(i) The electric power consumed by a device may be calculated by using either of the two expressions P = I2R or P = V2/R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. The seemingly different dependence of P on R in these expressions be explained by the fact that the if in the first expression , if resistance increases the current decreases as per the ohm’s law and thus the power consumed if effectively given by P = IV. In the second expression ,if resistance increases the voltage increases as per the ohm’s law and again the effective power is given by the expression P = VI.
(ii) (a) Tungsten is used almost exclusively for the filament of electric lamps due to the following reasons:
• High melting point: The high melting point of tungsten ensures that it does not melt easily on high temperature.
• Tensile strength: The tungsten has the greatest tensile strength. Great tensile strength ensures that they does not break under tension.
• Vapour pressure: The tungsten has a lowest vapour pressure. The vapour pressure is the indication of the evaporation rate. Lower the vapour pressure, lower is the boiling point.
(b) Copper and aluminium wires are used for electricity transmission due to following reason:
• Copper and aluminium have very less resistivity.
• Lower the resistivity, easier the flow of electricity.
Hence, copper and aluminium wires are used for electricity transmission because of their low resistivity.
***note: resistivity is defined as power of a material to resist or oppose the flow of electricity through it.
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