Page No 209: - 12) ELECTRICITY
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Question 1:
On what factors does the resistance of a conductor depend?
Answer:
The resistance of a conductor depends upon the following factors:
(a) Length of the conductor
(b) Cross-sectional area of the conductor
(c) Material of the conductor
(d) Temperature of the conductor
Question 2:
Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?
Answer:
Resistance of a wire, 
Where,
= Resistivity of the material of the wire
l = Length of the wire
A = Area of cross-section of the wire
Resistance is inversely proportional to the area of cross-section of the wire.
Thicker the wire, lower is the resistance of the wire and vice-versa. Therefore, current can flow more easily through a thick wire than a thin wire.
Question 3:
Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
Answer:
The change in the current flowing through the component is given by Ohm’s law as,
V = IR
Where,
Resistance of the electrical component = R
Potential difference = V
Current = I
The potential difference is reduced by half, keeping resistance constant.
Let the new resistance be R‘ and the new amount of current be I ‘.
Therefore, from Ohm’s law, we obtain the amount of new current.
Therefore, the amount of current flowing through the electrical component is reduced by half.
Question 4:
Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Answer:
The resistivity of an alloy is higher than the pure metal. Moreover, at high temperatures, the alloys do not melt readily. Hence, the coils of heating appliances such as electric toasters and electric irons are made of an alloy rather than a pure metal.
Question 5:
Use the data in Table 12.2 to Answer the following −
Table 12.2 Electrical resistivity of some substances at 20°C
|
− |
Material |
Resistivity (Ω m) |
|
Conductors |
Silver |
1.60 × 10−8 |
|
Copper |
1.62 × 10−8 |
|
|
Aluminium |
2.63 × 10−8 |
|
|
Tungsten |
5.20 × 10−8 |
|
|
Nickel |
6.84 × 10−8 |
|
|
Iron |
10.0 × 10−8 |
|
|
Chromium |
12.9 × 10−8 |
|
|
Mercury |
94.0 × 10−8 |
|
|
Manganese |
1.84 × 10−6 |
|
|
Constantan (alloy of Cu and Ni) |
49 × 10−6 |
|
|
Alloys |
Manganin (alloy of Cu, Mn and Ni) |
44 × 10−6 |
|
Nichrome (alloy of Ni, Cr, Mn and Fe) |
100 × 10−6 |
|
|
Glass |
1010 − 1014 |
|
|
Insulators |
Hard rubber |
1013 − 1016 |
|
Ebonite |
1015 − 1017 |
|
|
Diamond |
1012 − 1013 |
|
|
Paper (dry) |
1012 |
(a) Which among iron and mercury is a better conductor?
(b) Which material is the best conductor?
Answer:
(a) Resistivity of iron = 
Resistivity of mercury = 
Resistivity of mercury is more than that of iron. This implies that iron is a better conductor than mercury.
(b) It can be observed from Table 12.2 that the resistivity of silver is the lowest among the listed materials. Hence, it is the best conductor.
