The Zeroth and First Laws

Mechanical energy includes both kinetic and potential energy. Kinetic energy can be changed to potential energy and vice versa. Introduction

Total mechanical energy (E) is the sum of kinetic and potential energies. Changes in a system’s total mechanical energy (ΔE) are important. Introduction

due to the rapid, random motion of the molecular, atomic, and subatomic particles of matter can be subdivided into kinetic energy and potential energy Thermal Energy

average kinetic energy is proportional to the temperature of a substance Internal energy (U): sum of the particle kinetic and potential energies Thermal Energy

adiabatic boundary: no thermal energy can pass through diathermic: ideal conductor of thermal energy Zeroth Law

thermal equilibrium: objects have reached the same temperature Zeroth Law

Two systems that are in thermal equilibrium with a third must be in thermal equilibrium with each other Zeroth Law

If no net energy exchange occurs in (a), then none will occur in (b). Zeroth Law

The General Law of Conservation of Energy in general: First Law Q + W ncf = ΔU + ΔE

mathematical statement of the first law of thermodynamics: First Law Q = ΔU + W

The heat transferred to or from a system is equal to the sum of the change of the system’s internal energy and the work the system does on its surroundings. First Law

Heat Engines can do mechanical work by absorbing and discharging heat the simplest example is an expanding gas cylinder with piston

Heat Engines quasi-static process: gas expands without ever being far from thermal equilibrium gas pressure inside cylinder is in equilibrium with external pressure

Heat Engines work is done on the gas when it is compressed from V 1 to V 2 gas warms when it is compressed work done by gas on surroundings is negative

Heat Engines work done by gas when expanding or contracting against a constant pressure: W = P(V 2 – V 1 )

pressure against a gas is not always constant graphing pressure versus volume (P-V diagram) makes some equations easier to solve P-V Diagrams

Notice that the area under the curve representing the process on a P-V diagram is equal to the absolute value of the work done by the gas during the process!

P-V Diagrams The sign of the work depends on whether the gas gains or loses energy. Gas expands → does work on surroundings → sign is positive Gas contracts → surroundings do work on it → sign is negative

If a gas is to be useful as a machine, it must be able to expand repeatedly, following a cycle. Expansion Cycles

For a cycle, the absolute value of the work done is equal to the area enclosed by the path of the cycle on a P-V diagram. Clockwise path: + work CCW path: – work Expansion Cycles

The work done by a gas depends on the path of the process in a P-V diagram. Heat engines: positive Refrigerators: negative Expansion Cycles

Internal energy is path- independent: its change does not depend on the way the energy is added. Path-independent quantities are called state variables. State Variables

a piece of the universe isolated for study if it is not part of the system, it is part of the surroundings Thermodynamic Systems

can exchange both matter and energy with its surroundings Ex.: ice cube resting on a kitchen counter Open System

can exchange energy but not matter with its surroundings Ex.: expanding gas in a thermally conducting cylinder with a gas-tight piston Closed System

cannot exchange energy or matter with its surroundings Ex.: liquid in a perfectly insulated vacuum flask Isolated System

energy is conserved energy may be converted but none leaves or enters universe is the only true isolated system no practical system is isolated Isolated System

The First Law of Thermodynamics is a conservation law It can be stated as... Isolated System

In an isolated system, the total quantity of energy is constant, neither being created nor destroyed.

a change in the thermodynamic state of a system often categorized by which variables are held constant Thermodynamic Processes

Adiabatic process: exchanges no thermal energy between system and its surroundings Q = 0 Thermodynamic Processes ΔU = -W

Isothermal process: temperature of the system is constant no phase changes ΔU = 0 J Thermodynamic Processes Q = W

Isochoric process: volume of the system is constant W = 0 J Thermodynamic Processes Q = ΔU

Isobaric process: pressure of the system is constant W = PΔV Thermodynamic Processes Q = ΔU + PΔV

A process that allows the use of ideal gas relationships is known as an ideal gas process. Thermodynamic Processes

The Second and Third Laws

Heat Engines The surroundings must contain either a source for thermal energy, a sink (receiver) for thermal energy, or both. Heat reservoir— temperature cannot be changed significantly

Heat Engines Hot reservoir higher temperature than the system source of thermal energy for the system

Heat Engines Cold reservoir lower temperature than the system thermal energy sink for the system Both types are used to operate a heat engine.

Second Law of Thermodynamics Energy flows from an area of higher concentration to an area of lower concentration.

Heat Engines Requirements: hot reservoir cold reservoir working fluid (liquid or gas)

Heat Engines Overview: thermal energy absorbed from hot reservoir causes fluid to expand expansion causes mechanical work

Heat Engines Overview: fluid gives up thermal energy to cold reservoir and contracts fluid is heated to expand again

Early Steam Engines aeolipile Hero of Alexandria not cyclic Thomas Savery first practical steam engine—water pump

Early Steam Engines Thomas Newcomen James Watt used separate chambers to heat and cool steam helped begin the Industrial Revolution

Early Steam Engines James Watt double-acting piston additional mechanical improvements

The Carnot Cycle Reversible process: quasi- static process that leaves the system in exactly the same state after occurring twice, once normally and once in reverse

The Carnot Cycle Reversible cycle: leaves the system in the same state as it was before the entire process occurred most efficient means of converting thermal energy to mechanical work

The Carnot Cycle Carnot cycle is the most efficient cycle that can operate between two temperatures four-step, reversible cycle

The Carnot Cycle Step 1: isothermal expansion from V 1 to V 2 at temperature T H Step 2: adiabatic expansion from V 2 to V 3 ; temperature changes from T H to T C

The Carnot Cycle Step 3: isothermal compression from V 3 to V 4 at temperature T C Step 4: adiabatic compression from V 4 to V 1 ; temperature returns to T H

The Carnot Cycle P-V diagram:

Thermal Efficiency For Carnot engine, thermal efficiency (ε) is defined as: ε = × 100% T H – T C THTH

Thermal Efficiency To increase thermal efficiency (ε): raise temperature of hot reservoir lower temperature of cold reservoir

Thermal Efficiency To increase thermal efficiency (ε): both efficiency can never reach 100%

Heat Pumps can be used to move thermal energy from a cold reservoir to a hot reservoir air conditioning refrigeration

Second Law of Thermodynamics Energy flows from an area of higher concentration to an area of lower concentration. Thermal energy naturally flows from hot bodies to cold bodies. Energy cannot be completed converted to work in a cyclic process. You cannot get as much work out of a machine as you put into it. Perpetual motion machines are an impossibility.

Third Law of Thermodynamics Absolute zero is unattainable.

Entropy and its Consequences

Entropy (S) is a measurement of the randomness, or disorder, of the particles in a specific part of the universe. What is Entropy?

Second Law of Thermodynamics You cannot get as much work out of a machine as you put into it. Perpetual motion machines are an impossibility. Entropy increases in all natural processes.

All natural processes make the universe more disorderly. Disorder implies unusable energy; the energy still exists but can no longer do useful work. What is Entropy?

For a reversible process: What is Entropy? ΔS ≡ T ΔQΔQ Units: J/K

In reversible processes, the entropy of the universe remains constant. The change in a system’s entropy is balanced by the change in the entropy of the surroundings. What is Entropy?

In natural (irreversible) processes, the entropy of the universe increases. ΔS is positive. What is Entropy?

The most likely state of a system is one of disorder. Entropy is also related to the encoding of information. Entropy

God created the universe with an immense supply of usable energy. Only God can create or destroy. Conservation and Degeneration in Nature

The first law does not support nor refute the theory of evolution. Naturalistic evolutionary cosmology is thermo- dynamically impossible. Conservation and Degeneration in Nature

The second law would need to be almost constantly violated in order for evolution to occur. This has never been observed. Conservation and Degeneration in Nature

The heat death of the universe is hypothetical. The universe will not be left to itself by God. Conservation and Degeneration in Nature