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## Homework Statement

A thermistor T is included in the circuit shown. Calculate the resistance of resistor R such that the output of the op-amp will change polarity at 6

^{o}C if at that temperature the thermistor has resistance 3 kΩ

## Homework Equations

## The Attempt at a Solution

The inverting input = 6/14 V, so if the output changes polarity, the value of non-inverting input should be less or greater than 6/14 V, depends on whether it changes from positive to negative or vice versa.

My question is how to determine whether it changes polarity from positive to negative or vice versa? Because in my opinion, the answer will be different. To change from positive to negative, the non-inverting input should be less than 6/14 V while changing from negative to positive, the non-inverting input should be greater than 6/14 V.

The answer is 2.25 kΩ. Based on that, I think the polarity changes from negative to positive. Here's my working:

[tex]\frac{3}{3+R}>\frac{6}{14}[/tex]

Solving that, I got R < 4. How to determine the exact value of R?

Hence, basically I have two questions:

1. How to determine whether it changes polarity from positive to negative or vice versa?

2. How to determine the exact value of R?

Thanks