CLASS 10 ELECTRCITY
1. Define electric potential.
Answer: Electric potential at a point in an electric field is defined
as the work done in moving a unit positive charge from infinity to that point
in the electric field.
2. State the relation between work (W), change (q) and electric
potential (V).
Answer: V = W/q.
3. What
is the S.I. unit of electrical potential? (CBSE 2005, 2012)
Answer: volt.
4. Define 1 volt electric potential.
Answer: Electric
potential is said to be 1 volt if 1 Joule of work is done in moving 1 coulomb
charge from infinity to a point in the electric field.
5. Is
electric potential a scalar or a vector physical quantity?
Answer: Electric
potential is a scalar physical quantity.
6.What is
meant by potential difference between two points ? (CBSE 2011)
Answer: Work done
per unit charge in moving a unit positive charge from one point to another
point in an electric field is called potential difference between two points.
8. Name
the instrument used to measure the electric potential difference. (CBSE 2011,
2012)
Answer: Voltmeter.
9.Write
down the relation between the potential difference between two points A and B
in a conductor, work done W in moving a unit charge from point B to A and the
charge q.
Or
State the relation between work, charge and potential difference for an
electric circuit.
[CBSE (All India) 2009, 2013]
Or
Express work done in an electric field in terms of charge and
potential difference. (CBSE 2014)
Answer:
10.Define electric current.
Answer:
Electric current is defined as the amount of electric charge
flowing through any cross-section of a conductor per unit time.
11.Write down the relation between the electric current I passing
in a conductor, change Q flowing in the conductor and time t.
Or
Write the relation between coulomb and ampere. (CBSE 2014)
Answer:
12.State the relation between the current I passing in a conductor,
number of electrons (n) flowing through any cross-section of the conductor,
magnitude of charge on an electron (e) and time t.
Answer: I = ne/t.
13. State SI unit of electric current.
Answer: ampere (A)
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14. Define 1 ampere electric current.
Answer: Electric
current through a conductor is said to be 1 ampere if 1 coulomb electric charge
flows through a cross-section of a conductor in 1 second.
15. How is the direction of electric current related to the
direction of flow of electrons in a wire?
[CBSE (All India) 2009]
Answer: The direction of electric current in a wire is just
opposite to the direction of flow of electrons in the wire.
16. Name the
instrument used to measure electric current in a circuit. [CBSE (Delhi) 2008,
2011, 2012, 2015]
Answer: Ammeter.
17. How is ammeter connected in the circuit to measure electric current?
(CBSE 2011, 2012)
Answer: Ammeter is connected in series in an electric
circuit.
18. How is voltmeter connected in the circuit to measure electric
potential difference?
Answer: Voltmeter is connected in parallel across a
conductor or resistor in the electric circuit.
19. Define ohm’s law. [CBSE 2010, 2011, 2012, 2013, 2015]
Or
State the law that gives the relationship between the potential difference (V)
across the two ends of a conductor and the current (I) flowing through it.
(CBSE 2012)
Answer:
Ohm’s law states that the electric current flowing through a conductor is
directly proportional to the potential difference across the ends of the
conductor, provided the temperature and . other physical conditions of the
conductor remain the same.
20.Graph is plotted between the values of potential difference (V)
and current (I). What v conclusion do you draw about the relation between V and
I from this graph. State this relation in your words. (CBSE 2013)
Answer:
The potential difference (V) is directly proportional to the current (I).
21. State the formula showing how the current I in a conductor
varies with the potential difference V applied across it. [CBSE (Delhi)
2004(S), 2012]
Answer:
V ∝ I or V = IR or I = V/R .
22. Define electrical resistance of a conductor. (CBSE 2013)
Answer:
It is the property of a conductor to
oppose the flow of electric charge through it.
Resistance of a conductor, R = V/I , where V is the potential difference across
the conductor and I is the current flowing through the conductor.
23. State SI unit of resistance. [CBSE (Delhi) 2004(S), 2012]
Answer: ohm
(Ω).
24. “The resistance of a conductor is IΩ”. What is meant by this statement?
(CBSE 2010, 2011, 2012)
Or
Define 1 ohm resistance. (CBSE 2014)
Answer:
The resistance of a conductor is said to be 1Ω if a potential difference of IV
across the ends of the conductor makes a current of 1A to flow through it.
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25. What is the shape of the graph obtained by plotting potential
difference applied across a conductor against the current flowing through it?
Answer: A straight line passing through the origin and has
constant slope.
26. What is the name the physical quantity which is equal to V/I ?
Or
Name the physical quantity whose unit is volt-ampere.
Answer: y = R. Therefore, physical quantity is resistance of
a conductor.
27.What does the slope of V-I graph at any point represent ?
Answer:
Therefore, slope of V-I graph represents reciprocal of the resistance of a
conductor resistance of a conductor.
28. What happens to resistance of a conductor when temperature is
increased ?
Answer: Resistance of a conductor increases with the increase
in temperature.
29. The voltage — current (V-I) graph of a metallic circuit at two
different
temperature T1 and T2 is
shown. Which of the two temperatures is higher and why ? (CBSE 2010)
Or
The voltage-current (V-I) graph of a metallic conductor at two different
temperatures T1 and T2 is
shown in figure. At which temperature is the resistance higher ? (CBSE 2011,
2012)
Answer:
Slope of I-V graph = resistance of metallic conductor.
Since, slope of I-V graph at temperature T2 is greater than the slope of I-V graph at temperature T1, therefore, resistance at T2 is greater than resistance at T1 . Since, resistance of a metallic conductor increases with increase in temperature, therefore, T2 > T1.
30. How does the resistance of a wire vary with its cross-section
area ?
Answer: Resistance of a wire is inversely proportional to its
cross-sectional area. More is cross-sectional area of a conductor; less is the
resistance of the conductor.
31. State the relation between the resistance R of a conductor,
resistivity ρ of a conductor, length l of a conductor and area of cross-section
A of the conductor.
Or
Write an expression for the resistivity of a substance. (CBSE 2015)
Answer:
where R is the resistance, A is the area of cross-section and l is the length
of the substance.
32. What is electrical resistivity? [CBSE 2010, 2014, 2015]
Answer:
Electrical resistivity of a material is defined as the resistance of an object
(made of the material) of unit length and unit area of cross-section.
34. State SI unit of resistivity. [CBSE (Delhi) 2004 (S), 2014,
2015]
Answer: ohm-metre
(Ω-m).
35. A wire of resistivity ‘ρ’ is pulled to double its length. What
will be its new resistivity? (CBSE 2010, 2011)
Answer: New resistivity will also be ‘ρ’ because resistivity
of a wire does not depend on its length.
36.You have a metal, insulator and an alloy. Write these substances
in the ascending order of their electrical resistivity.
Answer Resistivity of metal < resistivity of alloy <
resistivity of insulator.
37.n resistance each of R are connected in series in a circuit.
What is the total effective resistance of the circuit ?
(CBSE 2010)
Answer: Reff =
R1 + R2 + R3 + … upto n = nR
38.n resistors each of resistance R are connected in parallel in an
electric circuit. What is the total effective resistance of the circuit ? (CBSE
2010 )
Answer:
39.n resistors each of resistance R are connected in parallel in an
electric circuit. What is the total effective resistance of the circuit? (CBSE 2010)
Answer:
40.n resistors each of resistance R are first connected in series
and then in parallel. What is the ratio of the total effective resistance of
the circuit in series combination and parallel combination? (CBSE 2013)
Answer:
41. You have two metallic wires of resistances 60 and 30. How will
you connect these wires to get the effective resistance of 20? (CBSE 2010,
2012)
Answer: In parallel.
42.Draw a schematic diagram of a circuit consisting of battery of
two cells each of 1.5 V, 50 resistor, 100 resistor and 150 resistor and a plug
key, all connected in series. [CBSE (All India) 2009, 2011, 2013]
Answer:
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43. What is heating effect of electric current?
Answer:
The production of heat in a conductor due to the flow of electric current
through it is called heating effect of electric current.
44. Write down the relation between heat
produced H in a conductor of resistance R through which current I passes for t
seconds.
Answer: H = i2Rt
45.State Joule’s law of
heating. (CBSE 2012, 2012, 2013, 2014, 2015)
Answer:
According to Joule’s law of heating, the amount of heat produced in a conductor
is
1.
directly proportional to the square of
electric current passing through the conductor,
2.
directly proportional to the resistance
of the conductor, and
3.
directly proportional to the time for
which electric current passes through the conductor.
46. Write a
mathematical expression for Joule’s law of heating. Name one device which works
on this principle.
(CBSE 2010, 2015)
Answer:
H = I2 Rt. An electric
heater and electric bulb work on Joule’s law of heating.
47. What is meant by
the statement that the rating of fuse in a circuit is 5A ?
Answer:
It
means maximum current of 5A can pass through the fuse without melting it.
48. Name the material
used for making the filament of a bulb.
Answer: Tungsten.
49. Define electric energy. (CBSE 2013,
2015)
Answer: It is defined as the
work done by a source of electricity to maintain electric current in an
electrical circuit.
50. Define electric
power.
Answer: It is defined as the
amount of electric energy consumed in an electric circuit per unit time.
51. Name the physical
quantity expressed as the product of potential difference and electric current.
Answer: Electric power.
52. State SI unit of
electric power.
Answer: Volt x ampere (or
Watt).
53. Name the unit used
in selling electrical energy to consumers.
Or
what is the commercial unit of energy?
Answer: Kilowatt hour (kWh).
54. Which one is having
lesser resistance: A 60 W bulb or a 40 W bulb?
Answer:
Hence, bulb of higher wattage will have less resistance. In other words,
resistance of 60W bulb is less than the resistance of 40 W bulbs.
55. What is the
difference between kilowatt and kilowatt hour? (CBSE 2012, 2014)
Answer:
Kilowatt is the unit of electric power and kilowatt hour is the commercial unit
of electric energy.
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56. Define electric
current. State and define its SI unit. (CBSE 2012, 2013, 2014, 2015)
Answer:
Electric current is defined as the amount of charge flowing through a
cross-section of a conductor in unit time.
SI unit of electric current is ampere (A).
Electric current through a conductor is said to be 1 ampere if one coulomb
charge flows through a cross-section of the conductor in one second.
57. List two
differences between a voltmeter and ammeter. (CBSE 2013)
Answer:
|
Ammeter |
Voltmeter |
|
1.
Ammeter measures electric current in the circuit. 2.
Ammeter is connected in series in an electric circuit. |
1.
Voltmeter measures the potential difference between two points on a
conductor. 2.
Voltmeter is connected in parallel across the ends of a conductor or
resistor. |
58. What is an electric circuit ?
Distinguish between an open and a closed circuit.
[CBSE (All India) 2009, 2010, 2011]
Answer:
electric circuit:
An
electric circuit is a closed conducting path containing a source of electric
energy (i.e., a cell or a battery) and a device or element or load (say, an
electric bulb) utilizing the electric energy.
The direction of electric current is opposite to the direction of the flow of
electrons in the conductor.
Open electric circuit: An electric circuit through which no electric current flows is known as open electric circuit.
The electric circuit shown in figure 10
(A) will be open circuit if the plug of the key is taken out or if the
connecting wire breaks from any point.
Closed circuit: An electric
circuit through which electric current flows continuously is known as closed
circuit (Figure 10 (B)).
59.What do the following symbols
represent in a circuit ? Write the name and one function of each.
(CBSE 2013)
Answer:
(i) It represents a battery. It maintains a potential difference across the circuit element for the flow of current in the circuit.
(ii) It represents an ammeter. Ammeter is used to measure the electric current in the circuit.
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60.What do the following symbols represent in a circuit ? Write the
name and one function of each ?
[CBSE 2011, 2012, 2013, 2015]
Answer:
(i) It represents a closed plug key. It is used to make the closed electric
circuit.
(ii) It represents a variable resistance. It is used to increase or decrease
the electric current in the circuit.
(iii) It represents a voltmeter. It is used to measure the potential difference
across a resistor in the circuit.
(iv) It represents a galvanometer. It is used to detect the presence of small
current in the circuit.
(v) It represents wire crossing (not connected with each other). The wires are
used to connect various components in the circuit.
61. Express Ohm’s law
both by a mathematical formula and by a graph line. (CBSE (Delhi) 2004 (S))
Answer:
Mathematical
formula of Ohm’s law is
V = IR
For a graph line,
62. List the factors on
which the resistance of a conductor depends. Write the expression to show the
relation of resistance with these factors. (CBSE 2013, 2014)
Answer:
Resistance of a factor depends on its length (l) and area of cross section (A).
63. What is likely to
happen and how it would affect the value of resistance if we pass the current
for a longer time ? (CBSE 2014)
Answer:
When current passes through a conductor for a longer time, the conductor is
heated due to Joule’s heating effect. Resistance of conductor increases with
increase in temperature due to heating effect.
64. Two wires, one of
copper and other of manganin, have equal lengths and equal resistances. Which
wire is thicker ? Given that resistivity of copper is
lower than that of manganin. (CBSE 2011, 2015)
Answer:
Since l and R of both the wires are same, so A α ρ . Since, resistivity of manganin
is higher than that of copper, so manganin wire is thicker than that of copper
wire.
65. Mention the
condition under which charge can move in a conductor. Name the device which is
used to maintain this condition in an electric circuit. (CBSE 2012, 2013)
Answer: The ends of the
conductor must be maintained at different voltages. This condition is
maintained in a conductor by a cell or a battery.
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66. In the circuit diagram shown, the two resistance wires A and B
are of the same length and same material, but A is thicker than B. Which
ammeter A1 or A2 will indicate higher reading for current?
Give reason.
(CBSE 2010, 2011)
Answer:
The current will be high if resistance is low.
p and l for both wires A and B are same but area of cross-section (A) of wire A is- more than the wire B. Therefore, resistance of wire A is less than the resistance of the wire B. Hence, ammeter A connected in series with the wire A will indicate higher reading for current.
67.In the circuit diagram shown, the two resistance wires A and B are of same area of cross-section and same material, but A is longer than B. Which ammeter A1 and A2 will indicate higher reading of current ? Give reason. (CBSE 2010, 2011)
Answer:
The current in the circuit is high if the resistance of the circuit is low. Since wire A is longer than B, so the resistance of wire B is less than the resistance of wire A. Hence, current flowing through wire B is greater than the current flowing through wire A. Therefore, ammeter A2 will indicate higher reading of current.
68.Two students perform
experiments on two given resistors R1 and R2 and
plot the following V-I graphs. If R1 > R2,
which of the two diagrams correctly represent the situation on the plotted
curves ? Justify your answer. (CBSE 2010, 2012)
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Answer:
Resistance of a conductor = slope of I-V graph. It means, resistance is high if
slope of I-V graph for it is steeper. Since R1 > R2, therefore, slope of
I-V graph for R1 must be steeper than the slope of
I-V graph for R2 Thus, diagram I represents the situation
correctly.
69.Is the circuit given below correct ? Justify your answer. (CBSE 2014)
Answer:
The given circuit is incorrect because in a circuit, ammeter is always connected in series and voltmeter is connected parallel to the resistor R1. The correct circuit is shown in figure.
70. List two
distinguishing features between the resistance and resistivity of a conductor.
(CBSE 2014)
Answer:
1.
Resistance of a conductor depends on the
length and area of cross-section of the conductor. On the other hand,
resistivity of a conductor is independent of length and area of cross-section
of the conductor.
2.
Unit of resistance is ohm. Unit of
resistivity is ohm-metre.
71. Draw a schematic diagram of a circuit consisting of a battery of 12V, three resistors of 5Ω, 10Ω and 20Ω connected in parallel, an ammeter to measure the total current through the circuit, voltmeter to measue the potential difference across the combination of resistors. [CBSE (All India) 2008]
Answer:
72.(a) Why an ammeter likely to bum out if you connect it in parallel ?
(b) Why is series arrangement not found satisfactory for domestic lights ?(CBSE 2010, 2011)
Answer:
(a) Ammeter is a low resistance device. If it is connected in parallel, a large current flows through it. Hence, large heat is produced and it may burn the ammeter.
(b) If domestic lights are connected in series, then all lights will be switched off even when only one light fuses.
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73.Draw diagrams to show series and parallel combinations of resistors. State three salient features each of both the combinations. (CBSE 2015)
Answer:
Salient features of series combination :
1.
The net resistance of the combination is
equal to the sum of the resistances of the individual resistor.
2.
The current flowing through each resistor
is the same.
3.
The voltage applied across the series
combination of resistors is equal to the sum of potential differences across
the individual resistors.
Salient
features of parallel combination :
1.
The reciprocal of the net resistance of
the combination is equal to the sum of the reciprocal of the resistances of the
individual resistors.
2.
Different amount of current flows through
each resistor.
3.
The potential difference across each
resistor is equal to the voltage applied across the combination.
74. Why is the tungsten used almost
exclusively for filament of electric lamps?
Or
Why is Tungsten used for the filament in electric bulb? (CBSE 2011, 2014, 2015)
Answer:
This is because
1.
melting point of tungsten is very high
(about 3380°C) and
2.
it does not oxidize (or burn) even at
higher temperatures.
75.Why do copper or aluminium
wires usually used for electricity transmission and distribution purposes ?
(CBSE 2012, 2013)
Or
Copper and aluminium wires are usually employed for electricity transmission.
Explain reason.
Answer:
The resistivities of copper and aluminium are very low, so electric current
flows easily through them. Hence, copper and aluminium wires are usually used
for electricity transmission and distribution purpose.
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76.Should the heating
element of an electric iron be made of iron, silver or nichrome wire 1
[CBSE (Foreign)2005, 2016]
Or
List two reasons why nichrome is used for making heating element of electrical
appliances. (CBSE 2015)
Answer:
It
should be made of nichrome wire because
1.
Resistivity of nichrome is greater than
that of iron and silver, so more heat is produced in the nichrome wire due to
the flow of current.
2.
Melting point of nichrome wire is greater
than that of iron and silver and hence it does not melt easily on heating.
3.
nichrome wire does not oxidise (or burn)
easily even at higher temperature.
77.Two identical immersion heaters are
to be used to heat water in a large container. Which one of the following
arrangement would heat the water faster,
1.
connecting the heaters
in series with the main supply,
2.
connecting the heaters
in parallel with the main supply ? (CBSE 2011)
Answer:
Heat produced in a heater, when connected to main supply,
When identical heaters (i.e. having same resistance) are connected in parallel,
their net resistance decreases as compared to when connected in series.
Therefore, heaters connected in parallel would heat the water faster as the
heat produced in parallel combination is more than the heat produced in series
combination, on electric energy and electric power
78.What is commercial
unit of electrical energy ? Express it in joules. (CBSE 2015)
Or
Define 1 kWh. How is this unit of energy related to 1 joule ? (CBSE 2011, 2015)
Or
Establish the relationship between 1 kWh and SI unit of energy (joule). (CBSE
2012, 2013, 2015)
Answer:
Commercial
unit of electrical energy is kWh. 1 kWh is the amount of electric energy
consumed by 1000 W electric appliance when operates for 1 hour. 1 kWh = 1000 W
x 3600 s = 1000 Js-1 x 3600 s = 3.6 x
106 J.
79.Two electric bulbs A
and B are marked 220V, 40W and 220 V, 60W respectively. Which one of the two
has greater resistance ? (CBSE 2010, 2011)
Answer:
Therefore, bulb A (40 W) has greater resistance than the bulb B (60 W)
80.Define electric power. A device of resistance R is connected
across a source ofV voltage and draws a current I. Derive an expression for
power in terms of voltage and resistance.
Answer:
Electric power is defined as the amount of electric work or electric energy per
unit time.
If W be the amount of electric energy consumed in a circuit in t seconds, then
the electric power is given by
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Thus,
electric power is defined as the product of potential difference applied across
the circuit and current flowing through it.
Units of Power:
81. State Ohm’s law. How can this law be
verified experimentally ? Does Ohm’s law hold good under all conditions.
Comment. (NCERT Question Bank, CBSE, 2010, 2012)
Answer:
For Ohm’s law: Ohm’s law states that the electric current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor, provided the temperature and . other physical conditions of the conductor remain the same.
For experimental verification: Verify Ohm’s law
Apparatus: A conductor of resistance R, an ammeter, a voltmeter, a battery, a variable resistance (or rheostat used to change the current in the circuit), connecting wires, a key and sand paper.
Procedure:
1.
1.
Connect the various components as shown
in figure 12.
2.
Close the key, so that current begins to
flow in the circuit.
3.
Note down the potential difference (V)
across the conductor PQ of resistance R shown by the voltmeter and the
corresponding current (I) shown by the ammeter.
4.
Now move the knob of rheostat so that the
current in the circuit increases.
5.
Again note down the potential difference
(V) across the conductor PQ of resistance R in the voltmeter and current in the
circuit shown by ammeter.
Repeat the experiment
at least five times by increasing the current in the circuit by moving the knob
of the rheostat in steps. For
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Observations:
|
S.No |
Potential Difference |
Current |
V/I |
|
1. |
|||
|
2. |
|||
|
3. |
|||
|
4. |
|||
|
5. |
|||
|
6. |
|||
|
7. |
Plot a graph between V and I by taking V along X-axis and I along Y-axis. We get a straight line passing through origin as shown in figure 11.
Conclusion: From the graph between V and I, we conclude that I α V, which is Ohm’s law. Hence Ohm’s law is verified experimentally.
Precautions: While verifying Ohm’s law experimentally, the following precautions should be taken :
1.
Current should not be allowed to pass
through the circuit continuously for a long time, which may cause the increase
in temperature of the conductor. Therefore, the plug of the key must be taken
out every time after noting the readings of ammeter and voltmeter.
2.
Connections should be tight.
3.
The conductor used in the experiment
should be such that its resistance is not changed with increase in temperature
of the conductor.
Ohm’s
law holds good if the temperature of the conductor remains the same.
82.How will you infer
with the help of an experiment that same voltage or potential difference exists
across three resistors connected in parallel arrangement to a battery ?
Answer:
Perform
an activity to investigate the relation between potential difference across
parallel combination of resistors and the potential difference across each
individual resistors,
Connect three
resistors of resistances R1, R2 and R3 in parallel. One
end of each resistor is joined at a common point ‘a’ and the other end of each
resistor is connected at another common point ‘b’.
1.
Connect the parallel combination of
resistors with a battery, a plug key K and an ammeter A as shown in figure
22(A).
3.
Note the reading of voltmeter. Let it be
V. This is the potential difference across the parallel combination of
resistors.
4.
Now, disconnect the voltmeter and connect
it across R1 as shown in figure 22(B).
5.
Note the reading of voltmeter. It is
found to be V.
6.
Disconnect the voltmeter and connect it
across R2. Note the reading of
voltmeter. It is found to be V.
7.
Again disconnect the voltmeter and
connect it across R3. Note the reading of
voltmeter. It is found to be V.
Conclusion: When resistors
are connected in parallel to each other, potential difference across each
resistor is equal to the potential difference across the parallel combination
of resistors.
83. Describe an
experiment to study the factors on which the resistance ofa conductor or a
conducting wire depends.
Answer:
For experiment:
Connect the various electrical components as shown in figure 15.
1.
Dependence of length of
a conductor:
Take a copper wire of length l and connect it between the terminals A and B.
Note the reading of ammeter. Now take another copper wire of same area of
cross-section but of length 2l. Connect it between the terminals A and B by
disconnecting the previous wire. Again, note the reading of ammeter. It will be
found that the reading of ammeter (i.e., electric current) in the second case
is half of the reading of ammeter in the first case. Since R = V/I, so
resistance of second wire is double than the resistance of the first wire.
Thus, resistance of a conductor is directly proportional to the length of the
conductor,
i.e resistance ∝
length of the conductor
Thus, more is the length of a conductor, more is its resistance.
Thus, the resistance of’a conductor is ‘inversely proportional to the area of
cross -section of the conductor.
2.
Dependence on area of
cross-section of a conductor:
Now take two copper wires of same length but of different area of
cross-sections. Let area of cross-section of first wire is more than the area
of cross-section of the second wire. Connect first wire between the terminals A
and B in the circuit shown in figure 15. Note the reading of ammeter. Now
disconnect the first wire and connect the second wire between the terminals A
and B. Again note the reading of the ammeter. It will be found that the reading
of ammeter (i.e. electric current) is more when first wire (i.e. thick wire) is
connected between A and B than the reading of the ammeter when second wire
(i.e. thin wire) is connected between the terminals A and B.
Thus, the resistance of a thin wire is more than the resistance of a thick
wire.
3.
Effect of the Nature of
material:
Take two identical wires, one of copper and other of aluminium. Connect the
copper wire between the terminals A and B. Note the reading of ammeter. Now,
connect the aluminium wire between the terminals A and B. Again note the
reading of ammeter. It is found that the reading of ammeter when copper wire is
connected in the circuit is more than the reading of the ammeter when aluminium
wire is connected in the circuit.
Therefore, resistance of copper wire is less than the resistance of aluminium
wire. Hence, resistance of a wire or a conductor depends upon the nature of the
material of the conductor.
4.
Effect of temperature
of conductor:
If the temperature of a metallic conductor connected in the circuit increases,
its resistance increases.
Thus, factors on which resistance of a conductor depends are :
1.
its length,
2.
its area of cross-section,
3.
the nature of its material and
4.
its temperature.
The
various factors affecting resistance of a conductor are given in table 1.
Table 1. Factors affecting resistance of
a conductor:
84.What is Joule’s
heating effect ? List applications of Joule’s heating effect in daily life.
(NCERT Question Bank, CBSE 2012, 2014, 2015)
Answer:
Joule’s law can be stated as : The amount of
heat produced in a conductor is
(i) directly
proportional to the square of the electric current flowing through it.
This
is
H ∝ I2
(ii) directly
proportional to the resistance of the conductor or resistor.
That is, H ∝
R
(iii) directly
proportional to the time for which the electric current flows through the
conductor or resistor.
That is, H ∝
t
Combining (i), (ii) and (iii), we get H ∝ I2 Rt.
or H = K I2Rt, where K is constant of
proportionality
If K = 1, then H = I2Rt joule
This is known as Joule’s law of heating.
Four Applications:
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1.
When electric appliances like electric
heater, electric iron and water heater etc. are connected to the main supply of
electricity, these appliances become hot but the connecting wires remain cold.
We know, heat produced is directly proportional to the resistance of the
material through which current flows. The element of electric heater is made of
nichrome. Since, resistance of nichrome is high, so a large amount of heat is
produced in the element of the electric heater. Thus, filament of electric
heater becomes red hot. However, heat produced in connecting wires made of
copper or aluminium is very small and hence they are not heated up.
2.
Filament of an electric bulb is made of a
thin wire of tungsten. The melting point of filament is high i.e., about 3380
°C. The filament of the bulb is enclosed in a glass envelope fixed over an
insulated support as shown in figure 28. The glass envelope of electric bulb is
filled with inactive gases like nitrogen and argon to increase the life of the
tungsten filament.
Since resistance of thin filament is very high, so a large heat is produced as
the electric current flows through the filament. Due to this large amount of
heat produced, filament of the bulb becomes white hot. Hence, the filament of
the bulb emits light and heat.
3.
Electric fuse is a safety device
connected in series with the electric circuit. Electric fuse is a wire made of
a material whose melting point is very low. Examples of the materials of making
fuse wire are copper or tin-lead alloy. When large current flows through a
circuit and hence through a fuse wire, a large amount of heat is produced. Due
to this large amount of heat, the fuse wire melts and the circuit is broken so
that current stops flowing in the circuit. This saves the electric circuit from
burning due to the passage of large current through it.
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