Day 1 Laws of Thermodynamics – ECET 2026 Mechanical

Concept Notes

Thermodynamics is the science of energy and its transformations. It deals with heat, work, and properties of matter. The laws of thermodynamics are fundamental principles that govern all thermal systems.

🔹 Zeroth Law of Thermodynamics

Statement: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
Meaning: It defines the concept of temperature.
Example: If body A = 50°C and body B = 50°C, both in contact with thermometer (C), then A and B are in equilibrium.

🔹 First Law of Thermodynamics (Law of Energy Conservation)

Statement: Energy can neither be created nor destroyed; it can only change from one form to another.
In thermodynamics → Heat and Work are energy interactions.

Equation (for closed system):
$\Delta Q = \Delta U + \Delta W$
where

ΔQ\Delta QΔQ = Heat supplied
ΔU\Delta UΔU = Change in internal energy
ΔW\Delta WΔW = Work done by system

Example: In an IC engine, fuel burning provides heat → increases internal energy → produces piston work.

🔹 Second Law of Thermodynamics

Statement (Kelvin-Planck): It is impossible to construct an engine that operates in a cycle and produces no effect other than absorption of heat from a single reservoir and conversion of the entire heat into work.
Statement (Clausius): It is impossible for heat to flow from a colder body to a hotter body without external work.

Entropy (S): A measure of disorder in a system.

$dS = \frac{\delta Q_{rev}}{T}$

Implication:

Heat cannot be fully converted to work.
Heat flows naturally from hot → cold.
Efficiency of heat engines is always < 100%.

Example: A refrigerator requires external work to transfer heat from cold food compartment to warm kitchen.

🔹 Third Law of Thermodynamics

Statement: As the temperature approaches absolute zero (0 K), entropy of a perfect crystal approaches zero.
Implication: It is impossible to reach absolute zero in finite steps.

Example: Cryogenic engineering → materials cannot practically reach 0 K.

⚙️ Formulas

First Law for Closed System:

$Q - W = \Delta U$

First Law for Cyclic Process:

$\Delta U = 0 \quad \Rightarrow \quad Q = W$

Work in Isothermal Process:

$W = nRT \ln \left( \frac{V_2}{V_1} \right)$

Work in Adiabatic Process:

$W = \frac{P_1 V_1 - P_2 V_2}{\gamma - 1}$

Entropy Change (Reversible):

$\Delta S = \int \frac{\delta Q_{rev}}{T}$

🔟 10 MCQs

Q1. Zeroth law of thermodynamics defines:
a) Heat
b) Temperature
c) Work
d) Entropy

Q2. The first law of thermodynamics is the law of:
a) Conservation of mass
b) Conservation of energy
c) Increase of entropy
d) Conservation of momentum

Q3. For a cyclic process, the change in internal energy is:
a) Zero
b) Positive
c) Negative
d) Infinite

Q4. In an adiabatic process:
a) Heat transfer = 0
b) Work transfer = 0
c) Internal energy change = 0
d) Temperature remains constant

Q5. Clausius statement of second law refers to:
a) Heat engine
b) Heat pump/refrigerator
c) Internal energy
d) Entropy

Q6. Efficiency of a Carnot engine depends on:
a) Working substance
b) Pressure
c) Reservoir temperatures
d) Type of fuel

Q7. According to second law, heat flows:
a) Cold to hot
b) Hot to cold
c) Any direction
d) Neither direction

Q8. Third law of thermodynamics deals with:
a) Zero entropy at absolute zero
b) Conservation of heat
c) Heat engines
d) Refrigeration

Q9. In an isothermal process, internal energy change is:
a) Zero
b) Positive
c) Negative
d) Infinite

Q10. Entropy is mathematically expressed as:
a) $dS = \frac{dQ}{dT}$
b) $dS = \frac{\delta Q_{rev}}{T}$
c) $dS = \frac{T dQ}{W}$
d) $dS = \frac{W}{T}$

✅ Answer Key

Q	Ans
1	b
2	b
3	a
4	a
5	b
6	c
7	b
8	a
9	a
10	b

🧠 Explanations

Q1: Zeroth law → defines temperature → (b).
Q2: First law = conservation of energy → (b).
Q3: In cyclic process, ΔU = 0 → (a).
Q4: Adiabatic process means no heat transfer → (a).
Q5: Clausius statement → refrigerator/heat pump principle → (b).
Q6: Carnot efficiency = function of T1, T2 → (c).
Q7: Heat flows naturally from hot → cold → (b).
Q8: Third law → entropy → 0 at 0 K → (a).
Q9: Isothermal → ΔU = 0 since T constant → (a).
Q10: Entropy formula = δQrev/T → (b).

🎯 Motivation / Why Practice Matters

The laws of thermodynamics are the backbone of Thermal Engineering. Every IC engine, power plant, and refrigeration system is designed using these principles.
👉 In ECET exams, questions often mix theory + numerical. If you master these laws now, you’ll answer Thermo, RAC, Power Plant, IC Engines questions with ease.

Consistent practice ensures speed + accuracy → key to scoring high in competitive exams.

📲 CTA

👉 For more ECET 2026 updates & free tests, Join our WhatsApp Group here:
https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9

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Day 1 Laws of Thermodynamics – ECET 2026 Mechanical