Question

a. Temperature coefficient of resistivity of a material can be positive, negative or zero. 

An electric heating device consists of a wire of material with an unknown temperature coefficient of resistivity and is connected to a constant voltage supply. 

Will the power delivered to the electrical device increase, decrease or remain constant with time as the temperature of the wire increases with usage? 

Give answers with explanations for all the three possible values of temperature coefficient of resistivity of the wire inside the device. 

b. The operating voltage of a water heater is 220V and that of a coffee whipper is 120V. With this information, can you say or not say, which of the two devices will draw lesser power? Give reason for your answer.

Answer 1

a. Power delivered to a device, P = V² /R

For positive temperature coefficient of resistivity of the material, with time, R will increase. So power delivered will decrease, for a constant voltage supply. 

For negative temperature coefficient of resistivity of the material, with time, R will decrease. So power delivered will increase, for a constant voltage supply. 

For a zero-temperature coefficient of resistivity of the material, with time, R will remain constant. So, power delivered will also stay constant, for a constant voltage supply.

b. Cannot say.

For determining the power drawn by an electric device, the current flowing through the device or the resistance of device is a required.