Physics---DC electricity

(a) Current through thermistor = 3/(200+1000) A = 3/1200 A
hence, p.d. across thermistor = (2/1200) x 200 v = 0.5 v

(b) When the temperature rises, the resistance of the thermistor decreases. This causes the p.d. across the thermistor to drop. When the p.d. drops to below 0.6 v, the electronic switch will turn the heater off.

(c) The heater is switched off when the p.d. across the thermistor is 0.6 v
Hence, p.d. across the 1000-ohm resistor
= (3 - 0.6) v = 2.4 v
Current flows through the 1000-ohm resistor
=2.4/1000 A = 2.4x10^-3 A
Thus, current through the thermistor = 2.4x10^-3 A (becuase of series connection)
Resistance of thermistor = 0.6/(2.4x10^-3) ohms = 250 ohms