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Q. 1. Define the SI unit of potential difference.
Ans. If amount of work done in bringing a
one coulomb charge from one point to the other is 1 joule then potential
difference between two points is said to be 1 volt.
Q. 2. Name the device that helps to
maintain a potential difference across a conductor.
Ans. A battery is used to maintain
potential difference across a conductor.
Q. 3. Name the physical quantity which is
same in all the resistors when they are connected in series.
Ans. Electric current.
Q. 4. State the relationship between 1
ampere and 1 coulomb.
Ans. 1 ampere = 1 coulomb/1 second
Q. 5. State in brief the meaning of an
electric circuit.
Ans. A closed conducting path through which
electric charge may flow.
Q. 6. Name the physical quantity whose unit
is volt/ ampere.
Ans. Resistance.
Q. 7. Through which of the two wires, the
electric current will flow more easily ? (i) a thick wire or (ii) a thin wire
of the same material, and of the same length when connected to the same source
?
Ans. A thick wire.
Q. 8. The resistance of a resistor is kept
constant and the potential difference across its two ends is decreased to half
of its former value. State the change that will occur in the current through
it.
Ans. Current will be half of its former
value.
Q. 9. What happens to the resistance of a
conductor when its area of cross-section is increased ?
Ans. Resistance is decreased.
Q. 5. How the voltmeter and ammeter are
connected in a circuit ?
Ans. A voltmeter is connected in parallel
with device to measure its voltage, while an ammeter is connected in series
with a device to measure its current.
Q. 6. State the factors on which at a given
temperature the resistance of a cylindrical conductor depends. State the SI
unit of resistivity.
Ans. Factors on which the resistance of a
cylindrical conductor depends : (i) Area of cross-section of the conductor.
(ii) Nature of the material. SI unit of resistivity is ohm-m.
Q. 9. On what factors does resistance of a
conductor depend ?
Ans. There are four factor that influence
the resistance in a conductor. Thickness, length, temperature, conductivity of
the material is used.
Q. 10. Three incandescent bulbs of 100 W
each are connected in series in an electric circuit. In another circuit another
set of three bulbs of the same wattage are connected in parallel to the same
source.
(a) Will the bulb in the two circuits glow
with the same brightness? Justify your answer.
(b) Now let one bulb the circuits get
fused. Will the rest of the bulbs continue to glow in each circuit? Give
reason.
Ans. (a) No. The resistance of the bulbs in
series will be three times the resistance of single bulb. Therefore, the
current in the series combination will be onethird compared to current in each
bulb in parallel combination. The parallel combination bulbs will glow more
brightly.
(b) The bulbs in series combination will stop
glowing as the circuit is broken and current is zero. However the bulbs in
parallel combination shall continue to glow with the same brightness.
Q. 11. Mention the condition under which
charges can move in a conductor. Name the device which is used to maintain this
condition in an electric circuit.
Ans. Charges can move if there is a
difference of electric pressure or potential difference along the conductor.
Electric cell or a battery consisting of two or more cells.
Q. 12. Write SI unit of resistivity.
Ans. Ohm metre.
Q. 13. What is commercial unit of energy ?
Ans. Kilowatt hour or kWh.
Q. 14. What is electric power ? State its
SI unit.
Ans. Rate at which energy is consumed is
called electric power. Its unit is watt.
Q. 15. On what principle is an electric
bulb based ?
Ans. Heating effect of current.
Q. 16. Mention one reason why tungsten is
exclusively used for making filaments of electric lamps.
Ans. Tungsten has a high resistivity and a
high melting point. So, most of the power consumed by this, is dissipated in
the form of heat.
Q. 17. How is an ammeter connected in a
circuit to measure the current flowing through it ?
Ans. In series.
Q. 18. What determines the rate at which
energy is delivered by a current ?
Ans. Electric power determines the rate at
which energy is delivered by a current.
Q. 19. In a circuit if two resistors of 5 W
and 10 W are connected in series. Compare the current passing through the two
resistors.
Ans. In a series connection of resistors,
same current passes through all the resistors. Hence, current will be same.
Ratio of the current will be 1 : 1.
Q. 20. Why are coils of electric toasters
and electric irons made of alloy rather than pure metals ?
Ans. Alloys are preferred to make coils of
toasters and irons because they have higher resistivity and do not oxidise
easily at high temperature.
Q. 21. Why the filament of bulb has high
melting point ?
Ans. A filament is made up of a substance
having high melting point because a bulb lights up heating the filament. The
heated filament is the source of light and if the melting point of filament’s
substance is low, it may melt.
Q. 22. Name the physical quantity that
determines the rate at which energy is delivered by an electric current. State
and define the unit of this physical quantity.
Ans. Quantity — Electric power. Unit —
Watt.
1 Watt is the power consumed by a device
that carries 1 A of current when operated at a potential difference of 1 V.
Q. 23. Explain two disadvantages of series
arrangement for household circuit.
Ans. (i) Current is constant in series
combination, so it is impractical to connect a bulb and an electric heater in
series. (ii) When one component fails,
the circuit is broken and none of the components work.
Q. 24. How does fuse wire protect
electrical appliances ?
Ans. The fuse breaks the circuit if a fault
in an appliance causes too much current flow. This protects the wiring and the
appliance if something goes wrong. The fuse contains a piece of wire that melts
easily. If the current going through the fuse is too great, the wire heats up
until it melts and breaks the circuit.
Q. 25. Give reasons for the following : (i)
Electric bulbs are usually filled with chemically inactive gases like nitrogen
and argon.
(ii) Fuse wire is placed in series with the
device.
Ans. (i) So that the filament does not burn
on heating.
(ii) It melts and stops the flow of any
unduly high electric current and breaks the circuit.
Q. 26. What is an electric fuse ? Why it is
used in electric circuits ? Should it be placed on natural wire or on live wire
? Justify your answer.
Ans. An electric fuse is a low resistance
resistor that provides protection in the event of a overload current. Rest
answer refer to know the terms fuse-wire. It is should be placed on live wire.
Q. 27. Which is the better way to connect
lights and other appliances in domestic wiring and why ?
Ans. Parallel connection is a better way to
connect lights and other appliance in domestic circuit. It is because— (a) When
we connect a number of devices in parallel combination, each devices in
parallel combination, each device gets the same potential as provided by the
battery and it keeps on working even if other devices stops working.
(b) Parallel connection is helpful when
each device has different resistances and requires different current for its
operation as in this case the current divides itself through different devices.
Q. 28. Explain the following : (i) The
elements of electric heating devices such as bread-toasters and electric iron
are made of an alloy rather than of a pure metal.
(ii) Series arrangement is not used for
domestic circuits.
(iii) Copper and aluminium wires are
usually employed for electricity transmission.
Ans. (i) Resistivity of an alloy is
generally higher and it does not oxidize easily.
(ii) In series arrangement, same current
will flow through all the appliances which is not required and the equivalent
resistance becomes higher, hence the current drawn becomes less.
(iii) They are extremely good conductors
having a low value of resistivity.
Q. 29. Semi-conductors are certain type of
metals which allow only partial current to pass through them in one direction
only. In a solar cell, the pieces (wafers) of semi-conductor materials containing
impurities are so arranged that potential difference develops between two
regions of the semi-conductors when light falls on it. A lead storage battery
is connected in the circuit which gets charged and can be used as and when
desired.
(i) How does conductivity of
semi-conductors increases ?
(ii) Name any four materials which act as a
semiconductor.
Ans. (i) Conductivity of semi-conductors
increases when light falls on them and certain impurities are added to
them.
(ii) Materials like Silicon, Selenium,
Cadmium sulphide and Copper sulphide acts as semi-conductors.
Q. 30. Though same current flows through
the electric line wires and the bulb filament, yet only the filament of bulb
glows. Give the reason behind it.
Ans. The resistance of electric line wires
is very, very less than that of the filament of the bulb. Therefore, the
current through high resistance filament produces more heat which makes it
glow.
Q. 31. The heating elements of electric
toasters and electric irons are made of an alloy rather than a pure metal. Why
?
Ans. The heating elements of electric
toasters and electric irons are made of an alloy rather than a pure metal due
to the following reasons : (i) Alloys do not oxidise readily at high
temperature.
(ii) Alloys have higher resistivity than
their constituent elements.
Q. 32. For domestic purposes, we connect
the electrical devices in parallel instead of connecting them in series. What
are the advantages of connecting electrical devices in parallel ?
Ans. By connecting the electrical devices
in parallel : (i) Each device gets the full or same voltage.
(ii) Each device gets proper current
depending on its resistance.
(iii) If one device is switched off / on,
other electrical appliances remain unaffected. That is why for domestic
purposes we connect the electrical devices in parallel instead of connecting
them in series.
Q. 33. Explain the following ;
(i) Why is tungsten used exclusively for
filament of electric lamps ?
(ii) Why are the conductors of electric
heating devices, such as bread toasters and electric iron made of an alloy
rather than a pure metal ?
(iii) Why is series arrangements not used
for domestic circuit ?
(iv) How does the resistance of a wire vary
with its area of cross section ?
(v) How many joules are there in one kilo
watt hour ?
Ans. (i) Tungsten has a high melting point
(3380°C). It does not melt at high temperatures. It retains as much of heat
generated, so that it becomes very hot and emits light.
(ii) Conductors of electric heating devices
are made up of alloys because alloys do not oxidise readily at high
temperatures, unlike metals. Also alloys have a greater resistivity as compared
to their constituent pure metals.
(iii) Because if one component fails, the
circuit is broken and none of the other components will work. Also components
need currents of widely different values to operate properly. But in a series
circuit, the current is constant throughout the electric device.
(iv) Resistance is inversely proportional
to the area of cross-section of the wire. Thus, if the wire is thick, then
resistance is less. If the wire is thin, then resistance is large.
(v) The commercial unit of electrical
energy is kilo-watt hour.
Q. 34. Kritika observed that the tube
lights in the corridor of her school were always switched on the whole day. She
brought the matter to the notice of her class teacher who talked to the
principal about it. The principal took immediate action.
(i) Kritika helped in a way to reduce air
pollution. Explain how ?
(ii) Kritika was appreciated by the
teachers and the principal for portraying which values ?
(iii) How can the consumption of
electricity be reduced in the school ?
Ans. (i) Power/Electricity production involves
burning of fossil fuel. Electricity save is fuel saved, less fuel burnt means
less air pollution.
(ii) Courage, appreciation and protection
of environment.
(iii) Students should turn off the lights
while going out of their class, they should not waste water as it also involves
the use of electricity many times.
Magnetic Effect of Current
Q. 1. What is meant by magnetic field ?
Ans. The region surrounding a magnet in
which the force of the magnet can be detected is said to be its magnetic field.
Q. 2. Define magnetic field of bar magnet.
Ans. Magnetic field around the bar magnet
is the region in which magnetic force due to bar magnet can be experienced.
Q. 3. What is the direction of the magnetic
field lines inside a bar magnet ?
Ans. Inside a bar magnet is South to North.
Q. 4. What is the direction of the magnetic
field lines outside a bar magnet ?
Ans. Outside a bar magnet is North to
South.
Q. 5. Define the term ‘induced current’.
Ans. It is the electric current which
originates in a conductor by use of fluctuating magnetic field around the
conductor
Q. 6. Why are magnetic field lines more
crowded towards the pole of a magnet ?
Ans. This indicates that the magnetic field
in that region is strong
Q. 7. Why does a compass needle show
deflection when brought near a current carrying conductor ?
Ans. Due to production of magnetic field
around the current carrying conductor.
Q. 8. Why does two magnetic field lines not
intersect ?
Ans. Magnetic field lines never intersect
because for intersection there must be two north directions at a point, which
is not possible.
Q. 9. When a current carrying conductor is
kept in a magnetic field, state the position when maximum force acts on it.
Ans. The force experienced by a current
carrying conductor placed in a magnetic field is the maximum when conductor is
kept perpendicular to the direction of the magnetic field.
Q. 10. List two sources of magnetic fields.
Ans. Magnet, moving charges, electric
current.
Q. 11. State the effect of a magnetic field
on the path of a moving charged particle.
Ans. It can deflect the path of the charged
particle.
Q. 12. State the effect on the strength of
magnetic field produced at a point near a straight conductor if the electric
current flowing through it increases.
Ans. The strength of the magnetic field
increases.
Q. 13. State the conclusions that can be
drawn from the observation that a current carrying wire deflects a magnetic
needle placed near it.
Ans. A magnetic field exists around it
Q. 14. Meena draws magnetic field lines of
field close to 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.
Ans. Strength of the magnetic field falls
as distance increases. This is indicated by the decrease in degree of closeness
of the lines of field.
Q. 15. Name the physical quantities which
are indicated by the direction of thumb and forefinger in the Fleming’s right
hand rule.
Ans. Direction of motion of the conductor
and direction of field.
Q. 17. State the observation made by
Oersted on the basis of his experiment with current carrying conductors.
Ans. Every current carrying conductor has a
magnetic field around it.
Q.18. Mention the angle between a current
carrying conductor and magnetic field for which the force experienced by this
current carrying conductor placed in magnetic field is largest.
Ans. The angle between current carrying conductor
and magnetic field is 90°.
Q. 19. Suggest one way of discriminating a
wire carrying current from a wire carrying no current.
Ans. Bring a magnetic needle near the wire,
a current carrying wire will produce a deflection in the needle whereas a wire
without a current will not.
Q. 20. A straight wire carrying electric
current is moving out of plane of paper and is perpendicular to it. What is the
direction and type of induced magnetic field ?
Ans. Induced magnetic field will be in the
form of concentric circles in the plane of paper.
Q. 21. How can it be shown that magnetic
field exists around a wire carrying current ?
Ans. By using magnetic compass, that shows
deflection.
Q. 22. Name and state the rule which
determine the direction of magnetic field around a straight current carrying
conductor.
Ans. Right Hand Thumb Rule : Imagine
holding the current carrying straight conductor in your right hand such that
the thumb points towards the direction of current. Then the fingers of right
hand wrap around the conductor in the direction of field lines of the magnetic
field.
Q. 23. When a current carrying conductor is
kept in a magnetic field, it experiences a force. List the factors on which
direction of this force depends.
Ans. It depends upon : (i) The direction of
current through the conductor.
(ii) The direction of magnetic field in
which the conductor is placed.
Q. 24. A magnetic compass shows a
deflection when placed near a current carrying wire. How will the deflection of
the compass get affected if thecurrent in the wire is increased ? Support your
answer with a reason.
Ans. The deflection increases. The strength
of magnetic field is directly proportional to the magnitude of current passing
through the straight conductor.
Q. 25. In the experiment to show that a
current carrying conductor when placed in the uniform magnetic field
experiences a force. What happens when : (i) You reverse the terminals of the
battery ? (ii) The direction of current is perpendicular to the direction of
magnetic field ? State your observation.
Ans. (i) On reversing the terminals of the
battery, the deflection of rod will be in opposite direction.
(ii) When the direction of current is
perpendicular to the direction of magnetic field, then the rod will experience
maximum force.
Q. 26. A compass needle is placed near a
currentcarrying wire. State your observation for the following cases, and give
reason for the same in each case : (i) Magnitude of electric current in the
wire is increased. (ii) The compass needle is displaced away from the wire.
Ans. (i) Deflection of compass needle
increases. Magnetic field strength is
directly proportional to the amount of current.
(ii) Deflection of compass needle
decreases. Magnetic field strength is inversely proportional to the distance
from wire.
Q. 27. What is meant by Solenoid ? How does
a current carrying Solenoid behave ? Give its main use.
Ans: A closely bound cylindrical coil of insulated
metallic wire. A current carrying solenoid behaves as an electromagnet. The
uniform magnetic field inside it may magnetise a steel rod permanently.
Detailed Answer : A coil of many circular
turns of insulated copper wire wrapped closely in the shape of a cylinder is
called a Solenoid. The field lines around a current-carrying solenoid is
similar to that produced by a bar magnet. This means that a current - carrying
solenoid behaves as having north pole and south pole. The strong magnetic field
produced inside a solenoid can be used to magnetise a piece of magnetic
material like soft iron when placed inside the coil.
Q. 28. What is an electromagnet ? How can
we determine north and south pole of an electromagnet with the help of
magnetised iron bar.
Ans. An electromagnet is a solenoid coil
that attains magnetism due to flow of current. It works on the principle of
magnetic effect of current. (i) By suspending magnetised bar and identify its
north and south poles.
(ii) By finding the polarity of electromagnet
using the property-like poles repel.
Q. 29. (i) What is meant by a magnetic
field ? Mention two parameters that are necessary to describe it completely.
(ii) If field lines of a magnetic field are crossed at a point, what does it
indicate ?
Ans. (i) The space around the magnet or
current carrying conductor within which its influence can be felt by the
magnetic substance. Magnitude and direction.
(ii) It would mean that at the point of
intersection, compass needle would point to 2 directions which is impossible.
Q. 30. Writer the three ways to produce
magnetic field.
Ans. Three ways to produce magnetic field
are : (i) Charge induced magnetic field.
(ii) A current carrying coil may do work. (iii)
The permanent magnet and an electromagnet.
Q. 31. What does the magnetic field pattern
inside the solenoid indicate ? State how this field be utilised to make an
electromagnet. List two ways by which strength of this magnet can be increased.
Ans. It indicates that the magnetic field
is same at all points inside the solenoid.
This property is utilised to magnetise a piece of magnetic material like
soft iron when placed inside the coil. Two ways by which strength of
electromagnet can be increased : Increasing the amount of electric current
through it. Increasing the number of turns of the coil.
Q. 32. Name, state and explain with an
example the rule used to determine the direction of force experienced by a
current carrying conductor placed in a uniform magnetic field.
Ans. Fleming’s
Left Hand Rule: The direction of force which acts on the current carrying
conductor placed in a magnetic field is given by Fleming’s left hand rule. It
states that if the forefinger, thumb and middle finger of left hand are
stretched mutually perpendicular and the forefinger point along the direction
of external magnetic field, middle finger indicates the direction of current,
then thumb points along the direction of force acting on the conductor. Example
: When an electron enters a magnetic field at right angles, the direction of
force on electron is perpendicular to the direction of magnetic field and
current according to this rule.
Q. 33. Can a freely suspended current
carrying solenoid stay in any direction ? Justify your answer. What will happen
when the direction of current in the solenoid is reversed ? Explain.
Ans. A current carrying solenoid behaves
like a magnet. When suspended freely, it will stay in north - south direction.
On reversing current its polarity will be reversed and so it will turn at 180°.
Q. 34. Explain briefly two different ways
to induce current in a coil. State the rule which determines the direction of
induced current.
Ans. Two ways to induce current in a coil :
(i) If a coil is moved in a magnetic field.
(ii) By bringing a bar magnet close to the
coil or taking away from it.
(iii) By rotating the coil in a uniform
magnetic field. (Any two) Statement of Fleming’s Right Hand Rule.
Q. 35. State and explain Fleming’s right
hand rule for the direction of induced current.
Ans. The direction of induced current in a
straight conductor is given by Fleming’s right hand rule. It states that spread
the thumb, forefinger and the middle finger of your right hand at right angle
to one another in such a way that the forefinger points in the direction of
magnetic field, thumb, points in the direction of motion of conductor then the
direction in which the middle finger points, gives the direction of induced
current.
Q. 36. Explain whether an alpha particle
will experience any force in a magnetic field if:
(i) it is placed in the field at rest.
(ii) it moves in the magnetic field
parallel to field lines.
(iii) it moves in the magnetic field
perpendicular to field lines.
Ans. (i) No, because, a charged particle at
rest does not interact with magnetic field.
(ii) No, because, the force is zero if
current and field are in the same direction.
(iii) Yes, because, the force is maximum
when current and magnetic field are maximum.
Q. 37. Find the direction of magnetic field
due to a current carrying circular coil held:
(i) Vertically in North-South plane and an
observer looking it from east sees the current to flow in anti-clockwise
direction.
(ii) Vertically in East-West plane and an
observe looking it from south sees the current to flow in anti-clockwise
direction.
(iii) Horizontally and an observer looking
at it from below sees current to flow in clockwise direction.
Ans. (i) Towards south in upward direction.
(ii) Towards west in upward direction.
(iii) Towards upward direction.
Q. 38. What are magnetic field lines ? List
three characteristics of these lines. Describe in brief an activity to study
the magnetic field lines due to a current flowing in a circular coil.
Ans. Representation of the magnetic field
path along which an imaginary free north pole would tend to move. The tangent
at any point on the magnetic field line gives the direction of the magnetic
field at that point.
(i) Emerge at north pole and merge at south
pole. Inside the magnet, the direction of field lines is from south pole of
magnet to its north pole and are closed curves.
(ii) At the points where the magnetic field
is stronger, field lines are crowded and vice-versa.
(iii) No two magnetic field lines can
intersect each other.
Q. 39. What are magnetic field lines ? List
any two characteristics of field lines. Draw the pattern of magnetic field
lines due to a current carrying circular loop.
Ans. A magnetic field line is the path
along which a free north pole tends to move.
Characteristics : (i) Outside a magnet, the
magnetic field lines are directed from N-pole of magnet towards S-pole.
However, inside a magnet field lines are directed from S-pole to N-pole.
(ii) The relative strength of magnetic
field lines is given by degree of closeness of the field lines. More crowded
field lines means a stronger field.
(iii) No two magnetic field lines can ever
intersect each other.
Q. 40. (i) What is a solenoid ? (ii) Draw
the pattern of magnetic field formed around a current carrying solenoid.
Compare this field to that of a bar magnet.
(iii) Explain an activity to show that a
current carrying conductor experiences a force when placed in a magnetic field.
Ans. (i) A coil of many circular turns of
insulated copper wire wrapped closely in the shape of a cylinder is called
solenoid.
(ii) Magnetic fields of a solenoid and a
bar magnet are similar.
(iii) A small aluminium rod suspended
horizontally from a stand using two connecting wires. Place a strong horse shoe
magnet in such a way that the rod lies between the two poles with the magnetic
field directed upwards. For this put the north pole of the magnet vertically
below and south pole vertically above the aluminium rod. Connect the aluminium
rod in series with a battery, a key and a rheostat. Pass a current through the
aluminium rod from one end to other. The rod is displaced towards left. When
the direction of current flowing through the rod is reversed, the displacement
of rod will be towards right.
Q. 41. (i) A coil of insulated copper wire
is connected to a galvanometer.
What happens if a bar magnet is :
(a) pushed into the coil ?
(b) withdrawn from inside the coil ?
(c) held stationary inside the coil ? Give
reasons for your observation.
(ii) Mention one more method of inducing
current in a coil.
Ans. (i) (a) When a bar magnet is pushed
into the coil of insulated copper wire connected to a galvanometer, an induced
current is set-up in the coil due to charge of magnetic field through it. As a
result, galvanometer gives a deflection (say towards left).
(b) When the bar magnet is withdrawn from
inside the coil, again an induced current is set up in the coil due to charge
of magnetic field through it. As a result galvanometer gives a deflection in
the reverse direction. (say towards right).
(c) If the bar magnet is held stationary
inside the coil, then there is no induced current in the coil, because there is
no change in magnetic field through it. As a result, galvanometer does not show
any deflection.
(ii) By changing current in another coil
placed near it.
Q. 42. (i) A stationary charge is placed in
a magnetic field. Will it experience a force ? Give reason to justify your
answer.
(ii) On what factors does the direction of
force experienced by a conductor when placed in a magnetic field depend ?
(iii) Under what conditions is the force
experienced by a current carrying conductor placed in a uniform magnetic field
maximum ?
(iv) Name and state the rule which gives
the direction of force experienced by a current carrying conductor placed in a
magnetic field.
Ans. (i) No, a magnetic field exerts a
force only on moving charges.
(ii) The direction of force depends on the
direction of current and direction of magnetic field.
(iii) The force is maximum when the
direction of current is at right angles to the direction of magnetic field.
(iv) Fleming’s Left Hand Rule. According to this rule—‘‘Stretch the thumb,
forefinger and middle finger of your left hand such that they are mutually
perpendicular to each other. If the first finger points in the direction of
magnetic field and the middle finger in the direction of current, then the
thumb will point in the direction of motion or the force acting on the
conductor.’’
Q. 43. A student fixes a sheet of white
paper on a drawing board. He places a bar magnet in the centre of it. He
sprinkles some iron filings uniformly around the bar magnet. Then he taps the
board gently and observes that the iron filings arrange themselves in a
particular pattern.
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(i) Why do the iron
filings arrange in a pattern ?
(ii) What does the lines along which the
iron filings align represent ?
(iii) What does the crowding of iron
filings at the end of the magnet indicate ?
(iv) How does strength of magnetic field is
indicates ?
Ans. (i) Due to the force exerted by the
magnet within its magnetic field.
(ii) The lines represent magnetic field
lines.
(iii) Crowding of iron filings at the ends
of the magnet indicates that the magnetic field is strongest near the poles of
the magnet.
(iv) The strength of magnetic field is
indicated by the closeness of the field lines. Closer the lines, more will be
the strength and farther the lines, lesser will be the field strength.
Q. 44. Name any two sources of direct
current.
Ans. Some sources of direct current are
cell, battery and DC generator, etc.
Q. 45. Mention the advantage of A.C. over
D.C. for long distance transmission.
Ans. A.C. can be easily transmitted over
long distances without much loss of energy.
Q. 46. What is the frequency of A.C. in
India ?
Ans. The frequency of alternating current
in India is 50 Hz.
Q. 47. List any one point of difference
between A.C. and D.C.
Ans. As the name suggests, direct current
always flows in one direction and alternating current reverses its direction
periodically.
Q. 48. Name the type of current : (i) Used
in household supply (ii) Given by a cell.
Ans. (i) Alternating current (ii) Direct
current.
Q. 49. An alternating current has a
frequency of 50 Hz. How many times does it change its direction in one second ?
Ans. 100 times
Q. 50. Who discovered the electromagnetic
induction ?
Ans. Electromagnetic induction was first
discovered by Michael Faraday.
Q. 51. Name the device used to prevent
damage to the electrical appliances and the domestic circuit due to
overloading.
Ans. Electric fuse.
Q. 52. State the value of potential
difference between the live wire and the neutral wire in our country.
Ans. 220 V.
Q. 53. What is Short Circuit ?
Ans. Short Circuit is simply a low
resistance connection between the two conductors supplying electrical power to
any circuit. This results in excessive current flow in the power source through
the ‘short’ and may even cause the power source to be destroyed.
Q. 54. State a difference between the wires
used in the element of an electric heater and in a fuse.
Ans. The melting point of the wire used in
heater elements is high while a fuse wire has low melting point.
Q. 55. How is the type of current that we
receive in domestic circuit different from the one that runs a clock ?
Ans. In domestic circuit, it is A.C. while
clock cell gives D.C.
Q. 56. State the use of earth wire in
domestic electric circuit.
Ans. To ensure that any leakage of current
to the metallic body of the appliance does not give shock to the user.
Q. 57. Mention the colour convention for
live, neutral and earth wires.
Ans. Live wire — Red, Neutral Wire — Black,
Earth wire — Green.
Q. 58. In domestic electric circuit, with
which wire do we connect a fuse ?
Ans. Live wire.
Q. 59. What is the role of the two
conducting stationary brushes in a simple electric motor ?
Ans. The brushes are connected to the
battery and touch the outer side of two halves of the split ring whose inner
sides are insulated and attached to the axle.
Q. 60. Explain the terms : Overloading and
shortcircuiting.
Ans. Overloading occurs when or two many
appliances of high power ratings are connected to a single electric circuit and
switched on simultaneously. As a result, the circuit draws large current beyond
its capacity. Short-circuiting takes place in domestic power supply if the live
wire and the neutral wire come into direct contact, due to damage of insulation
of live wire.
Q. 61. When does Short Circuit occurs ?
Ans. A common short circuit occurs when the
positive and negative terminals of a battery are connected with a
low-resistance conductor, like a wire. With a low resistance in the connection,
a high current will flow, causing the delivery of a large amount of energy in a
short period of time.
Q. 62. What is a solenoid ? Mention two
ways to increase the strength of the field of a solenoid.
Ans. A coil of many circular turns of
insulated copper wire wrapped closely in the shape of cylinder is called a
solenoid. Two ways to increase the strength of the field of a solenoid:
(i) By increasing the number of turns. (ii)
By increasing current.
Q. 63. Mention the provision of two
different current ratings in our domestic circuits. Explain with reason, the
advantage of such a provision.
Ans. The two different current ratings
provided in domestic circuits are 5A and 15A. This is because different electrical
appliances have different power ratings, so they draw different currents when
connected in the mains. Some appliances need smaller currents, while some other
need heavy currents.
Q. 64. Write the use of safety device used
in electric circuit. Ans. Electric Fuse. A fuse is a small conducting wire of
low melting point which protects the household electrical system from getting
damaged due to unusual high current. The unusual high current may be due to
short circuit, power fluctuation etc.
Q. 65. 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 ?
Ans. Electric fans, mixers, washing
machines, computer drives etc. Motors convert electrical energy into mechanical
energy whereas generators convert mechanical energy into electrical energy.
Q. 66. Explain the function of an earth
wire. Why is it necessary to earth metallic appliances ?
Ans. Earth wire is used as a safety measure
especially for those appliances that have a metallic body, for example –
electric press, toaster etc. The metallic body is connected to the earth wire,
which provides a low resistance conducting path for the current. Thus it
ensures that any leakage of current to the metallic body of the appliances
keeps its potential to that of the earth and the user may not get a severe
shock.
Q. 67. List two precautions to be taken to
avoid overloading in the domestic circuit. Also state one difference between
overloading and short circuiting.
Ans. Precautions : (i) A fuse of
appropriate rating should be used in the circuit.
(ii) Too many appliances should never be
connected to a single socket.
Overloading : Occurs when a large number of
higher current drawing appliances are switched on at the same time and the
total current drawn through the circuit exceeds its rated value. Short
circuiting : Occurs when the live wire comes in direct contact with neutral
wire.
Q. 68. What is overloading and short
circuiting ? What is the function of earth wire ?
Ans. If too many electrical appliances of
high power rating are switched on at the same time, they draw large current
from the circuit. This is called overloading. If the live wire and neutral wire
come in contact either directly or via conducting wire, the situation is called
short circuiting. To avoid risk of electrical shock, the metal body of the
appliances is earthed. Earthing means to connect the metal case of the appliance
to earth by a means of a metal wire called earth wire.
Q. 69. What is short circuiting ? State one
factor/ condition that can lead to it. Name a device in the household that acts
as a safety measure for it. State the principle of its working.
Ans. Situation in which live and neutral
wire come in direct contact, abruptly increasing the current in the circuit is
called short–circuiting. Factor – Insulation of wire is damaged / fault in
appliance. Safety device – electric fuse. Electric fuse is an application of
Joule’s heating. If there is a current larger than the specified value the
temperature of fuse wire increases and it melts to break the electric circuit
and stop the flow of unduly high electric current.
Q. 70. What is earth wire ? How it works in
our domestic circuit ?
Ans. The earth wire gives a safe route for
the current if the live wire touches the outer casing. Live wire is A wire
carrying electric current. The metallic body is connected to the earth wire,
which provides a low resistance-conducting path for the current. It ensures
that any leakage of current of the metallic body the appliance will flow to the
earth only and the user may not get a severe shock.
Q. 71. (a) Give the significance of the following
in a domestic circuit: (i) electric meter (ii) earthing (b) List two
precautions that should be taken to avoid overloading.
Ans. (a) (i) Electric meter : It is used to
record the consumption of electrical energy in kWh in the circuit. (ii)
Earthing : It prevents electric shock. The earthwire is joined to the metal
case of the appliance and provides a low resistance conducting path for the
current.
(b) (i) Too many appliances should not be
connected to a single socket.
(ii) Fuse of current rating 5A and 15A
should be separately used in the domestic circuit.
Q. 72. 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 ?
Ans. Fuse is used for protecting appliances
due to shortcircuiting or overloading. The fuse is rated for a certain maximum
current and blows off when a current more than the rated value flows through
it. If a fuse is replaced by one with larger ratings, the appliances may get
damaged while the protecting fuse does not burn off. This practice of using
fuse of improper rating should always be avoided.
Q. 73. List three factors which can cause
overloading of domestic electric circuits.
Ans. Three factors which can cause overloading
are : (i) When the live wire and the neutral wire come into direct contact.
(ii) Accidental hike in the supply voltage.
(iii) By connecting too many appliances to
a single socket.
Q. 74. Write one difference between direct
current and alternating current. Which one of the two is mostly produced at
power stations in our country ? Name one device which provides alternating
current. State one important advantage of using alternating current.
Ans. Direct current does not change its
direction with time whereas alternating current reverses its direction
periodically. Most power stations produce AC in India/an AC generator. Electric
power can be transmitted over long distances without much loss of energy.
Q. 75. Name the three types of wires used
in household circuits. Out of these three which wire is used as a safety
measure especially for those appliances that have metallic body. State the
colour of insulation used for this wire. How it ensures the safety of the user
?
Ans. (i) Live wire (ii) Neutral wire (iii)
Earth wire,
Earth wire, colour of insulation – green.
The metallic body connected to the earth wire keeps its potential to that of
the earth’s zero potential and if there is any leakage the user does not get
any kind of electric shock.
Q. 76. Distinguish between alternating
current and direct current. Explain why alternating current is preferred over
direct current for transmission over long distances.
Ans. Alternating Current : If the current
changes direction after equal intervals of time, it is called alternating
current. The positive and negative polarities of AC are not fixed. Direct
Current : If the current always flows in the same direction, it is called
direct current. It can be obtained from a cell or a battery. The positive and
negative polarities of DC are fixed for long distance transmission. AC is
preferred as it caused minimum loss of energy during transmission.
Q. 77. A coil of insulated copper wire is
connected to a galvanometer. What would happen if a bar magnet is : (i) Pushed
into the coil ?
(ii) Withdrawn from inside the coil ?
(iii) Held stationary inside the coil ?
Ans. (i) Due to change in magnetic flux
linked with coil, the galvanometer shows deflection (say towards right).
(ii) Due to change in magnetic flux linked
with coil, the galvanometer shows deflection (say towards left opposite to that
in case one).
(iii) As it is stationary, no change in
magnetic flux linked with coil, so galvanometer shows no deflection.
Q. 78. It is necessary to connect an earth
wire to electric appliances having metallic covers. Why? How will you identify
earth wire in household circuit ?
Ans. The earth wire provides a low
resistance conducting path for the current. In case of any leakage of current
to the metallic body of any appliance, the earth wire allows the current to
flow into the earth and user saved from electric shock. Earth wire has green
insulation.
Detailed Answer : The earth wire is
connected to a metallic plate deep inside the earth. In this way, the metallic
body of appliance is connected to the earth, which provides a low resistance
conducting path for electric current. Hence, any leakage of current to the
metallic body of appliance keeps the potential to that of earth. The user might
not get a severe electric shock on touching such an appliance. Earth wire has
green insulation, so it can be identified.
Q. 79. (i) What is meant by the terms
alternating current and direct current ?
(ii) Name a source of alternating current
and a source of direct current.
(iii) Mention the frequency of AC supply in
India. (iv) State two important advantages of alternating current over direct
current.
Ans. (i) Refer to know the terms.
(ii) Source of AC current — electric
generator. Source of DC current — electric cell.
(iii) Frequency of alternating current in
India is 50 Hz. (iv) (a) Alternating current reverses its direction
periodically but direct current always flow in one direction. (b) AC electric
power can be transmitted over long distances without much less of energy while
DC not.
Q. 80. Explain the meanings of the words
“electro magnetic‘‘ and ‘induction‘ in the term electromagnetic induction. List
three factors on which the value of induced correct produced in the circuit
depends. Name and state the rule to determine the direction of induced current.
State one practical application of this phenomenon in everyday life.
Ans. ‘Electromagnetic‘ stands for the
property by which magnetism developed by a current of electricity. ‘Induction‘
stands for the action or process of bringing about or giving rise to something.
Induced current depends on : (i) The strength of magnetic field used.
(ii) The rate of change of magnetic flux
associated with the coil.
(iii) The conductor in the form of coil of
many turns of insulated wire. Rule : Fleming right Hand Rule Statement : Refer
to Quick Review. Application : Electric Generator.
Q. 81. Describe any five safety measures
that should be taken while dealing with electric appliances connected in
domestic electric circuit.
Ans. (i) To avoid shocks from electrical
appliances use proper-earthing arrangement.
(ii) Replace old worn out and damaged wires
with new set.
(iii) Put the main switch off when removing
any fault in electric circuits.
(iv) Wear rubber shoes and gloves while
dealing with replacement of any appliance.
Q. 82. The deflection of the compass needle
increases as it is moved towards the pole of a bar magnet. Why ?
Ans. The magnetic field due to a bar magnet
is highest near its two poles. Therefore, the force exerted by the bar magnet
on the compass needle is larger in such a region. That is why the deflection of
the compass needle increases when it is moved towards the pole of a bar magnet.
Q. 83. Sita wants to determine the
direction of a force, experienced by a straight current carrying conductor
placed in a magnetic field which is perpendicular to it. Which rule will she
apply to determine it ?
Ans. Sita will apply Fleming’s left hand
rule. According to Fleming’s left hand rule, when the forefinger, the central
finger and the thumb of the left hand are stretched mutually perpendicular to
each other and if the forefinger points in the direction of the magnetic field,
the central finger in the direction of current, then the thumb points in the
direction of force on the conductor.
Q. 84. All the household electric
appliances are connected in parallel. List two advantages of this type of
arrangement.
Ans. Two advantages : (i) To have equal
potential difference across each appliance.
(ii) Each appliance has separate switch to
ON / OFF the flow of current through it, so that each circuit can work
independently.
Q. 85. How would the strength of magnetic
field due to current carrying loop be affected if : (i) Radius of the loop is
reduced to half its originalvalue ?
(ii) Strength of current through the loop
is doubled ?
Ans. (i) Strength of magnetic field gets
doubled and increases.
(ii) Strength of magnetic field gets
doubled and increases.
Q. 8. Is it possible to change the polarity
of an electromagnet ? Give any two uses of electromagnets.
Ans. Yes, we can change the polarity of an
electromagnet by reversing the direction of current. Uses of electromagnets :
(i) In electric bells (ii) For separating magnetic substances such as iron and
other metallic substances from metallic scrap.
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