Unit - 1
Basic concept
Q 1) Determine the temperature if 200 J of heat is released by the body of mass 6 Kg and has a specific heat of 0.8 J/KgoC.
A1)
Given:
Heat released Q = 200 J,
Mass m = 6 Kg,
Specific Heat c = 0.8 J/KgoC
The temperature is given by ΔT = Q / mc
= 200 / 6 x 0.8
ΔT= 41.66oC
Q 2) Determine the heat released when the temperature changes by 40oC by a body of mass 3 Kg which has a specific heat of 0.7 J/kgoC
A2)
Given:
Temperature change = 40oC
Mass m = 3 kg,
Specific heat c = 0.7 J/kgoC
The Heat released is given by formula Q = mc ΔT
= 3 × 0.7 × 40
Q = 84 J
Q 3) Explain Internal Energy:
A3)
As usual, we get change in internal energy = U2 – U1 = m x Cv (T2 – T1).
Another way of determination of change in internal energy is very common in isentropic operation.
By first law of thermodynamics as applied to non-flow process,
Heat supplied = change in internal energy + work done; but heat supplied is zero
Change in internal energy = – work done.
Thus, we get an important relation in an isentropic process. This relation can be stated as “Change in internal energy is numerically equal to work done”. When the work is done by the gas, it loses internal energy and it gains internal energy when the work is done on the gas.
Q 4) Explain process?
A4) Process:
Thermodynamics process represents a transition in which a system changes from one state to another. When the path is completely specified then the change of state is called a process. A Process is defined as the transformation of the system from one fixed state to another fixed state .When any one of the properties changes, the working substance or system is said to have undergone a process.
Some of the processes are identified by special names as given below:
- Isobaric process (constant pressure process)
- Isochoric process (constant volume process)
- Isothermal process (constant temperature process)
- Isentropic process (constant entropy process)
- Adiabatic process(perfectly insulated process)
Q 5) Explain temperature?
A5) Temperature is the degree or intensity of the heat present in a substance or a system, expressed based on the comparative scale and shown by a thermometer.
In other words, Temperature is the measure of hotness or coldness of a body measured using Celsius, Kelvin, and Fahrenheit scales.
The change in temperature is based on the amount of heat released or absorbed. The S.I unit of temperature is Kelvin.
The Temperature formula is given by,
Δ T = Q / mc
Where,
Δ T = temperature difference,
Q = amount of heat absorbed or released,
m = mass of the body,
c = specific heat of the body.
Q 6) Explain macroscopic and microscopic approach ?
A6)
MACROSCOPIC APPROACH | MICROSCOPIC APPROACH |
In this approach, a certain quantity of matter is considered without taking into account the events occurring at the molecular level. | The matter is considered to be comprised of a large number of tiny particles known as molecules, which move randomly in a disordered fashion. The effect of molecular motion is considered. |
An analysis is concerned with the overall behaviour of the system. | Knowledge of the structure of matter is essential in analysing the behaviour of the system. |
This approach is used in the study of classical thermodynamics. | This approach is used in the study of statistical thermodynamics. |
A few properties are required to describe the system. | Large numbers of variables are required to describe the system. |
The properties like pressure, temperature, etc. needed to describe the system, can be easily measured. | The properties like velocity, momentum, kinetic energy, etc. needed to describe the system, cannot be measured easily. |
The properties of the system are their average values. | The properties are defined for each molecule individually. |
This approach requires simple mathematical formulas for analysing the system. | No. Of molecules is very large so it requires an advanced statistical and mathematical method to explain any change in the system |
Q 7) Explain the difference between work and heat?
A7)
Work Heat
Interaction | Mechanical | Thermal |
Requires | Force and Displacement | Temperature difference |
Process | Macroscopic pushes and pulls | Microscopic collisions |
Positive value | W > 0 when a gas is compressed. Energy is transferred into system. | Q > 0 when the environment is at a higher temperature than the system. Energy is transferred into system. |
Negative value | W < 0 when a gas expands. Energy is transferred out of system. | Q < 0 when the system is at a higher temperature than the environment. Energy is transferred out of system. |
Equilibrium | A system is in mechanical equilibrium when there is no net force or torque on it. | A system is in thermal equilibrium when it is at the same temperature as the environment. |
Q 8) Explain the limitation of first law of thermodynamics
A8)
Limitations of First Law of Thermodynamics
i. The limitation of the first law of thermodynamics is that it does not say anything
About the direction of flow of heat.
Ii. It does not say anything whether the process is a spontaneous process or not.
Iii. The reverse process is not possible. In actual practice, the heat doesn’t convert completely into work. If it would have been possible to convert the Whole heat into work, then we could drive ships across the ocean by extracting Heat from the water of the ocean.
Q 9) What is intensive property and extensive property ?
A9)
Intensive property: If the property is independent of mass of the system is called an intensive property.
Example - Pressure, temperature, density, velocity, height, viscosity are the example of intensive property.
Extensive property: If the property is proportional to the mass of the system it is called an extensive property.
Example- volume, surface area, potential energy, kinetic energy, internal energy, electric charge.
Q 10) Explain thermodynamics equilibrium ?
A10)
A system is said to be in the states of thermodynamics equilibrium if the value of the property is the same at all the points in the system. Or the system is said to exist in thermodynamics equilibrium when no change in any macroscopic property is registered, if the system is isolated from its surrounding.
The system is said to be in thermodynamic equilibrium if the conditions for following three equilibrium is satisfied:
1. Mechanical equilibrium
2. Chemical equilibrium
3. Thermal equilibrium
