Thermodynamics

1. LESSON 1: PHYSICS IN DAILY
LIFE

2. OBJECTIVES
At the end of this lesson, we will be able to:
•Describe how thermodynamics works in a refrigerator.
•Explain the role of evaporation and condensation in cooling.
•Identify ways thermodynamics improves life at home.

4. What is Thermodynamics?
• is the branch or fundamental of physics that deals with heat,
work, temperature, and energy, and how they relate to each
other within a system
• Thermodynamics is the study of heat, energy, and work.
• It is the mechanism that allows your air-conditioner to work,
causes hot coffee to turn cold, and most especially, powers
your refrigerator!

5. The Three Laws of Thermodynamics
• 1st Law – Law of Conservation of Energy
🧃 Energy cannot be created or destroyed, only changed from one form
to another.
• 2nd Law – Law of Entropy / Direction of Heat Flow
☕ Heat always flows from hot to cold unless energy (work) is used to
reverse it.
• 3rd Law – Law of Absolute Zero
🧊 It is impossible to reach a temperature of absolute zero (–273.15°C),
where all motion stops.

6. 1st
Law of Thermodynamics
• Energy cannot be created or destroyed, only changed from one
form to another.
• Example: Charging your phone changes electrical energy
into stored energy.

7. 2nd Law – Law of Entropy / Direction of Heat
Flow
• ☕ Heat always flows from hot to cold unless energy (work) is
used to reverse it.
• The Second Law says:
• "In any natural process, the total entropy (disorder) of a system
tends to increase.“
• This means: Heat flows from hot to cold, not the other way around.
Entropy (disorder) always increases unless we add energy to keep
things organized.
Entropy is the reason why ice melts, coffee cools, and rooms get
messy.

9. 3rd Law – Law of
Absolute Zero
• 🧊 It is impossible to reach a temperature of
absolute zero (–273.15°C), where all motion stops.
Example: Even your freezer can’t make things
perfectly still or "infinitely cold."
• The Third Law tells us that perfect cold
(absolute zero) is impossible to reach and that
some motion and disorder always exist, even in
extremely cold systems.

10. How a Refrigerator Works
(Basic Idea of Thermodynamics)
• A refrigerator keeps your food cold by
removing heat from the inside of the
fridge and releasing it outside into the
surrounding air. It doesn’t produce cold—
it moves heat from a cooler area (inside
the fridge) to a warmer area (your
kitchen).
• This is how Thermodynamics is applied in
our daily living.

11. ⚙️Main Components:
Refrigerant – the working fluid
Compressor – compresses the gas (raises its temperature)
Condenser Coils – where the gas releases heat and turns into liquid
Expansion Valve – reduces pressure and temperature
Evaporator Coils – where the liquid refrigerant absorbs
heat and becomes gas again

12. Cycle of Operation
(Thermodynamic Cycle – Reverse Carnot Cycle):
• 1. Compression (Compressor):
• The compressor squeezes the refrigerant gas, increasing its pressure
and temperature.
• Thermodynamic Principle: First Law of Thermodynamics (energy
input as work increases internal energy).
• 2.Condensation (Condenser Coils at the Back):
• The hot, high-pressure gas flows through the condenser coils.
• It releases heat to the kitchen air and condenses into a high-pressure
liquid.
• Physics involved: Heat transfer through conduction and convection.
• Thermodynamics: Heat flows from hot to cold (Second Law of
Thermodynamics).

13. Cycle of Operation
(Thermodynamic Cycle – Reverse Carnot Cycle):
• 3. Expansion (Expansion Valve):
• The liquid refrigerant passes through a small nozzle or valve.
• It rapidly expands, causing its pressure and temperature to drop.
• Thermodynamics: Expansion = drop in pressure and temperature
(Joule–Thomson effect).
• 4. Evaporation (Evaporator Coils Inside the Fridge):
• The cold, low-pressure liquid refrigerant absorbs heat from inside the
fridge and evaporates into a gas.
• This removes heat from your food and cools the fridge.
• Thermodynamics: This process is endothermic (absorbs heat).
• Physics: Latent heat of vaporization helps absorb a lot of heat with
little temperature change.

14. Cycle of Operation
(Thermodynamic Cycle – Reverse Carnot Cycle):
• 5. Back to Compressor – the cycle repeats.

16. THERMODYNAMICS IN SIMPLE
TERMS
•Compressor squeezes the refrigerant → it gets hot and high pressure
•This hot gas goes through the condenser coils at the back → it releases
heat to the room
•It turns into a liquid and passes through the expansion valve → it becomes
cold
•The cold liquid goes inside the evaporator coils (inside the fridge) → it
absorbs heat from the food and air
•Now it turns back into a gas and goes to the compressor again — the cycle
repeats

17. COOL AIR LEFT
INSIDE THE REF
HOT AIR
RELEASED
OUTSIDE
EVAPORATION
CONDENSATION

19. LET’S TAKE IT THIS WAY
• Think of the refrigerator as a heat taxi:
🚕 The refrigerant is the taxi
🔥 Heat is the passenger
🧊 Inside the fridge: the taxi picks up heat
Outside the fridge: the taxi
️
🌬️ drops it off

20. EVAPORATION
CONDENSATIO
N
HEAT
RELEASED
HEAT
ABSORBED

21. Thermodynamic Laws at Play:
•First Law of Thermodynamics (Energy Conservation):
•The fridge does not destroy heat; it moves it using electrical work input
(from the plug).
•Second Law of Thermodynamics:
•Heat flows naturally from hot to cold, but in a refrigerator, we force it to
flow from cold to hot using external work (compressor).
•That’s why refrigerators consume electricity.

22. ANY QUESTIONS?