poissons ratio

 Poisson's ratio:

Introduction:

The Poisson's ratio is identified (defined) as the ratio of lateral strain to axial strain.

(or)

It can also be stated as the ratio of unit strain in lateral (transverse) direction to

the unit strain in axial direction. its denoted by

the symbol μ.

μ =Lateral Strain/Axial Strain

Derivation of Poisson's Ratio

Consider, a steel bar of length (L) , and diameter D, as shown in the below

figure.

When this steel bar is not subjected to any load, its dimensions (L and D) remain the same.

When a tensile load is applied to the bar, the bar habits a change in its dimensions. The tensile

load act-the bar, increases the length of the bar in axial direction. Due to the increase in length,

the diameter of e bar decreases and thus the cross sectional area of the bar changes i.e., the bar exhibits a change in its dimensions in lateral or transverse direction. Thus due to load, the bar experiences strain in axial direction as well as in lateral direction.

Axial strain, Σa =Change in length/original length =ΔL/L

Lateral strain, Σl = Change in diameter/original diameter = -ΔD/D

Difference between RTD and thermistor

Difference between RTD and thermistor:-

RTD

Thermistor

1. RTD is made up of metals. 2. Metals have Positive Temperature Coefficient (PTC) of resistance. Hence, the resistance of RTD increases with an increase in temperature and decreases with a decrease in temperature. 3. The resistance temperature characteristics of RTD's are linear. 4. It is less sensitive to temperature compared to thermistor. 5. But, it has-a wide operating temperature range i.e., minus 200 to plus 650°C. 6. RTD's are relatively larger in size. 7. They are costlier as compared to thermistor. 8. They have low self resistance. 9. RTD's provide high degree of accuracy and long term stability. 10. They are used in laboratory and industrial applications.

Thermistor is made up of semiconductor Materials Semiconductor materials have Negative Temperature Coefficient (NTC) of resistance. Hence, the resistance of a thermistor decreases with an increase in temperature and increases with a decrease in temperature. The resistance temperature characteristics of thermistor are highly nonlinear. It has large temperature coefficient of resistance i.e. thermistor highly sensitive to temperature compared to RTD. It has low operating temperature range compared to RTD i.e., minus 100 to plus 300°C. Size of thermistors are small . They are not costlier as compared to RTD. They have high self resistance. Thus, they require shielding cables to minimize interference problems. Thermistors also provide an accuracy of ± 0.01°C. They are widely used for dynamic temperature measurement.