First Law of Thermodynamics, Conservation of Energy, Work and Heat Transfer(10a)

What is thermodynamics?

the study of energy, how it changes from one form to another, how it relates to the properties of matter, how it is transferred by heat and work, and how efficiently it can be used to perform work.

Which forms of energy are most important in applied thermodynamics, and what do they include?

• Mechanical energy → includes kinetic, potential, and strain energy (e.g., a weight oscillating on a spring).

• Thermal energy → found on the microscopic level in the motion and interactions of atoms and molecules; together, these make up the internal energy of matter.

What is a closed system in thermodynamics?

• Example: A piston–cylinder with all openings and valves shut, trapping its contents.

• The same atoms of matter remain inside, and the boundaries are impermeable.

What is an open system in thermodynamics?

An open system allows matter to flow in or out across permeable or imaginary boundaries.

• Atoms of matter can be exchanged, so the system’s mass may change.

• Example: A piston–cylinder with its valves open, allowing matter to enter or leave.

Which thermodynamic properties can be measured directly?

The measurable thermodynamic properties are:

• Pressure (p)

• Temperature (T)

• Volume (V)

• Mass (m)

Which thermodynamic properties can be derived from mass and volume?

• Density (ρ): mass per unit volume

• Specific volume (v): the inverse of density (volume per unit mass)

Which three thermodynamic properties can be determined from other property values?

The following properties are derived from other thermodynamic variables:

• Internal energy (U)

• Enthalpy (H)

• Entropy (S)

What is pressure in thermodynamics?

Pressure is the force exerted per unit area at a boundary.

On what types of boundaries does a gas exert pressure?

• Real boundaries (e.g., the walls of a container)

• Imaginary boundaries within the enclosed volume

What are the common units of pressure?

• The SI unit of pressure is the pascal (Pa), equal to N/m².

• Pressure is also commonly expressed in bar.

How can the gauge pressure of a gas be determined using a water column?

Gas pressure acts at the boundary between the gas and the water.
A higher gas pressure supports a taller water column.
By knowing:

• the weight of the water in the column (w)

• the cross-sectional area of the column (A)

you can calculate the gauge pressure of the gas.

Why is it called gauge pressure?

It is called gauge pressure because it is measured relative to the surrounding (ambient) air pressure, which acts on the opposite end of the water column.

What does “pressure” usually refer to in thermodynamic calculations?

In calculations, absolute pressure is usually required, and it is referred to simply as pressure (without stating “absolute”).

What is temperature and what units are used to measure it?

Temperature measures the energy possessed by particles at the molecular level.
It can be expressed in:

• degrees Celsius (°C)

• kelvin (K)

What does the Kelvin temperature scale represent?

The Kelvin scale is an absolute temperature scale that reflects the link between a gas’s temperature and the energy in the motion and vibration of its molecules.

• High temperature: energetic molecular movement

• Low temperature: slower movement

• Absolute zero: molecular motion is stilled

What are key characteristics of temperatures measured in kelvin (K)?

• Kelvin temperatures are always positive.

• 0 K represents absolute zero.

• The freezing point of water at 1.013 bar is 273.15 K.

What is the difference between extensive and intensive properties?

• Extensive properties describe the total amount of something in a system.

  • Examples: mass, volume

• Intensive properties do not depend on system size and give no indication of total quantity.

What does it mean for a system to be in a defined state?

A system is in a defined state if all property values can be specified at all locations, though this does not necessarily mean it is in thermodynamic equilibrium.

When is a closed system in thermodynamic equilibrium?

A closed system is in thermodynamic equilibrium when it is isolated from its surroundings and its properties remain constant and unchanging.

What conditions are required for thermodynamic equilibrium?

• Thermal equilibrium: uniform temperature

• Mechanical equilibrium: uniform pressure

• Chemical equilibrium: uniform chemical composition

In an open system, properties may vary with location, but each location can be treated as having its own local thermodynamic equilibrium.

What is a process in thermodynamics?

A process occurs when the state of a system or its working fluid changes.
For a closed system, the initial and final states of a process are typically states of thermodynamic equilibrium.

What makes a process reversible or irreversible in thermodynamics?

A process is reversible if all intermediate states between the initial and final conditions are states of thermodynamic equilibrium.

• This allows the change to be defined exactly and reversed, restoring both the system and surroundings to their initial conditions.

Any process that does not meet this condition is irreversible.

What is a common cause of irreversibility in thermodynamic processes?

A common cause is the generation of kinetic energy in fluids or gases (e.g., rapid piston movement), which is later dissipated by friction, making the process irreversible.

Why are process or state diagrams useful in thermodynamics?

Process/state diagrams help summarize complex process descriptions in a clear and concise way.
They show how the state of a system or working matter changes during a process, making the information easier to understand.

How many properties define the state of a closed system, and how can reversible processes be plotted?

• For a closed system in thermodynamic equilibrium, only two properties are needed to define the state.

• Therefore, a reversible process can be plotted on a two-dimensional diagram using any pair of independent properties as the axes.

What happens during a reversible expansion where pv = constant?

• An expansion means the volume (v) increases → so v₁ < v₂.

• Since p · v = constant, pressure must decrease as specific volume increases.

• Therefore, p₂ < p₁ during the process.

How is this process described?
“A closed system undergoes a reversible constant-volume process where pressure rises from 1 bar to 3 bar, followed by a reversible constant-pressure compression where specific volume decreases from 5 m³/kg to 2 m³/kg.”

• Constant-volume (isochoric) process:

  • Volume stays constant

  • Pressure increases from 1 bar → 3 bar

• Constant-pressure (isobaric) compression:

  • Pressure stays constant

  • Specific volume decreases from 5 m³/kg → 2 m³/kg

Can multiple processes be plotted on the same thermodynamic diagram?

Yes. Both processes—and any sequence of reversible processes—can be plotted on the same process diagram, showing how the system’s state changes step by step.

How do constant-volume and constant-pressure processes appear on a pv diagram?

• A constant-volume process (from state 1 → 2) is shown as a vertical line.

• A constant-pressure process appears as a horizontal line.

• If the specific volume decreases along the horizontal line, the process is a compression.

Why is the power law relationship important in describing gas processes?

The power law defines how pressure and temperature vary during many gas processes.
Being able to manipulate power-law relationships is important because they are commonly used to describe and analyse the behaviour of gases.

How is this sequence of processes described?
“A reversible expansion during which p·vⁿ = constant and pressure decreases from 5 bar to 3 bar, followed by an irreversible compression back to the original specific volume, ending at 4 bar.”

How is this sequence of processes described?
“A reversible expansion during which p·vⁿ = constant and pressure decreases from 5 bar to 3 bar, followed by an irreversible compression back to the original specific volume, ending at 4 bar.”

How do we represent an irreversible process on a process diagram?

• Previous examples were reversible, so all intermediate states were known.

• For an irreversible process, only the initial state (2) and final state (3) can be plotted.

• The path between them is drawn as a dashed line because the intermediate states cannot be defined.

What is a thermodynamic cycle?

A system undergoes a cycle if it experiences a series of processes that return it to its original state—the initial and final states are the same.
On a state diagram, a cycle appears as a closed contour, with points marking the start and end of each process.

What makes a thermodynamic cycle reversible or irreversible?

• A cycle is reversible only if every process within it is reversible.

• If any process is irreversible, then the entire cycle is irreversible, even if the initial and final states match.

How do changes in the state of a closed system occur, and what are work and heat?

• Changes in a closed system occur through work transfer and heat transfer—the two ways energy crosses system boundaries.

• Work and heat are not forms of energy and not properties of a system.

• In mechanics, work is done when a force moves through a distance

How are work and heat defined in thermodynamics?

• Work: energy transferred across a system boundary when the system’s state changes because a boundary moves under a force.

• Heat: energy transferred across a boundary when the system’s state changes due to a temperature difference between the system and its surroundings.

What is heat transfer, and what is special about the first law of thermodynamics?

What does the First Law of Thermodynamics state for a closed system undergoing a cycle?

For a closed system taken through a cycle, the sum of the net work transfer and the net heat transfer is zero.

What does the First Law of Thermodynamics state for a closed system undergoing a cycle?

For a closed system taken through a cycle, the sum of the net work transfer and the net heat transfer is zero.

What is the mechanical expression for work, and what does it imply?

Work is expressed as F · dx, where F is the force and dx is the incremental distance moved in the direction of that force.
This expression does not specify how the force is applied or how it moves—only that movement under a force results in work done.

How does a paddle wheel do work on a system?

As the paddle wheel turns, the system boundary moves with its surface.
The rotating wheel exerts a force F on the surrounding matter, equal and opposite to the resisting force, causing work transfer into the system.

Why is piston movement to the left taken as positive in thermodynamic diagrams?

Piston movement to the left is considered positive because it represents a compression process, which corresponds to work done on the system.

What force balance exists on a mass less, freely moving piston?

If a piston has no mass and is free to move, the external force on its outer surface is always exactly balanced by an equal force exerted by the system on its inner surface.

How does a system’s energy change during the processes of a cycle?

Even though a full cycle results in no net energy change, the system’s energy can increase or decrease during individual processes as work and heat transfers occur.
In a thermodynamic system, these changes appear as changes in the internal energy of the system’s matter.

What are internal energy and specific internal energy, and what does the First Law say about their change?

• Internal energy (U): measured in joules (J)

• Specific internal energy (u): measured in J/kg
  Both are properties of a system.

First Law — Corollary 1:
For a closed system, the change in internal energy equals the sum of the heat transferred and the work done during any change of state.