Chapter 11 Electricity Questions-03
QUESTIONS
1. On what factors does the resistance of a conductor depend?
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Answer
The resistance of a conductor depends upon the following factors:
$\rightarrow$ Length of the conductor
$\rightarrow$ Cross-sectional area of the conductor
$\rightarrow$ Material of the conductor
$\rightarrow$ Temperature of the conductor
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?
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Answer
The current will flow more easily through thick wire. It is because the resistance of a conductor is inversely proportional to its area of cross - section. If thicker the wire, less is resistance and hence more easily the current flows.
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?
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Answer
According to Ohm’s law
$V=I R$ $\Rightarrow I=V / R \ldots$
Now Potential difference is decreased to half
$\therefore$ New potential difference $V^{\prime}=V / 2$
Resistance remains constant
So the new current $I^{\prime}=V^{\prime} / R$
$=(V / 2) / R$
$=(1 / 2)(V / R)$
$=(1 / 2) I=I / 2$
Therefore, the amount of current flowing through the electrical component is reduced by half.
4. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
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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.
5. Use the data in Table 11.2 to answer the following –
(a) Which among iron and mercury is a better conductor?
(b) Which material is the best conductor?
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Answer
Table 12.2 Electrical resistivity of some substances at $20^{\circ} C$
| - | Material | Resistivity $(\Omega m)$ |
|---|---|---|
| Conductors | Silver | $1.60 \times 10^{-8}$ |
| Copper | $1.62 \times 10^{-8}$ | |
| Aluminium | $2.63 \times 10^{-8}$ | |
| Tungsten | $5.20 \times 10^{-8}$ | |
| Nickel | $6.84 \times 10^{-8}$ | |
| Iron | $10.0 \times 10^{-8}$ | |
| Chromium | $12.9 \times 10^{-8}$ | |
| Mercury | $1.84 \times 10^{-6}$ | |
| Manganese | $49 \times 10^{-6}$ | |
| Constantan (alloy of Cu and Ni) |
||
| Alloys | Manganin (alloy of Cu, Mn and Ni) |
$44 \times 10^{-6}$ |
| :— | :— | :— |
| Nichrome (alloy of Ni, Cr, Mn and Fe) |
$100 \times 10^{-6}$ | |
| Glass | $10^{10}-10^{14}$ | |
| Insulators | Hard rubber | $10^{13}-10^{16}$ |
| Ebonite | $10^{15}-10^{17}$ | |
| Diamond | $10^{12}-10^{13}$ | |
| Paper (dry) | $10^{12}$ |
(a) Resistivity of iron $=10.0 \times 10^{-8} \Omega$
Resistivity of mercury $=94.0 \times 10^{-8} \Omega$
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.
