Thermodynamics

Definition

Thermal Equilibrium occurs when two objects in contact no longer exchange heat because they have reached the same temperature.

The Zeroth Law of Thermodynamics states that if two systems (A and B) are each in thermal equilibrium with a third system (C), then they are in thermal equilibrium with each other.

Explanation

Think of it as the "Law of Equality." If System A is at the same "heat level" as System C, and System B is also at the same "heat level" as System C, then A and B must be the same as each other, even if they never touch. This law is the reason why thermometers work!

Key Points

• Heat only flows when there is a temperature difference.
• Equilibrium means net heat transfer is zero.
• The Zeroth Law provides the basis for the concept of temperature.

Examples

• A spoon in hot tea: If you leave a metal spoon in hot tea, eventually the spoon and the tea will reach the same temperature.
• Room Temperature: A cold glass of water will eventually reach the same temperature as the air in a warm room.

Real-Life Use: The Thermometer

The most important use of the Zeroth Law is the thermometer. When you put a thermometer in your mouth, the thermometer (System C) reaches thermal equilibrium with your body (System A). This allows us to measure temperature reliably.

Meaning & Definitions

• Heat (Q): The transfer of energy due to a temperature difference.
• Work (W): The energy transferred when a force moves an object (e.g., gas pushing a piston).
• Internal Energy (U): The total energy stored inside a system (kinetic and potential energy of molecules).

Simple Explanation

Imagine blowing up a balloon. The air you blow in carries Heat (energy). As the balloon expands, it does Work by pushing the rubber walls. The energy is now stored as Internal Energy, making the molecules move faster.

Examples

• Rubbing Hands: Doing Work converts into Internal Energy, which we feel as Heat.
• Boiling Water: Adding fire adds Heat, increasing the Internal Energy of water molecules.

Real-Life Use: Bicycle Pumps

When you use a manual pump, it gets hot because the Work you do increases the Internal Energy of the air, manifesting as Heat.

Formula

ΔU = Q - W

(Where ΔU is change in internal energy, Q is heat added, and W is work done by the system.)

Meaning & Definition

The First Law is the Law of Conservation of Energy. Energy cannot be created or destroyed, only transformed.

Key Points

• Energy in = Change in Internal Energy + Work done.
• It is a mathematical way to track every Joule of energy.

Examples

• Steam Engine: Heat from coal increases internal energy, which then does work by pushing a piston.
• Ice Melting: Adding heat increases internal energy (breaking bonds), but if no movement occurs, W = 0.

Real-Life Use: Internal Combustion Engines

In a car, fuel explosion provides Heat (Q). This increases Internal Energy (ΔU), causing gas expansion that does Work (W) to move the pistons.

Definitions

• Isothermal Process: Occurs at a constant temperature (ΔT = 0).
• Adiabatic Process: No heat is exchanged with surroundings (Q = 0).

Comparison Table

Real-Life Use: Aerosol Sprays

When spraying deodorant, gas expands so fast that no heat enters (Adiabatic expansion). This uses internal energy, making the spray feel cold.

Definition

The total Entropy (disorder) of an isolated system always increases over time. It dictates the direction of energy flow.

Key Points

• Entropy: A measure of randomness.
• Directionality: Heat flows from Hot $\rightarrow$ Cold spontaneously.

Examples

• Coffee Cooling: A hot cup of coffee will always cool down to room temperature, never spontaneously gets hotter.
• Broken Egg: It is easy to increase disorder (break an egg) but impossible to reverse it spontaneously.

• Reversible: Can be undone such that both system and surroundings return to original states (Ideal).
• Irreversible: Cannot be undone without permanent changes (Real-world, due to friction).

Problem 1: The First Law

Question: A system absorbs 800 J of heat from its surroundings and performs 300 J of work on its surroundings. What is the change in the internal energy of the system?

Solution:
1. Identify: Q = +800 J, W = +300 J
2. Formula: ΔU = Q - W
3. Calculation: ΔU = 800 - 300 = 500 J
Answer: 500 J

Problem 2: Isothermal Work

Question: In an isothermal expansion of an ideal gas, what happens to the internal energy (ΔU)?

Solution:
1. In isothermal, ΔT = 0.
2. For ideal gas, U depends on T.
Answer: ΔU = 0

1. The Zeroth Law of Thermodynamics defines the concept of:
   a) Entropy | b) Temperature | c) Internal Energy | d) Work
   Answer: (b)
2. In an adiabatic process, which of the following is true?
   a) ΔT = 0 | b) Q = 0 | c) W = 0 | d) ΔU = 0
   Answer: (b)
3. The First Law of Thermodynamics is a restatement of:
   a) Conservation of Mass | b) Conservation of Energy | c) Law of Entropy
   Answer: (b)
4. Which process occurs at a constant temperature?
   a) Adiabatic | b) Isobaric | c) Isothermal
   Answer: (c)
5. Entropy is a measure of:
   a) Heat | b) Work | c) Disorder
   Answer: (c)

1. State the Zeroth Law and explain its importance in thermometry.
   Ans: If A=C and B=C in thermal equilibrium, then A=B. It allows a thermometer to measure other objects.
2. Differentiate between Isothermal and Adiabatic processes.
   Ans: Isothermal has constant T and allows heat exchange; Adiabatic has no heat exchange and T changes.
3. Why is a 100% efficient heat engine impossible?
   Ans: The Second Law states that some heat must always be rejected to a cold sink, increasing total entropy.
4. Define Internal Energy. Is it a state function?
   Ans: It is the total microscopic energy of a system. Yes, it is a state function as it only depends on the current state.

• Isothermal: "I am Tame" $\rightarrow$ Isothermal = Temperature stays same.
• Adiabatic: "Adiabatic = Away with Heat" $\rightarrow$ No heat enters or leaves.
• First Law: ΔU = Q - W (Internal energy is what's left after work is done).
• Entropy: "Everything gets Entangled" $\rightarrow$ Disorder increases.