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Question

# (a) What is meant by saying that the potential difference between two points is 1 volt? Name a device that helps to measure the potential difference across a conductor. (b)Why does the connecting cord of an electric heater not glow hot while the heating element does? (c) Electrical resistivities of some substances at 20°C are given below: Silver 1.60 × 10−8 Ωm Copper 1.62 × 10−8 Ωm Tungsten 5.20 × 10−8 Ωm Iron 10.0 × 10−8 Ωm Mercury 94.0 × 10−8 Ωm Nichrome 100 × 10−6 Ωm Answer the following questions in relation to them: (i) Among silver and copper, which one is a better conductor? Why? (ii) Which material would you advise to be used in electrical heating devices? Why?

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Solution

## (a) It means that the 1 joule of energy is used up in transferring a charge of 1-coulomb between two points. A voltmeter is used to measure the potential difference across a conductor and it is always connected in parallel. (b) The heating element of the heater is made up of an alloy which has very high resistance. So, when current flows through the heating element, it becomes too hot and glows red due to the heating effect of current. But the resistance of cord which is usually of copper or aluminium is very low so it does not glow. (c) The conductivity of a material is indirectly proportional to its resistivity. (i) Out of silver and copper, silver has less resistivity, that's why it is a good conductor as compared to copper. (ii) The material used in electrical heating devices must have high resistance. So, a material with high resistivity should be used from the table. Nichrome has the highest resistivity, it would have the highest resistance among all the given materials. Hence, it should be used for electrical heating devices.

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