What is the First Law of Thermodynamics? | Simple Explanation for Class 10

S6-SA4-0462

What is the First Law of Thermodynamics (Chemistry)?

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed, only transferred or changed from one form to another. In simple terms, the total amount of energy in a closed system always stays the same.

Simple Example

Quick Example

Imagine you have 100 rupees in your wallet. You spend 30 rupees on a samosa and 20 rupees on chai. You now have 50 rupees left. The total money (100 rupees) didn't disappear; it just changed form – some went to the shopkeeper, and some is still with you. Similarly, energy just changes forms, it doesn't vanish.

Worked Example

Step-by-Step

Let's say a gas in a container absorbs 100 Joules (J) of heat from its surroundings and, at the same time, does 20 J of work on the surroundings (like pushing a piston). We want to find the change in the internal energy of the gas.

Step 1: Understand the formula: The First Law of Thermodynamics is expressed as: ΔU = Q - W, where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done BY the system.
---Step 2: Identify the given values. Heat absorbed (Q) = +100 J (positive because heat is added to the system). Work done BY the system (W) = +20 J (positive because the system does work).
---Step 3: Substitute the values into the formula. ΔU = 100 J - 20 J.
---Step 4: Calculate the change in internal energy. ΔU = 80 J.

Answer: The change in the internal energy of the gas is 80 Joules.

Why It Matters

This law is fundamental to understanding how energy works in everything from our mobile phones to rockets. Engineers use it to design efficient engines and power plants, while scientists in AI/ML and biotechnology apply it to understand energy flow in complex systems. It's crucial for careers in renewable energy, space technology, and even developing new medicines.

Common Mistakes

MISTAKE: Thinking that if a system does work, its internal energy increases. | CORRECTION: When a system does work (W is positive), it uses its internal energy, so its internal energy decreases. The formula is ΔU = Q - W, meaning work done BY the system reduces internal energy.

MISTAKE: Confusing heat 'added to' the system with heat 'removed from' the system. | CORRECTION: Heat added to the system (Q) is positive. Heat removed from the system (Q) is negative. Pay close attention to the direction of heat transfer.

MISTAKE: Assuming energy can be 'lost' or 'gained' in total. | CORRECTION: Energy can only be transformed from one form to another (e.g., chemical to heat, or electrical to mechanical). The total energy in a closed system always remains constant.

Practice Questions

Try It Yourself

QUESTION: A system absorbs 50 J of heat and does 15 J of work. What is the change in its internal energy? | ANSWER: 35 J

QUESTION: If the internal energy of a system decreases by 60 J, and it releases 20 J of heat to the surroundings, how much work was done by or on the system? | ANSWER: 40 J of work was done ON the system (W = -40 J, so -60 = -20 - W, W = -40)

QUESTION: A battery in a mobile phone provides 500 J of electrical energy. If 400 J is converted into light and sound energy, and 50 J is lost as heat, how much internal energy change occurred in the phone's components that didn't contribute to light, sound, or heat? | ANSWER: 50 J (This implies the remaining 50J is stored or used for other internal processes, maintaining the total energy balance.)

MCQ

Quick Quiz

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

Energy can be created but not destroyed.

Energy can be destroyed but not created.

The total energy in a closed system 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 a closed system remains constant, which is the core principle of the First Law of Thermodynamics. Options A and B contradict the law, and Option D describes the Second Law of Thermodynamics.

Real World Connection

In the Real World

Think about your pressure cooker at home. When you heat the water, it absorbs energy (heat). This heat increases the water's internal energy, turning it into steam. The steam then does work by pushing against the whistle, releasing pressure. The total energy (heat, work, internal energy) is always conserved, just changing forms within the cooker system.

Key Vocabulary

Key Terms

THERMODYNAMICS: The study of heat and its relation to other forms of energy and work. | INTERNAL ENERGY: The total energy contained within a thermodynamic system. | HEAT (Q): Energy transferred due to a temperature difference. | WORK (W): Energy transferred when a force causes displacement. | SYSTEM: The part of the universe being studied.

What's Next

What to Learn Next

Great job understanding the First Law of Thermodynamics! Next, you should explore the Second Law of Thermodynamics, which talks about the direction of energy flow and the concept of entropy. This will help you understand why some processes happen naturally and others don't, building on your knowledge of energy conservation.