The graph is used to read off the temperature at 1500 ohm.
c) Potential at A is 3V . Potential at B is also 3V when the voltmeter read zero. That means the resistance of thermistor is equal to 1200 ohm. Hence read off from graph its temperature.
ii) If the voltmeter reads 1.2 V , the potential at B could be 1.2 higher or lower than A. Hence the resistance of thermistor is either greater than 1200 ohm or less than 1200 ohm . Hence two different temperature.
2. To calc the lower temperature, by looking at the graph it must be having a greater resistance. Hence greater than 1200 ohm.
The working is done mathematically now,
current through thermistor = current through 1200ohm (belowB) No current from voltmeter.
1.2V higher than 3V must be 4.2 V across the thermistor.
6V - 4.2 V = 1.8 V across the 1200 ohm
the equation of same current :
4.2 / Rt = 1.8/1200
Rt= 2800 ohm
Read off graph temp = 282K
The principle of this is potential divider . The current is the same down one line , so p.d is proportional to resistance
