Thermodynamics

Introduction to Thermodynamics

Thermodynamics is the study of energy, heat, temperature and work. Mechanical engineers use thermodynamics to understand how energy moves, changes form and affects machines and physical systems.

1. What is Energy?

Energy is the ability to perform work or produce a change. Energy can exist in several different forms.

• Thermal Energy – Energy associated with temperature.
• Kinetic Energy – Energy caused by motion.
• Potential Energy – Stored energy caused by position.
• Chemical Energy – Energy stored in chemical bonds.
• Electrical Energy – Energy associated with electric charge.

Energy cannot be created or destroyed. It can only be transferred or converted from one form into another. This principle is known as the conservation of energy.

2. Heat and Temperature

Heat and temperature are related, but they do not mean the same thing.

Temperature describes how hot or cold a substance is. Temperature is related to the average kinetic energy of the particles inside the substance.

Common temperature units include:

• Degrees Celsius – °C
• Degrees Fahrenheit – °F
• Kelvin – K

Kelvin is the SI temperature unit commonly used in engineering equations.

Heat is energy transferred from one object or system to another because of a temperature difference.

Heat naturally moves from a region of higher temperature to a region of lower temperature.

3. Thermal Equilibrium

When two objects at different temperatures come into contact, heat moves from the hotter object to the colder object.

Heat transfer continues until both objects reach the same temperature. This condition is called thermal equilibrium.

4. Thermodynamic Systems

Before analyzing a thermodynamics problem, engineers identify the system.

A system is the object, machine or region being studied. Everything outside the system is called the surroundings.

The real or imaginary surface separating the system from the surroundings is called the boundary.

There are three main types of thermodynamic systems:

• Open System – Allows both mass and energy to cross the system boundary.
• Closed System – Allows energy to cross the boundary, but the amount of mass inside the system remains constant.
• Isolated System – Does not allow mass or energy to cross the boundary.

Turbines, compressors and pumps are commonly analyzed as open systems. A sealed piston-cylinder device is commonly analyzed as a closed system.

5. Thermodynamic Properties

A property is a measurable characteristic that describes the condition of a thermodynamic system.

Common thermodynamic properties include:

• Pressure
• Temperature
• Volume
• Mass
• Density
• Internal Energy

The condition of a system at a particular moment is called its state.

6. Intensive and Extensive Properties

Thermodynamic properties can be classified as either intensive or extensive.

• Intensive Properties – Do not depend on the amount of matter present. Examples include temperature, pressure and density.
• Extensive Properties – Depend on the amount of matter present. Examples include mass, total volume and total internal energy.

7. Pressure

Pressure is the amount of force applied over a specific area.

Formula:

P = F / A

• P = Pressure
• F = Applied Force
• A = Area

Pressure is measured in Pascals.

One Pascal is equal to one Newton per square meter.

1 Pa = 1 N/m²

8. Work in Thermodynamics

In thermodynamics, work is a transfer of energy that occurs when a force causes movement.

Examples of thermodynamic work include:

• Expanding gas pushing a piston
• Steam rotating a turbine shaft
• A compressor shaft compressing air
• An engine producing mechanical motion

Work is represented by the symbol W and is normally measured in Joules.

9. Internal Energy

Internal energy is the microscopic energy stored inside a substance.

Internal energy includes energy associated with:

• Molecular motion
• Molecular vibration
• Molecular rotation
• Interactions between particles
• Chemical bonds

Internal energy is represented by the symbol U.

10. The First Law of Thermodynamics

The First Law of Thermodynamics applies the conservation of energy to thermodynamic systems.

Energy entering a system must either leave the system or remain stored inside it.

Formula:

ΔU = Q − W

• ΔU = Change in Internal Energy
• Q = Heat Added to the System
• W = Work Done by the System

In this sign convention, heat entering the system is positive and work performed by the system is positive.

If more heat enters the system than leaves as work, the internal energy of the system increases.

11. Specific Heat Capacity

Specific heat capacity describes the amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius or one Kelvin.

Formula:

Q = mcΔT

• Q = Heat Transferred
• m = Mass
• c = Specific Heat Capacity
• ΔT = Change in Temperature

Materials with a high specific heat capacity require more energy to increase their temperature.

12. Methods of Heat Transfer

Heat can be transferred in three main ways:

• Conduction – Heat transfer through direct contact or through a material.
• Convection – Heat transfer caused by the movement of a liquid or gas.
• Radiation – Energy transfer through electromagnetic waves.

A metal spoon becoming hot in soup is an example of conduction. Warm air rising from a heater is an example of convection. Energy traveling from the Sun to Earth is an example of radiation.

13. Phase Changes

A substance can exist as a solid, liquid or gas. A phase change occurs when a substance changes from one phase to another.

Common phase changes include:

• Melting – Solid to liquid
• Freezing – Liquid to solid
• Vaporization – Liquid to gas
• Condensation – Gas to liquid
• Sublimation – Solid directly to gas

Energy transferred during a phase change can be calculated using:

Formula:

Q = mL

• Q = Heat Transferred
• m = Mass
• L = Latent Heat

14. Efficiency

Efficiency compares the useful energy output of a machine to the amount of energy supplied to it.

Formula:

η = Useful Energy Output / Energy Input

Efficiency is commonly expressed as a percentage.

Real machines are not 100 percent efficient because some energy is transferred to the surroundings, often as waste heat.

15. Units, Conversions and Rearranged Formulas

Thermodynamics problems often require engineers to use consistent units and rearrange equations to solve for different unknown values.

Common Energy Units

• J = Joule, the SI unit of energy, heat and work.
• kJ = Kilojoule.
• 1 kJ = 1,000 J

To convert Joules to kilojoules, divide by 1,000.

To convert kilojoules to Joules, multiply by 1,000.

Common Pressure Units

• Pa = Pascal.
• kPa = Kilopascal.
• 1 Pa = 1 N/m²
• 1 kPa = 1,000 Pa

To convert Pascals to kilopascals, divide by 1,000.

To convert kilopascals to Pascals, multiply by 1,000.

Specific Heat Capacity Units

In the equation Q = mcΔT, the standard units are:

• Q = Heat transferred, measured in Joules (J)
• m = Mass, measured in kilograms (kg)
• c = Specific heat capacity, measured in J/(kg·°C) or J/(kg·K)
• ΔT = Temperature change, measured in °C or K

Temperature change can be calculated using:

ΔT = T_final − T_initial

Latent Heat Units

In the equation Q = mL, the standard units are:

• Q = Heat transferred, measured in Joules (J)
• m = Mass, measured in kilograms (kg)
• L = Latent heat, measured in Joules per kilogram (J/kg)

Efficiency as a Decimal or Percentage

Efficiency may be written as a decimal or a percentage.

To convert a percentage to a decimal, divide by 100.

30% = 30 / 100 = 0.30

To convert a decimal to a percentage, multiply by 100%.

0.35 × 100% = 35%

Useful Rearranged Formulas

The equations from this lesson can be rearranged depending on the unknown value.

• F = PA – Finds force from pressure and area.
• A = F / P – Finds area from force and pressure.
• Q = ΔU + W – Finds heat using the First Law.
• W = Q − ΔU – Finds work using the First Law.
• ΔT = Q / (mc) – Finds temperature change.
• c = Q / (mΔT) – Finds specific heat capacity.
• m = Q / (cΔT) – Finds mass in a specific heat problem.
• L = Q / m – Finds latent heat.
• m = Q / L – Finds mass during a phase change.
• Useful Energy Output = η × Energy Input
• Energy Input = Useful Energy Output / η

16. Why Thermodynamics Matters

Mechanical engineers use thermodynamics to understand and improve systems that transfer or convert energy.

Examples include:

• Automobile engines
• Aircraft engines
• Steam and gas turbines
• Power plants
• Refrigerators
• Air-conditioning systems
• Heat pumps
• Boilers
• Solar-energy systems
• Manufacturing equipment

17. Key Takeaways

• Thermodynamics is the study of energy, heat, temperature and work.
• Energy can be transferred or converted but cannot be created or destroyed.
• Heat is energy transferred because of a temperature difference.
• Temperature describes how hot or cold a substance is.
• Open systems exchange mass and energy.
• Closed systems exchange energy but maintain a constant amount of mass.
• The First Law of Thermodynamics is based on conservation of energy.
• Specific heat capacity describes how much energy is needed to change temperature.
• Heat is transferred through conduction, convection and radiation.
• Efficiency compares useful energy output to total energy input.

Thermodynamics provides the foundation for understanding engines, refrigeration systems, power plants, turbines and many other mechanical engineering technologies.

Real World Applications of Thermodynamics

Thermodynamics is used whenever engineers study how energy, heat, temperature, pressure, and work move through or affect a system. It helps engineers design engines, refrigeration systems, turbines, power plants, heating systems, and many other machines that transfer or convert energy.

1. Automobile Engines

Automobile engines convert chemical energy from fuel into thermal energy and then into mechanical work. Engineers study combustion, pressure, temperature, and efficiency to improve engine performance.

Concepts used:

• Chemical energy: energy stored in fuel.
• Heat and work: combustion produces heat that helps move pistons.
• Efficiency: compares useful mechanical output to energy input.

2. Refrigerators

Refrigerators remove heat from the inside compartment and transfer it to the surrounding room. A refrigerant circulates through the system and changes pressure, temperature, and phase.

Concepts used:

• Heat transfer: heat moves out of the refrigerated space.
• Phase changes: refrigerant evaporates and condenses.
• Work: the compressor uses energy to move and compress the refrigerant.

3. Air-Conditioning Systems

Air-conditioning systems cool buildings by removing heat from indoor air and releasing it outdoors. Engineers design these systems to control temperature while using as little energy as possible.

Concepts used:

• Convection: moving air carries heat through the building.
• Heat transfer: heat moves from indoor air to the refrigerant.
• Efficiency: determines how effectively the system uses electricity.

4. Power Plants

Many power plants convert thermal energy into mechanical work and then into electrical energy. Heat may be used to produce steam that rotates a turbine connected to a generator.

Concepts used:

• Energy conversion: thermal energy becomes mechanical and electrical energy.
• Work: expanding steam rotates the turbine.
• Efficiency: measures how much input energy becomes useful electrical output.

5. Steam and Gas Turbines

Turbines use high-energy fluids such as steam or hot gas to rotate blades and produce shaft work. Engineers analyze the energy entering and leaving the turbine to improve performance.

Concepts used:

• Open systems: mass and energy flow through the turbine.
• Work: the moving fluid turns the turbine shaft.
• Internal energy: the fluid's stored microscopic energy changes.

6. Heat Pumps

Heat pumps transfer heat from one location to another. They can move heat into a building during cold weather and remove heat during warm weather.

Concepts used:

• Heat transfer: energy moves between indoor and outdoor spaces.
• Work input: a compressor requires electrical energy.
• Phase changes: refrigerant evaporates and condenses during operation.

7. Boilers and Water Heaters

Boilers and water heaters transfer energy from fuel or electricity into water. The added heat raises the water temperature or changes liquid water into steam.

Concepts used:

• Specific heat capacity: determines the energy needed to raise water temperature.
• Conduction and convection: transfer heat through the system.
• Phase change: additional energy can convert water into steam.

8. Aircraft and Rocket Engines

Aircraft and rocket engines release energy through combustion and use expanding gases to produce thrust. Engineers study high temperatures, pressures, heat transfer, and energy efficiency.

Concepts used:

• Chemical energy: fuel stores energy before combustion.
• Thermal energy: combustion creates extremely hot gases.
• Work and energy conversion: expanding gases help produce thrust.

9. Solar-Energy Systems

Solar thermal systems collect radiation from the Sun and use it to heat water, air, or another working fluid. Engineers design collectors and storage systems to capture and use this energy efficiently.

Concepts used:

• Radiation: energy travels from the Sun through electromagnetic waves.
• Heat transfer: collected energy heats a working fluid.
• Thermal storage: energy may be stored for later use.

10. Manufacturing and Industrial Equipment

Manufacturing processes often involve heating, cooling, melting, freezing, drying, or compressing materials. Engineers use thermodynamics to control temperatures and energy use during production.

Concepts used:

• Specific heat capacity: predicts energy needed for temperature changes.
• Phase changes: materials may melt, solidify, or vaporize.
• Efficiency: helps reduce wasted energy and operating costs.

11. Computer and Electronics Cooling

Computers and electronic devices produce heat while operating. Engineers use fans, heat sinks, cooling liquids, and thermal materials to prevent components from overheating.

Concepts used:

• Conduction: heat moves from components into heat sinks.
• Convection: moving air or liquid carries heat away.
• Thermal equilibrium: temperatures move toward a balanced condition.

12. Building Insulation

Insulation slows heat transfer between a building and the outdoor environment. Engineers select materials that help buildings remain warmer in winter and cooler in summer.

Concepts used:

• Conduction: insulation reduces heat flow through walls and roofs.
• Convection: sealed spaces reduce moving-air heat transfer.
• Efficiency: less energy is needed for heating and cooling.

13. Piston-Cylinder Devices

Piston-cylinder devices are used in engines, compressors, and laboratory equipment. A gas inside the cylinder can expand or compress while heat and work are transferred.

Concepts used:

• Closed systems: the contained mass may remain constant.
• Pressure: gas pressure acts on the piston area.
• Work: piston movement transfers mechanical energy.

14. Food Storage and Cooking

Cooking and food storage both involve thermodynamics. Ovens transfer heat into food, while refrigerators and freezers remove heat to slow spoilage.

Concepts used:

• Conduction: heat moves through cookware and food.
• Convection: moving air or water transfers heat.
• Phase changes: water may freeze, melt, or vaporize.

15. Future Experiment Demonstrations

This section can later include your own demonstrations, such as comparing how quickly different materials heat up, observing conduction through metal, measuring cooling over time, or testing insulation materials.

Concepts used:

• Specific heat capacity: different materials heat at different rates.
• Heat transfer: energy moves through conduction, convection, and radiation.
• Thermal equilibrium: objects gradually approach the same temperature.

Thermodynamics Experiment Video

This video demonstrates thermodynamics by showing how heat naturally transfers from a hotter object to a colder object until both reach the same temperature. Using only two cups of water and a thermometer, this simple experiment illustrates heat transfer and thermal equilibrium.

Experiment Idea

Fill one cup with warm water at approximately 65°C and another cup with cold water at approximately 15°C. Measure and show both temperatures on camera. Pour the two cups together, stir the mixture, and measure the final temperature. The water should settle at approximately 40°C, demonstrating that heat naturally transfers from the warmer water to the cooler water until thermal equilibrium is reached.

Concepts Demonstrated

• Heat naturally flows from a higher-temperature object to a lower-temperature object.
• Temperature changes as energy is transferred between the two cups of water.
• The final temperature settles between the two starting temperatures.
• Thermal equilibrium occurs when both portions of water reach the same temperature.
• Thermodynamics explains how energy is transferred without being created or destroyed.