


The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown
in the following ray diagram.



Question 23: An object is placed at a distance of 10 cm from a convex mirror of focal length 15
cm. Find the position and nature of the image.
Answer: Focal length of convex mirror, f = +15 cm
Object distance, u = −10 cm
According to the mirror formula,

The magnification produced by a plane mirror is +1. It shows that the image formed by the plane
mirror is of the same size as that of the object. The positive sign shows that the image formed is
virtual and erect.

Question 24: An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex
mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.
Answer: Object distance, u = −20 cm
Object height, h = 5
Radius of curvature, R = 30 cm
Radius of curvature = 2 × Focal length
R = 2 f
f = 15 cm
According to the mirror formula,

The positive value of image height indicates that the image formed is erect. 
Therefore, the image formed is virtual, erect, and smaller in size.

Question 25: An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal  length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.
Answer: Object distance, u = −27 cm
Object height, h = 7 cm
Focal length, f = −18 cm
According to the mirror formula,

The negative value of image height indicates that the image formed is inverted.

Question 26: Find the focal length of a lens of power −2.0 D. What type of lens is this?
Answer: 

A concave lens has a negative focal length. Hence, it is a concave lens.

Question 27: A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Answer: 

A convex lens has a positive focal length. Hence, it is a convex lens or a converging lens.

( From www.ncrtsolutions.in )

CLASS 10 Science CHAPTER 13 Magnetic Effects Of Electric Current (NCERT Solution)

CLASS 10 Science                                                                                                  CHAPTER 13                                                                                                            Magnetic Effects Of Electric Current (NCERT Solution)

Question 1: Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle is a small bar magnet. When it is brought near a bar magnet, its magnetic field lines interact with that of the bar magnet. Hence, a compass needle shows a deflection when brought near the bar magnet.

Question 2: Draw magnetic field lines around a bar magnet.
Answer: Magnetic field lines of a bar magnet emerge from the north pole and terminate at the
south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north
pole, as shown in the given figure.



Question 3: List the properties of magnetic lines of force.
Answer: The properties of magnetic lines of force are as follows.
(a) Magnetic field lines emerge from the north pole.
(b) They merge at the south pole.
(c) The direction of field lines inside the magnet is from the south pole to the north pole.
(d) Magnetic lines do not intersect with each other.

Question 4: Why don’t two magnetic lines of force intersect each other?
Answer: If two field lines of a magnet intersect, then at the point of intersection, the compass needle points in two different directions. This is not possible. Hence, two field lines do not intersect each other.

Question 5: Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Answer: Inside the loop = Pierce inside the table
Outside the loop = Appear to emerge out from the table
For downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.



Question 6: The magnetic field in a given region is uniform. Draw a diagram to represent it. 
Answer:


The magnetic field lines inside a current-carrying long straight solenoid are uniform.

Question 7: Choose the correct option. The magnetic field inside a long straight solenoid-carrying current
(a) is zero
(b) decreases as we move towards its end
(c) increases as we move towards its end
(d) is the same at all points
Answer: (d) The magnetic field inside a long, straight, current-carrying solenoid is uniform. It is
the same at all points inside the solenoid.

Question 8: Which of the following property of a proton can change while it moves freely in a
magnetic field? (There may be more than one correct answer.)
(a) mass
(b) speed
(c) velocity
(d) momentum
Answer: (c) and (d) When a proton enters in a region of magnetic field, it experiences a magnetic force. As a result of the force, the path of the proton becomes circular. Hence, its velocity and momentum change.

Question 9: State Fleming’s left-hand rule.
Answer: Fleming’s left hand rule states that if we arrange the thumb, the centre finger, and the
forefinger of the left hand at right angles to each other, then the thumb points towards the direction
of the magnetic force, the centre finger gives the direction of current, and the forefinger points in
the direction of magnetic field.

Question 10: What is the principle of an electric motor?
Answer: The working principle of an electric motor is based on the magnetic effect of current. A
current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

Question 11: What is the role of the split ring in an electric motor?
Answer: The split ring in the electric motor acts as a commutator. The commutator reverses the
direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

Question 12: Explain different ways to induce current in a coil.
Answer: The different ways to induce current in a coil are as follows:
(a) If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil.
(b) If a magnet is moved relative to a coil, then an electric current is induced in the coil.

Question 13: State the principle of an electric generator.
Answer: An electric generator works on the principle of electromagnetic induction. It generates
electricity by rotating a coil in a magnetic field.

Question 14: Name some sources of direct current.
Answer: Some sources of direct current are cell, DC generator, etc.

Question 15: Which sources produce alternating current?
Answer: AC generators, power plants, etc., produce alternating current.

Question 16: Choose the correct option.
Answer: A rectangular coil of copper wires is rotated in a magnetic field. The direction of the
induced current changes once in each
(a) two revolutions          (b) one revolution
(c) half revolution           (d) one-fourth revolution
(c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced
current in the coil changes once in each half revolution. As a result, the direction of current in the
coil remains the same.

Question 17: Name two safety measures commonly used in electric circuits and appliances.
Answer: Two safety measures commonly used in electric circuits and appliances are as follows:
(i) Each circuit must be connected with an electric fuse. This prevents the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current through that circuit, hence protecting the appliances connected to the circuit.
(ii) Earthing is a must to prevent electric shocks. Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.

Question 18: An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a
current rating of 5 A. What result do you expect? Explain.
Answer: Current drawn by the electric oven can be obtained by the expression,
P = VI
I = P/V
Where,
Current = I
Power of the oven, P = 2 kW = 2000 W
Voltage supplied, V = 220 V
I = 2000/220 = 9.09 A
Hence, the current drawn by the electric oven is 9.09 A, which exceeds the safe limit of the circuit.
Fuse element of the electric fuse will melt and break the circuit.

Question 19: What precaution should be taken to avoid the overloading of domestic electric  circuits?
Answer: The precautions that should be taken to avoid the overloading of domestic circuits are as
follows:
(a) Too many appliances should not be connected to a single socket.
(b) Too many appliances should not be used at the same time.
(c) Faulty appliances should not be connected in the circuit.
(d) Fuse should be connected in the circuit.

Question 20: Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of concentric circles centred on the wire
Answer: (d) The magnetic field lines, produced around a straight current-carrying conductor, are
concentric circles. Their centres lie on the wire.

Question 21: The phenomenon of electromagnetic induction is
(a) the process of charging a body
(b) the process of generating magnetic field due to a current passing through a coil
(c) producing induced current in a coil due to relative motion between a magnet and the coil
(d) the process of rotating a coil of an electric motor
Answer: (c) When a straight coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction.

Question 22: The device used for producing electric current is called a
(a) generator
(b) galvanometer
(c) ammeter
(d) motor
Answer: (a) An electric generator produces electric current. It converts mechanical energy into
electricity.

Question 23: The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Answer: (d) An AC generator has two rings called slip rings. A DC generator has two half rings
called commutator. This is the main difference between both the types of generators.

Question 24: At the time of short circuit, the current in the circuit
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously
Answer: (c) When two naked wires of an electric circuit touch each other, the amount of current
that is flowing in the circuit increases abruptly. This causes short-circuit.

Question 25: State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
Answer: (a) False An electric motor converts electrical energy into mechanical energy.
(b) True A generator is an electric device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.
(c) True A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel
lines.
(d) False Live wire has red insulation cover, whereas earth wire has green insulation colour in the domestic circuits.

Question 26: List three sources of magnetic fields.
Answer: Three sources of magnetic fields are as follows:
(a) Current-carrying conductors
(b) Permanent magnets
(c) Electromagnets

Question 27: How does a solenoid behave like a magnet? Can you determine the north and south
poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Answer: A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines
are produced around the solenoid when a current is allowed to flow through it. The magnetic field
produced by it is similar to the magnetic field of a bar magnet. The field lines produced in a current
carrying solenoid is shown in the following figure.
In the above figure, when the north pole of a bar magnet is brought near the end connected to the
negative terminal of the battery, the solenoid repels the bar magnet. Since like poles repel each
other, the end connected to the negative terminal of the battery behaves as the north pole of the
solenoid and the other end behaves as a south pole. Hence, one end of the solenoid behaves as a
north pole and the other end behaves as a south pole.

Question 28: When is the force experienced by a current-carrying conductor placed in a magnetic
field largest?
Answer: The force experienced by a current-currying conductor is the maximum when the direction of current is perpendicular to the direction of the magnetic field.

Question 29: Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer: The direction of magnetic field is given by Fleming’s left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current (opposite to the direction of electron) and direction of deflection/force i.e., either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from back wall to the front wall. The direction of magnetic force is rightward. Hence, using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

Question 30: Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer: The direction of magnetic field is given by Fleming’s left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current (opposite to the direction of electron) and direction of deflection/force i.e., either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from back wall to the front wall. The direction of magnetic force is rightward. Hence, using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

Question 31: Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer: An electric motor converts electrical energy into mechanical energy. It works on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field. The following figure shows a simple electric motor.


When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length MN and at the same time, an upward force acts on length ST. As a result, the coil rotates anti-clockwise.
Current in the length MN flows from M to N and the magnetic field acts from left to right, normal
to length MN. Therefore, according to Fleming’s left hand rule, a downward force acts on the length MN. Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST.
These two forces cause the coil to rotate anti-clockwise.
After half a rotation, the position of MN and ST interchange. The half-ring D comes in contact with
brush A and half-ring C comes in contact with brush B. Hence, the direction of current in the coil
MNST gets reversed.


The current flows through the coil in the direction TSNM. The reversal of current through the coil
MNST repeats after each half rotation. As a result, the coil rotates unidirectional. The split rings
help to reverse the direction of current in the circuit. These are called the commutator.

Question 32: Name some devices in which electric motors are used?
Answer: Some devices in which electric motors are used are as follows:
(a) Water pumps
(b) Electric fans
(c) Electric mixers
(d) Washing machines

Question 33: A coil of insulated copper wire is connected to a galvanometer. What will happen if
a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary
inside the coil?
Answer: A current induces in a solenoid if a bar magnet is moved relative to it. This is the principle of electromagnetic induction.
(i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced momentarily in the coil. As a result, the needle of the galvanometer deflects momentarily in a particular direction.
(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced momentarily in the coil in the opposite direction. As a result, the needle of the galvanometer deflects momentarily in the opposite direction.
(iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. Hence, galvanometer will show no deflection.

Question 34: Two circular coils A and B are placed closed to each other. If the current in the coil
A is changed, will some current be induced in the coil B? Give reason.
Answer: Two circular coils A and B are placed close to each other. When the current in coil A is
changed, the magnetic field associated with it also changes. As a result, the magnetic field around 
coil B also changes. This change in magnetic field lines around coil B induces an electric current
in it. This is called electromagnetic induction.

Question 35: State the rule to determine the direction of a 
(i) magnetic field produced around a straight conductor-carrying current, 
(ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and
(iii) current induced in a coil due to its rotation in a magnetic field.
Answer: (i) Maxwell’s right hand thumb rule
(ii) Fleming’s left hand rule
(iii) Fleming’s right hand rule

Question 36: Explain the underlying principle and working of an electric generator by drawing a
labelled diagram. What is the function of brushes?
Answer: An electric generator converts mechanical energy into electrical energy.
The principle of working of an electric generator is that when a loop is moved in a magnetic field,
an electric current is induced in the coil. It generates electricity by rotating a coil in a magnetic
field. The following figure shows a simple AC generator.


MNST → Rectangular coil
A and B → Brushes
C and D → Two slip rings
X → Axle, G → Galvanometer
If axle X is rotated clockwise, then the length MN moves upwards while length ST moves downwards. Since the lengths MN and ST are moving in a magnetic field, a current will be induced in both of them due to electromagnetic induction. Length MN is moving upwards and the magnetic field acts from left to right. Hence, according to Fleming’s right hand rule, the direction of induced current will be from M to N. Similarly, the direction of induced current in the length ST will be from S to T.
The direction of current in the coil is MNST. Hence, the galvanometer shows a deflection in a particular direction. After half a rotation, length MN starts moving down whereas length ST starts moving upward. The direction of the induced current in the coil gets reversed as TSNM. As the direction of current gets reversed after each half rotation, the produced current is called an alternating current (AC).
To get a unidirectional current, instead of two slip rings, two split rings are used, as shown in the
following figure.


In this arrangement, brush A always remains in contact with the length of the coil that is moving
up whereas brush B always remains in contact with the length that is moving down. The split rings
C and D act as a commutator.
The direction of current induced in the coil will be MNST for the first rotation and TSNM in the
second half of the rotation. Hence, a unidirectional current is produced from the generator called
DC generator. The current is called AC current.

Question 37: When does an electric short circuit occur?
Answer: If the resistance of an electric circuit becomes very low, then the current flowing through
the circuit becomes very high. This is caused by connecting too many appliances to a single socket
or connecting high power rating appliances to the light circuits. This results in a short circuit.
When the insulation of live and neutral wires undergoes wear and tear and then touches each
other, the current flowing in the circuit increases abruptly. Hence, a short circuit occurs.

Question 38: What is the function of an earth wire? Why is it necessary to earth metallic
appliances?
Answer: The metallic body of electric appliances is connected to the earth by means of earth wire
so that any leakage of electric current is transferred to the ground. This prevents any electric shock
to the user. That is why earthing of the electrical appliances is necessary.

Most Important Ques Asked In Every kind of Exams