What is the First Law of Thermodynamics?

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Grade Level:

Class 12

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Definition

What is it?

The First Law of Thermodynamics is a fundamental principle in physics that states energy cannot be created or destroyed, only transformed from one form to another. It essentially says that the total amount of energy in an isolated system always stays constant.

Simple Example

Quick Example

Imagine you have a mobile phone battery charged to 100%. When you use your phone to watch a video, the chemical energy stored in the battery changes into light energy for the screen, sound energy for the speakers, and some heat energy. The total energy remains the same; it just changes form.

Worked Example

Step-by-Step

PROBLEM: A system absorbs 100 Joules (J) of heat from its surroundings and does 30 J of work on the surroundings. What is the change in internal energy of the system?

STEP 1: Identify the given values. Heat absorbed (Q) = +100 J (positive because heat is added to the system). Work done BY the system (W) = -30 J (negative because the system is doing work).
---STEP 2: Recall the First Law of Thermodynamics equation: Change in Internal Energy (ΔU) = Q + W.
---STEP 3: Substitute the values into the equation: ΔU = 100 J + (-30 J).
---STEP 4: Calculate the change in internal energy: ΔU = 100 J - 30 J = 70 J.
---ANSWER: The change in internal energy of the system is 70 Joules.

Why It Matters

This law is crucial for understanding how machines work, from car engines to refrigerators. Engineers use it to design more efficient power plants and electric vehicles. Even in medicine, understanding energy transfer helps in developing new treatments and diagnostic tools.

Common Mistakes

MISTAKE: Thinking energy is 'lost' when a device heats up. | CORRECTION: Energy is never lost; it's converted to heat energy, which might not be the desired form but still exists.

MISTAKE: Confusing the sign convention for heat and work (e.g., positive for work done BY the system). | CORRECTION: Heat ADDED to the system (Q) is positive, heat REMOVED is negative. Work done BY the system (W) is negative, work done ON the system is positive.

MISTAKE: Believing that a system can produce more energy than it consumes. | CORRECTION: The First Law strictly forbids this; you can't get something for nothing in terms of energy.

Practice Questions

Try It Yourself

QUESTION: If a gas releases 50 J of heat and 20 J of work is done ON the gas, what is the change in its internal energy? | ANSWER: ΔU = -50 J + 20 J = -30 J

QUESTION: A kettle uses 1500 J of electrical energy to heat water. If 1200 J of heat goes into the water, how much energy is 'lost' to the surroundings as heat or sound? | ANSWER: The 'lost' energy is 1500 J - 1200 J = 300 J. This energy isn't truly lost but converted to other forms like heat escaping the kettle or sound.

QUESTION: A bicycle rider expends 1000 J of energy. If 300 J is used to move the bicycle forward and 500 J is lost as heat due to friction, how much energy is converted into other forms (e.g., sound, air resistance) that are not accounted for? | ANSWER: Total energy expended = 1000 J. Energy for movement = 300 J. Energy lost as friction = 500 J. Remaining energy = 1000 J - 300 J - 500 J = 200 J. This 200 J is converted into other forms.

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.

Energy can be transformed from one form to another but cannot be created or destroyed.

The total energy of a system always decreases.

The Correct Answer Is:

Option C correctly states that energy is conserved – it changes form but its total amount remains constant. Options A, B, and D contradict the principle of energy conservation.

Real World Connection

In the Real World

Think about an electric vehicle (EV) charging. The electrical energy from the charging station isn't lost; it's converted and stored as chemical energy in the EV's battery. When you drive, this chemical energy transforms into kinetic energy to move the car, and some heat due to friction and other processes. Companies like TATA Motors and Mahindra are constantly applying this law to make EVs more efficient in India.

Key Vocabulary

Key Terms

ENERGY: The ability to do work or produce heat | SYSTEM: The specific part of the universe we are studying | INTERNAL ENERGY: The total energy contained within a system | HEAT: Energy transferred due to temperature difference | WORK: Energy transferred when a force moves an object

What's Next

What to Learn Next

Great job understanding energy conservation! Next, you should explore the Second Law of Thermodynamics. It builds on this concept by explaining the direction of energy flow and why some energy transformations are more efficient than others, which is vital for understanding engines and refrigerators.