What is the First Law of Thermodynamics?
The study of relations between work, heat and temperature and their relations with energy, entropy, and physical properties of matter. Thermodynamics explain how matter is affected by the process when thermal energy is converted to or from the other forms of energy, and the process itself.
The energy that is released from heat is known as thermal energy. This generated heat allows the movement of particles within an object and as the speed of these particles increase, more heat is generated.
There are four laws of thermodynamics which are as follows-
Let us study the first law of thermodynamics in detail.
First Law of Thermodynamics- Explanation
The first law of thermodynamics states that the quantity of the heat absorbed, when some amount of heat is given to a system that is capable of doing external work, is equal to the sum of the increase in internal energy of the system due to a rise in temperature and external work done during expansion.
The first law of thermodynamics is generally represented by the equation-
\[\Delta U = Q -W\]
Where \[\Delta U\] = change in internal energy of the thermodynamic system
\[\Delta Q\] = heat given to the system
\[\Delta W\] = work done on the system
Differential form of the first law of thermodynamics equation-
dU - dQ - dW
The first law of Thermodynamics is also called ‘Law of Conservation of Energy’. The law of conservation of energy states that “Energy can neither be destroyed nor be created, it can only be transferred from one form to another”.
Significance of First Law of Thermodynamics
Significances that the first law of thermodynamics has are as follows-
The relation between heat and work is established by the first law of thermodynamics.
Both Work and Heat are equivalent to each other.
The exact equivalent amount of energy of the surrounding will be lost or gained, if any system gains or loses energy.
Applied heat is always equal to the sum of work done and change in internal energy.
The energy is constant for an isolated system.
Applications of First Law of Thermodynamics
The first law of thermodynamics is commonly used in heat engines.
Refrigerators is another example where the first law of thermodynamics is used.
Sweating is a great example of the first law of thermodynamics since the heat of the body is transferred to sweat.
When an ice cube is put in a drink, the ice cubes absorb the heat of the drink which makes it cool.
Limitations of First Law of Thermodynamics
The first law of thermodynamics does not state anything about the heat flow direction.
The process is not reversible.
It is difficult to distinguish whether the process is spontaneous or not.
FAQs on First Law of Thermodynamics
1. 750 calories of heat is added to a system and the system performs 550 calories of work. Calculate the change in Internal Energy.
Here, ΔQ = 750 Cal, ΔW = 550 Calories
We know that ΔU = ΔQ - ΔW
So, ΔU = (+) 750 - 550 = + 200 Calories is the increase in internal energy.
2. What is the CP/CV Ratio?
The CP/CV ratio is the ratio of specific heats, known as the adiabatic index.
3. 0.8 moles of O2 are heated from 30 to 150°C at constant pressure.
Calculate:
(i) Q
(ii) ΔU
(iii) W
(Given CP = 7 cal/mol-K)
Here, n = 0.8, ΔT = 150 - 30 = 120 °C
(i) Q
We know at constant pressure,
Q = nCPΔT = 0.8 x 7 x 120 = 672 Cal
(ii) ΔU
dU = nCVΔT
Since CV = CP - R
For a diatomic molecule, R = 2 cal/mol-K
⇒ CV = 5 cal/mol-K ⇒ dU = 0.8 x 5 x 120 = 480 Cal
(iii) W
dW = dU - dQ ⇒ 480 - 672 = - 592 Cal, i.e. work is done by the system.
4. A sample of an ideal gas (γ = 1.4) is heated at constant pressure. If 120 J is supplied to it, then find dU and dW.
Here, dQ = 120 J, n =1
dU = CVdT = CP(Cv/Cp)dT = CpdT/γ = dQ/γ= 86 J
Now, dW = dQ - dU = 120 - 86 = + 34 J, i.e. work is done on the system.
5. What are the four processes that are involved in a closed system?
The processes that are involved in a closed system are as follows-
Adiabatic Process- A type of thermodynamic process where there is no transfer of heat or mass between the environment and the system. Example- Rapid contraction and expansion of a gas.
Isothermal process- A type of thermodynamic process in which there is no change in the temperature that is it remains constant. Example- Refrigerator
Isobaric Process- A type of thermodynamic process in which there is no change in the process that is it remains constant. Example- water boiling and converting to steam
Isochoric Process- A type of thermodynamic process in which there is no change in the volume which means the volume is constant. Example- Pressure cookers
6. What are the sign conventions used for Heat and Work in Thermodynamics?
The sign conventions for Energy and Work are as follows-
If energy enters into the system, it is +
If energy leaves out of the system, it is -
If work done is ‘by’ the system, it is +
If work done is ‘on’ the system, it is -
The sign conventions for Heat and Work are as follows-
When Q is +, heat flows into the system
When Q is -, heat flows out of the system
When W is +, energy leaves the system
When W is -, energy enters the system
7. What does the Zeroth Law of Thermodynamics state?
The zeroth law of Thermodynamics states that “If two thermodynamics systems are in thermal equilibrium with a third body then they are also in equilibrium with each other. When two bodies are in contact with each other but separated by a barrier that is permeable to heat, there is no transfer of heat from one to another, the bodies are said to be in thermal equilibrium. For example in a thermometer, when the liquid or metal reaches thermal equilibrium with the substance which is being measured, the temperature-sensitive property which is present in the thermometer indicates the temperature of the body measured.
To know more about the Zeroth Law, students can visit Vendatu’s study material on the Zeroth Law of Thermodynamics, the link to which is given above.
8. What is the meaning of entropy in Thermodynamics?
The measure of energy that is not available to do work is known as entropy. In the case of a reversible process, the change in entropy is zero while in the case of an irreversible process there is an increase in entropy. Entropy can only be increased or remain zero, it can never be decreased. Unlike energy, entropy is never conserved, it always increases.
9. What are the properties of the first law of thermodynamics?
Properties of first law of thermodynamics are:
The first law of thermodynamics is given by U=Q-W, where U is a change in internal energy, Q is the sum of all transfers of heat in or out, and Wis is the sum of all the work done by or on the system.
Q and W are both energies in transit. Only U represents the capability of being stored.
Change in the internal energy depends only upon the state of the system.
Some specialized types of heat transfer, work done and changes in internal energies are photosynthesis in plants and metabolism in living organisms.