2.1 Temperature Definition of thermal equilibrium and Zeroth law

Thermodynamics

Unit 2

Temperature

2.1 Temperature, Definition of thermal equilibrium and Zeroth law

Temperature:

The temperature of a substance is defined as measure of hotness or degree of coldness of a body. We know that energy flows from higher level to lower level in the universe. So heat flows from higher temperature body to lower temperature body.

Temperature is not a measure of quantity of energy possessed by the body but rather, it indicates level of internal energy possessed by the body

The absolute temperature is the temperature measured above the point of absolute zero. Absolute temperatures are expressed by the capital letter T.

Thermal Equilibrium:

Thermal equilibrium is a state where no heat is transferred between two bodies in contact or between system and surrounding.

Zeroth Law of Thermodynamics:

If Body 1 is in thermal equilibrium with Body 3 and Body 2 is in thermal equilibrium with Body 3, it implies that Body 1 is also in thermal equilibrium with Body 2.

2.2 Temperature scales

Temperature Scales:

There are mainly four types of temperature scales.

Fahrenheit Scale: It is commonly used in the US. Founded by German scientist Daniel Gabriel Fahrenheit. It can express negative temperatures. Following are the important points on this scale

Freezing point of water: 32° F
Boiling point of water: 212° F
Coldest possible temperature: -459.67° F (absolute zero)

2. Celsius Scale: Most commonly used system and scale in the world. Also termed as Centigrade scale since it divides the difference in freezing and boiling point of water into 100 divisions. It can also express negative temperatures. Following are the important points on this scale

Freezing point of water: 0° C

Boiling point of water: 100° C

Coldest possible temperature: -273.15° C (absolute zero)

Fahrenheit to Celsius formula Celsius = (Fahrenheit - 32) / 1.8

3. Kelvin Scale: It has been adapted from the Celsius scale. It was designed to set the absolute zero temperature at 0. This scale does not use degree scale. Following are the important points on this scale

Freezing point of water: 273.15 K

Boiling point of water: 373.15 K

Coldest possible temperature: 0 K (absolute zero)

You can convert from Celsius to Kelvin by adding 273.15 to Celsius temperature.

4. Rankine Scale: It has been adapted from the Fahrenheit scale. It was designed to set the absolute zero temperature at 0. Following are the important points on this scale

Freezing point of water: 491.67 R

Boiling point of water: 671.67 R

Coldest possible temperature: 0 R (absolute zero)

You can convert from Fahrenheit to Rankine by adding 459.67 to Fahrenheit temperature.

2.3 Various Thermometers- Definition of heat

Thermometers:

A thermometer is a device that is used to measure the temperature of a system.

Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes.

Mercury bulb thermometer is the most commonly used, since mercury has a very high co-efficient of thermal expansion.

Various types of thermometers that are used are tabulated below with their principle of operation.

Heat (Q):

Heat is a form of energy that flows between bodies or parts of body due to difference in temperature.

Heat will always flow from body at higher temperature to lower temperature.

It is measured in Joules or Calories.

In thermodynamic system, heat produced by the system or heat rejected is taken as negative and heat absorbed by the system is taken as positive.

The total amount of heat transferred in a process is shown in the graph below.

It is the area under the curve 1-2 and can be found by integration as given above.

2.4 Examples of heat/work interaction in systems- First Law for Cyclic & Non-cyclic processes

First law of thermodynamics for cyclic and non-cyclic process:

There are different laws of thermodynamics while the first law of thermodynamics deals with the processes that are cyclic and non-cyclic.

For cyclic process:

The process is which start at a certain point and moves across a path and ends at a point where it had begun and then continues over and over again.

This state is given by the net usage of heat or heat transfer, this is the law for a cyclic process.

For Non-cyclic process:

In case of the non-cyclic processes, there are two aspects that determine the process instead of the heat transfer, in addition the non-cyclic process has work done.

So, this process has a Total energy that would be sum of net energy of heat transfer and work done. Q = E + W

2.5 Concept of total energy E

Concept of Total Energy:

The total energy content of a system is the sum of Kinetic energy, Potential energy and the internal energy contained in the system.

E = KE + PE + IE

E = ½ mv2 + mgh + U

In most of the systems, KE and PE are negligible as compared to Internal Energy of the system which is due to vibrational motion of the atoms.

Hence, in general, E = U

Then Q = U + W

In differential form, dQ = dU + dW

In integral form,

2.6 Enthalpy.

Enthalpy:

One of the fundamental quantities which occurs in thermodynamics is the sum of internal energy (u) and pressure volume product (pv). This sum is called Enthalpy (h).

h = u + pv = u + w

The enthalpy of a fluid is the property of the fluid, since it consists of the sum of a property and the product of the two properties.

Since enthalpy is a property like internal energy, pressure, specific volume and temperature, it can be introduced into any problem whether the process is a flow or a non-flow process.

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