What is First Law of Thermodynamics? | Simple Explanation for Class 12

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What is First Law of Thermodynamics (Chemistry)?

Grade Level:

Class 12

AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics

Definition

What is it?

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred or changed from one form to another. This means the total amount of energy in the universe always stays the same. For a system, any change in its internal energy comes from heat added to it or work done on it.

Simple Example

Quick Example

Imagine you have a phone battery. When you charge it, electrical energy goes into the battery and gets stored as chemical energy. When you use the phone, this chemical energy changes into light (screen), sound (speakers), and heat (phone warms up). The total energy just changes forms; it doesn't disappear or get created out of nowhere.

Worked Example

Step-by-Step

Let's say a gas in a cylinder is heated, and it expands, doing work.

Step 1: The gas absorbs 100 Joules (J) of heat from its surroundings (Q = +100 J).
---Step 2: As the gas expands, it pushes a piston, doing 30 Joules (J) of work on the surroundings (W = -30 J, as work is done BY the system).
---Step 3: We want to find the change in the internal energy (ΔU) of the gas.
---Step 4: Using the First Law of Thermodynamics: ΔU = Q + W
---Step 5: Substitute the values: ΔU = 100 J + (-30 J)
---Step 6: Calculate: ΔU = 70 J

Answer: The internal energy of the gas increased by 70 Joules.

Why It Matters

Understanding this law is crucial for designing efficient engines in EVs, optimizing power plants in Climate Science, and even understanding energy flow in living organisms for Biotechnology. Engineers and scientists in various fields use this principle daily to create new technologies and solve complex problems.

Common Mistakes

MISTAKE: Thinking that a system can create its own energy. | CORRECTION: Energy must always come from somewhere else or be converted from another form; it cannot be generated from nothing.

MISTAKE: Confusing the signs for heat (Q) and work (W). | CORRECTION: Remember the convention: Heat absorbed by the system is positive (+Q). Heat released by the system is negative (-Q). Work done BY the system is negative (-W). Work done ON the system is positive (+W).

MISTAKE: Assuming the First Law only applies to heat and work. | CORRECTION: The First Law applies to all forms of energy. Heat and work are just two common ways energy is exchanged in chemistry.

Practice Questions

Try It Yourself

QUESTION: A system releases 50 J of heat and has 20 J of work done on it. What is the change in internal energy? | ANSWER: ΔU = Q + W = (-50 J) + (20 J) = -30 J

QUESTION: If a reaction absorbs 120 J of heat and its internal energy increases by 90 J, how much work was done by or on the system? | ANSWER: ΔU = Q + W => 90 J = 120 J + W => W = 90 J - 120 J = -30 J. Work of 30 J was done BY the system.

QUESTION: A gas expands from 1 L to 5 L against a constant external pressure of 2 atm. During this expansion, the gas absorbs 300 J of heat. Calculate the change in internal energy of the gas. (Note: 1 L.atm = 101.3 J) | ANSWER: Work (W) = -P_ext * ΔV = -2 atm * (5 L - 1 L) = -2 atm * 4 L = -8 L.atm. Convert W to Joules: W = -8 * 101.3 J = -810.4 J. ΔU = Q + W = 300 J + (-810.4 J) = -510.4 J.

MCQ

Quick Quiz

Which of the following statements correctly describes the First Law of Thermodynamics?

Energy can be created but not destroyed.

Energy can be destroyed but not created.

The total energy of the universe remains constant.

Heat always flows from a colder body to a hotter body.

The Correct Answer Is:

Option C correctly states that the total energy in the universe is conserved, meaning it stays constant. Options A and B are incorrect because energy cannot be created or destroyed. Option D describes the Second Law of Thermodynamics, not the First Law.

Real World Connection

In the Real World

The First Law of Thermodynamics is fundamental to how our refrigerators and air conditioners work at home. They don't create cold; instead, they use electricity (work) to move heat from inside to outside, following the principle of energy conservation. Similarly, when you cook food on a gas stove, the chemical energy in the LPG is converted into heat energy to cook your meal.

Key Vocabulary

Key Terms

INTERNAL ENERGY: The total energy contained within a thermodynamic system. | HEAT: Energy transferred due to a temperature difference. | WORK: Energy transferred when a force acts over a distance. | SYSTEM: The part of the universe being studied. | SURROUNDINGS: Everything outside the system.

What's Next

What to Learn Next

Now that you understand energy conservation, you're ready to explore the Second Law of Thermodynamics. This will teach you about the direction of energy flow and why some processes happen naturally while others don't, which is super important for understanding engines and chemical reactions!