Electricity Worksheet-20
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Two wires are of the same material but of different lengths and areas of cross-section. Will their resistivity be same or different ?
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Why is nichrome used as a heating element ?
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What is the difference between a resistor and resistance ?
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Why are constantan and manganin used for making standard resistances ?
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Why is lead-tin alloy used for making fuse ?
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Why are copper and aluminium wires used as transmission lines for electric power ?
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What are superconductors ?
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A wire having a resistance of 18 Ω is cut into three equal parts. What is the equivalent resistance if these three parts are joined in parallel ?
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What is the ratio of equivalent resistance of series combination of n equal resistance to the equivalent resistance in parallel combination of these n resistances ?
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What is the resistance of a human body ? How much voltage is dangerous to it ?
Answer:
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Resistivity depends on the material of the conductor and not on its dimensions. Hence, the two wires will have the same resistivity. But their resistances are different.
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Nichrome has high resistivity and high melting point. It does not react with air when it is red hot (800°C).
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A resistor is an object whereas resistance is its property due to which it obstructs the flow of electrons through it.
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The resistivity of these materials is moderate and almost independent of temperature.
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It has low melting point and a fuse made of this alloy melts when the current exceeds a particular
level.
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These metals possess low resistivity and as such there is less power loss (I2R) due to heating effect of current.
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Superconductors are materials which have zero resistance at a particular temperature, for example., mercury is superconductor at 4.2 K.
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Resistance of each part, r = 18/3 = 6 Ω
Since these three parts are joined in parallel, the equivalent resistance, R = r/3 = 6/3 = 2 Ω
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Equivalent resistance of series combination of n equal resistances where each resistance equal to r,
Rs = nr
Equivalent resistance of parallel combination of n equal resistance each resistance equal to r,
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The resistance of a dry human body is approximately 50k Ω and in case it is wet, the resistance is about 10k Ω. A current of 1 mA is enough to send a shock through a human body. A voltage of about 100 V is injurious and 220 V may prove fatal.