# Heat and temperature

Heat and temperature

Heat is a form of energy that follows from a body of higher temperature to one of lower temperature.

Temperature is a degree of hotness or coldness of a body.

Thermometry:

The science of temperature and its measurement is called thermometry.

Scales of thermometer:

There are different temperature scales thermometers in use. They are as;

1. Celsius scale: In this scale lower fixed point is marked as o and upper fixed point marked as 1oo. The interval between the two fixed points is divided into 100 equal parts. Each parts or degree represents 1oC.

2. Fahrenheit scale: In this scale lower fixed point is marked as 32 and upper fixed point marked as 212. The interval between the two fixed points is divided into 180 equal parts. Each parts or degree represents 1o F.

3. Reaumur scale: In this scale lower fixed point is marked as 0 and upper fixed point marked as 80. The interval between the two fixed points is divided into 80 equal parts. Each parts or degree represents 1oR.

4: Absolute temperature scale: In this scale lower fixed point is marked as 237K and upper fixed point marked as 373K. The interval between the two fixed points is divided into 100 equal parts. Each parts or division represents 1K.

Absolute scale of temperature;

Absolute temperature scale: It is used to measure the temperature. Using absolute temperature scale we can measure the temperature 273k-373K. In this scale lower fixed point is marked as 237K and upper fixed point marked as 373K. The interval between the two fixed points is divided into 100 equal parts. Each parts or division represents 1K.

Zeroth law of thermodynamics;

When a hot substance is placed in contact with a cold substance, heat flows from hot to the cold substances. Finally, the heat flow ceases and both attain the same temperature. The bodies are then said to be the thermal equilibrium.

Two systems are said to be thermal equilibrium if they have same temperature. Consider two systems A and B separated to each other by an insulating wall but each being in contact with third system C through conducting wall as shown in the figure.

After sometime, the systems A will be in the thermal equilibrium with system C and similarly system B will be in thermal equilibrium with system C. This experiment shows that if two systems are in the thermal equilibrium with a third system, then they must be in thermal equilibrium with each other.

This is called the zeroth law of thermodynamics

2.

Derive the relation between Celsius, Fahrenheit and Kelvin scale of temperature.

There are two points in all temperatures scales and different intervals between them. If C, F, R and K be the temperature of body in Celsius, Fahrenheit Reaumur and Kelvin scale respectively,

Then we have

$\frac{{{\rm{C}} - 0}}{{100 - 0}} = \frac{{{\rm{F}} - 32}}{{212 - 32}}$=$\frac{{{\rm{R}} - 0}}{{80 - 0}} = \frac{{{\rm{K}} - 273}}{{373 - 273}}$

$\frac{{\rm{C}}}{{100}} = \frac{{{\rm{F}} - 32}}{{180}}$   =$\frac{{\rm{R}}}{{80}} = \frac{{{\rm{K}} - 273}}{{100}}$

$\frac{{\rm{C}}}{5} = \frac{{{\rm{F}} - 32}}{9}$   =$\frac{{\rm{R}}}{4} = \frac{{{\rm{K}} - 273}}{5}$

The conversion between the temperature in absolute scale, T and Celsius scale, θ is

T= θ+273.15

So ice point, T= θ+273.15 = 273.15K

These points are written approximately as 273 K and 373 k. the absolute zero temperature is equal to 0K θ+ 273 or θ = -273oC.

Since the temperature difference in the Kelvin scale is same, the temperature difference of 1oCis equal to the temperature difference of 1K and so 0o C-1 can be replaced by K-1.

This is relation between Celsius, Fahrenheit and Kelvin scale of temperature.

Temperature on the basis of zeroth law of thermodynamics:

According to zeroth law of thermodynamics, when a hot substance is placed in contact with a cold substance, heat flows from hot to the cold substances. Finally, the heat flow ceases and both attain the same temperature. The bodies are then said to be the thermal equilibrium.

Thermal equilibrium;

According to zeroth law of thermodynamics, when a hot substance is placed in contact with a cold substance, heat flows from hot to the cold substances. Finally, the heat flow ceases and both attain the same temperature. The bodies are then said to be the thermal equilibrium.

Two systems are said to be thermal equilibrium if they have same temperature.

Figure15_2; Ayam book Page 357

Consider two systems A and B separated to each other by an insulating wall but each being in contact with third system C through conducting wall as shown in the figure.

After sometime, the systems A will be in the thermal equilibrium with system C and similarly system B will be in thermal equilibrium with system C. This experiment shows that if two systems are in the thermal equilibrium with a third system, then they must be in thermal equilibrium with each other. This is called the zeroth law of thermodynamics.

Construction of a mercury thermometer: Figure; construction of a mercury thermometer

Mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam (1714). In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium with a temperature standard such as an ice/water mixture, and then with another standard such as water/vapor, and the tube is divided into regular intervals between the fixed points. Construction is done by following as shown in the figure;

1. Mercury filled in a clean glass capillary tube

2. Marking the lower fixed point

3. Making the upper fixed point

4. Calibration of a thermometer.

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