question_answer

In the given diagram, a piece of pure Semiconductor S in series with a variable resistor R and source of constant voltage V. Would you increase or decrease the value of R to keep the reading of ammeter A constant, when semiconductor S is heated? Give reason. If semiconductor is replaced by metal and it is heated then how the value of R should be changed to keep reading of ammeter A unchanged?

 

Answer:

We shall increase the value of resistance R, joined in series with pure semiconductors in order to maintain the ammeter reading constant when semiconductor is heated. On heating the semiconductor, more charge carriers are available due to higher thermal energy and consequently, resistance of the semiconductor piece decreases. In order to maintain constant current, the total resistance of the circuit must remain constant. Hence, external resistance will raise. If semiconductor is replaced by a metal and it is heated, its resistance will increase. To maintain the same current in the circuit, we must decrease the value of R.