Physics unit 4

A sensor circuit has an ideal 12 V supply, a fixed resistor R and a temperature-sensitive resistor X in series. The measured output V is the voltage across X. Thus V = 12X/(R+X), with R and X in the same resistance units. X falls with temperature: at 10, 20 and 30 °C its resistance is 6, 3 and 1.5 kΩ respectively. Initially R = 3 kΩ. A high-resistance meter draws negligible current. In a later run, a recording device of resistance 3 kΩ is connected across X; the parallel combination then has resistance Xeff = 3X/(3+X) kΩ. Electrical heating is negligible unless explicitly discussed.