First Law of Thermodynamics: Overview, Questions, Preparation

Updated on May 11, 2021 03:13 IST

Introduction

As we derived the concept of temperature using the Zeroth law of thermodynamics. Now we move on to other laws of thermodynamics. The First Law of thermodynamics states that energy can be converted from one form into another form, but cannot be created or destroyed under any circumstances.

Table of Contents

The First Law of Thermodynamics
First Law of Thermodynamics for Class 11
Illustrative Examples
FAQs on First Law of Thermodynamics

The First Law of Thermodynamics

First Law is the law of conservation of energy applied to any system in which the energy transfer from or to the surroundings is taken into consideration.

△Q = Heat supplied to the system by the surroundings.
△W = Work done by the system.
△U = Change in Internal energy of the system.

△Q = △W+ △U

△U = △Q - △W

Explanation

When gas trapped in a cylinder with a moving piston. The movement of the piston allows the gases to expand or compress, but not allow them to escape. Those gas molecules have kinetic energy. We know that internal energy is the sum of the kinetic energy and energy between gas molecules.

If the temperature of the gas increases, the gas molecules speed up, and the internal energy increases. So △U is positive. And if the temperature of gas decreases, △U is negative. So, temperature and internal energy are proportional.
We can increase the internal energy by transferring the heat into a gas by placing the container over a burner. If heat enters the gas, Q is positive. And Q is negative when heat exits from the gas.
The collision of the piston downwards causes the gas molecules to move faster, and internal energy increases. If the gas expands, work done on the gas is negative.

First Law of Thermodynamics for Class 11

The First Law deals with the properties of variables and exhibits relationships between them. The probability of asking long-answer types of questions and problems are more from this topic. Numericals for 2 marks and the First Law explanation for 5 marks.

Illustrative Examples

1. What is the change in the internal energy for 1g of water when going from liquid to vapour phase?

Solution: The measured latent heat is △Q = 2256J, which has volume 1 cm3 in the liquid phase and 1671 cm3 in the vapour phase.
△W = p(Vg- Vl) =1.013х 10⁵х1670х10⁻⁶ =169.2J

△U = △Q - △W = 2256 -169.2 = 2086.8J

2. A gas has constant pressure. The surroundings of the system lose 50J of heat and work done by the system is 424J. Calculate the internal energy.

Solution: In the first law, energy is not created nor destroyed. The energy lost by surroundings is the gain of the system. So, q and w are positive.

△U = q+w = 50+424 =474 J
3. An electric heater supplies heat to a system at a rate of 100 W. If the system performs work at a rate of 75 Joules per second, at what rate is the internal energy increasing?

Solution: △U = △Q - △W =100-75 =25 J/s

FAQs on First Law of Thermodynamics

1. In changing the state of a gas adiabatically from an equilibrium state A to another equilibrium state B, an amount of work equal to 22.3 J is done on the system. If the gas is taken from state A to B via a process in which the net heat absorbed by the system is 9.35 cal, how much is the net work done by the system in the latter case ? (Take 1 cal = 4.19 J)

The work done, W = 22.3 J

Being an adiabatic process, ΔQ = 0
ΔW = -22.3 J : since the work is done on the system
From the 1^st law of thermodynamics, we know ΔQ = ΔU + ΔW, where ΔU is the change of internal energy of the gas
ΔU = 22.3 J When the gas goes from state A to state B via a process, the net heat absorbed by the system is:
ΔQ = 9.35 cal = 9.35 J = 39.1765 J
Heat absorbed ΔQ = ΔU + ΔW
ΔW = ΔQ - ΔU== 39.1765 – 22.3 = 16.8765 J
Therefore, work done by the system is 16.8765 J

2. How is the First Law related to Human Metabolism?

Metabolism is the conversion of food into heat transfer and work. If the body is considered a system, the first law is used to examine the work and energy in activities ranging from sleep to exercise

3. What are the limitations of the First Law of thermodynamics?

It fails to explain the direction of the flow of heat. This law only quantifies the energy transfer during the process. It does not explain the feasibility of the reaction.

4. What are open, closed and isolated system thermodynamics?

An open system can exchange both energy and matter with the surroundings. Closed can exchange energy, but isolated cannot exchange both.

5. What violates the First Law?

Perpetual motion machines are devices that continuously supply energy without receiving any kind of energy.

6. What is internal energy?

Internal energy is the sum of the kinetic energy and energy stored in chemical bonds between molecules. So, heat and work are modes of energy transfer to a system resulting in a change in its internal energy.