The First Law of Thermodynamicsstates that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.
Table of Contents
Introducing State Variables
Explaining the First Law of Thermodynamics
Sign Conventions
First Law of Thermodynamics Solved Examples
Frequently Asked Questions – FAQs
Introducing State Variables
Thermodynamic state variables are the macroscopic quantities that determine a system’s thermodynamic equilibrium state. A system not in equilibrium cannot be described by state variables. State variables can further be classified as intensive or extensive variables. Intensive variables are independent of the dimensions of the system like pressure and temperature, while extensive variables depend on dimensions of the system like volume, mass, internal energy etc.
Explaining the first law of thermodynamics
The first law of thermodynamics relates to heat, internal energy, and work.
The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed, but it can be changed from one form to another.
According to this law, some heat given to the system is used to change the internal energy while the rest is used in doing work by the system.
So we can infer from the above equation that the quantity (ΔQ – W) is independent of the path taken to change the state. Further, we can say that internal energy increases when the heat is given to the system and vice versa.
Sign Conventions
The table below shows the appropriate sign conventions for all three quantities under different conditions:
ΔU (change in internal energy)
Q (heat)
W (work done on the gas)
is “+” if temperature increases
is “+” if heat enters gas
is “+” if gas is compressed
is “-” if temperature decreases
is “-” if heat exits gas
is “-” if gas expands
is “0” if temperature is constant
is “0” if no heat is exchanged
is “0” if volume is constant
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First Law of Thermodynamics Solved Examples
1. Calculate the change in the system’s internal energy if 3000 J of heat is added to a system and a work of 2500 J is done.
Solution: The following sign conventions are followed in the numerical: Solution: The following sign conventions are followed in the numerical:
2. What is the change in the internal energy of the system if 2000 J of heat leaves the system and 2500 J of work is done on the system? Solution: The change in the internal energy of the system can be identified using the formula:
ΔU = Q-W
Substituting the values in the following equation, we get
The first law of thermodynamics states that energy can neither be created nor destroyed. It merely transforms from one form to another.
Q2
Who stated the first law of thermodynamics?
Rudolf Clausius and William Thomson stated the first law of thermodynamics.
Q3
Can the first law of thermodynamics be violated?
A machine called a Perpetual Motion Machine of the first kind violates the first law by creating energy.
Q4
Why is the first law of thermodynamics important to the environment?
The first law states that energy can neither be created nor destroyed it can only e transformed from one form to another. Sun is the only source of energy for all living organisms on Earth. This solar energy is converted into chemical energy by plants through the process of photosynthesis. These energies obtained by the plants do not go back into the solar system. Rather, it is passed on to herbivores that feed on green plants. Some part of the energy obtained by the herbivores is utilized by carnivores or transferred to the decomposers when the herbivores die.
Q5
What are the limitations of the first law of thermodynamics
The first law of thermodynamics does not quantify the energy transfer that takes place, failing to explain the feasibility of the thermal process.
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The first law of thermodynamics is given as ΔE=q+w, where ΔE is the change in internal energy of a system, q is the net heat transfer (the sum of all heat transfer into and out of the system), and w is the net work done (the sum of all work done on or by the system).
The first law of thermodynamics states that the total energy of a system remains constant, even if it is converted from one form to another. For example, kinetic energy—the energy that an object possesses when it moves—is converted to heat energy when a driver presses the brakes on the car to slow it down.
For example, it takes approximately 1 calorie of heat to increase the temperature of 1 gram of water by 1 K. Since there are 18 grams of water in 1 mole, the molar heat capacity of water is 18 calories per K, or about 75 joules per K. The total heat capacity C for n moles is defined by C = nc.
The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed, but it can be changed from one form to another.
The correct answer to the above question is: a) Energy cannot be created or destroyed. The first law of thermodynamics is also known as the law of energy conservation and states that energy within a closed system can be neither created out of nothing nor altogether destroyed.
The first law of thermodynamics is given as ΔE=q+w, where ΔE is the change in internal energy of a system, q is the net heat transfer (the sum of all heat transfer into and out of the system), and w is the net work done (the sum of all work done on or by the system).
Other simple examples include throwing a ball from the top of a building to the ground (potential energy to kinetic energy), Photosynthesis reaction ( light energy to chemical energy), Combustion of wood (chemical energy to heat energy), etc.
The first law of thermodynamics is a statement about the law of conservation of energy for a thermodynamic system. It states that energy can neither be created nor be destroyed and introduces the concept of internal energy.
This is the basic idea of the First Law of Thermodynamics. There are many different ways of stating this law, but one way is: The change in the total energy of a system is equal to the net input (= input minus output) of energy into the system. This includes all forms of energy, both macroscopic and microscopic.
The first explicit statement of the first law of thermodynamics, by Rudolf Clausius in 1850, referred to cyclic thermodynamic processes, and to the existence of a function of state of the system, the internal energy.
From 1st Law:dU=đQREV−PdVFrom The Definition of Entropy:dS=đQREVT→đQREV=TdS⇒dU=TdS−PdV. Equation 8.1. 1 is called the fundamental equation of thermodynamics since it combines the first and the second laws.
The first law of thermodynamics states the relationship between the change in total internal energy of a system, the heat addition, and the work done. This can be mathematically expressed as ΔU = Q - W. Here, ΔU is the change in internal energy, Q is the heat added to the system, and W is work done by the system.
The laws of thermodynamics are deceptively simple to state, but they are far-reaching in their consequences. The first law asserts that if heat is recognized as a form of energy, then the total energy of a system plus its surroundings is conserved; in other words, the total energy of the universe remains constant.
Power Plants: Power plants are prime examples of the First Law of Thermodynamics at work. For example, in thermal power plants, the chemical energy in coal is converted into thermal energy which heats water turning it into high pressured steam.
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