ThermodynamicsThermodynamics

Thermodynamic systems and Thermodynamic systems and concepts—topic 10.1concepts—topic 10.1

What IS What IS thermodynamics?thermodynamics?

Thermodynamics is the study of processes in Thermodynamics is the study of processes in which thermal energy is transferred as heat which thermal energy is transferred as heat and as mechanical work.and as mechanical work.

Developed around the time that the first steam Developed around the time that the first steam engines were being produced to do work (19engines were being produced to do work (19 thth century)century)

Deals with the macroscopic properties of Deals with the macroscopic properties of variables such as pressure, volume, variables such as pressure, volume, temperature and change in internal energytemperature and change in internal energy

There are 4 laws of thermodynamics—we’re There are 4 laws of thermodynamics—we’re going to focus primarily on the first two…going to focus primarily on the first two…

Thermodynamic SystemsThermodynamic Systems Open systemOpen system: mass can enter and can : mass can enter and can

exit freelyexit freely Closed systemClosed system: mass is not free to be : mass is not free to be

added or removed from the systemadded or removed from the system Isolated systemIsolated system: no energy (of any : no energy (of any

kind) can enter or leave the systemkind) can enter or leave the system

Thermal energy Thermal energy transfer…transfer…

Thermal energy, in the form of heat, can be Thermal energy, in the form of heat, can be transferred between a system and its transferred between a system and its environment (the areas surrounding a system) environment (the areas surrounding a system) in three ways:in three ways: Through a temperature difference—thermal energy Through a temperature difference—thermal energy

is transferred from the higher temperature to the is transferred from the higher temperature to the lower temperature until equilibrium is reached.lower temperature until equilibrium is reached.

Doing work Doing work onon the system (i.e. compressing a the system (i.e. compressing a piston of gas)piston of gas)

Allowing work to be done on the surrounding Allowing work to be done on the surrounding environment environment byby the system (i.e. allowing a piston of the system (i.e. allowing a piston of gas to expand and push out)gas to expand and push out)

Heat vs. WorkHeat vs. Work

Heat:Heat: a process in which thermal energy a process in which thermal energy is transferred due to a temperature is transferred due to a temperature differencedifference

WorkWork: the process in which thermal : the process in which thermal energy is transferred by means that are energy is transferred by means that are not dependent on temperature not dependent on temperature differencesdifferences

Internal EnergyInternal Energy Review: internal energy is defined as…Review: internal energy is defined as…

The sum of the total potential energy and the The sum of the total potential energy and the kinetic energy of the particles in a systemkinetic energy of the particles in a system

Previously, we only looked at how the kinetic Previously, we only looked at how the kinetic energy would change the internal energy energy would change the internal energy (when temperature changes…)(when temperature changes…)

Changes to internal energy will occur if…Changes to internal energy will occur if… Work is done on the system Work is done on the system Work is done by the systemWork is done by the system Thermal energy is added to the systemThermal energy is added to the system Thermal energy is removed from the systemThermal energy is removed from the system

““State” of a SystemState” of a System

A thermodynamic “state” of a system is defined A thermodynamic “state” of a system is defined by looking at macroscopic properties of that by looking at macroscopic properties of that system (pressure, volume, temperature, mass, system (pressure, volume, temperature, mass, internal energy)internal energy)

Internal energy is dependent on the “state” of Internal energy is dependent on the “state” of the systemthe system

If a system has If a system has changed statechanged state, we are simply , we are simply referring to the fact that something has referring to the fact that something has changed one or more of the macroscopic changed one or more of the macroscopic propertiesproperties

Thermodynamic Processes:Thermodynamic Processes:

Anything that is done to a system that Anything that is done to a system that causes it to change state causes it to change state For example: For example:

Heat a gas—temperature, pressure, and/or Heat a gas—temperature, pressure, and/or volume will changevolume will change

Compressing a gas (doing work on it)Compressing a gas (doing work on it)

PressurePressure

Pressure is the amount of force applied to a Pressure is the amount of force applied to a substance or an object per unit area. substance or an object per unit area.

For example, the pressure that a gas exerts on For example, the pressure that a gas exerts on a piston is proportional to the force exerted by a piston is proportional to the force exerted by the gas and inversely proportional to the the gas and inversely proportional to the surface area of the piston:surface area of the piston:

Units = Pascal (= 1 N mUnits = Pascal (= 1 N m-2-2)) A

FP

Work done on a pistonWork done on a piston

When a gas expands and pushes against a When a gas expands and pushes against a piston, the force will be determined by looking piston, the force will be determined by looking at the pressure of the gas and the surface area at the pressure of the gas and the surface area of the piston:of the piston:

The force exerted on the piston will move it a The force exerted on the piston will move it a certain distance ( certain distance ( l l ))

Therefore, the force exerted by the gas has Therefore, the force exerted by the gas has done work on the piston…done work on the piston…

APF

Work done on a piston…part Work done on a piston…part 22

The work done on the piston by the gas is The work done on the piston by the gas is equal to the product of the applied force and equal to the product of the applied force and the distance the piston was moved:the distance the piston was moved:

Let’s make a few assumptions about this Let’s make a few assumptions about this situation:situation: The distance that the piston moved was very smallThe distance that the piston moved was very small Therefore, the pressure can be assumed to have Therefore, the pressure can be assumed to have

remained constantremained constant If pressure is constant, then the applied force is also If pressure is constant, then the applied force is also

constantconstant

lFdFW

Work done on a piston…part Work done on a piston…part 33

If we know the following:If we know the following:

And we know the following are true And we know the following are true based on definitions of pressure and based on definitions of pressure and volume:volume: , so, so

and:and:

A

FP

lFdFW

APF

lAV

Work done a piston…pt. 4Work done a piston…pt. 4Then we can deduce the following:Then we can deduce the following:

If the work done is positive, then the work If the work done is positive, then the work was done BY the piston (volume was was done BY the piston (volume was increased)increased)

If the work done is negative, then the work If the work done is negative, then the work was done ON the piston (volume was was done ON the piston (volume was decreased)decreased)

)( 12 VVPVPW

lAPlFW

First Law of First Law of ThermodynamicsThermodynamics

““The heat added to a closed system The heat added to a closed system equals the change in the internal equals the change in the internal energy of the system plus the work energy of the system plus the work done by the system”done by the system”

This law is essentially a statement of the This law is essentially a statement of the law of conservation of energy that takes law of conservation of energy that takes into consideration both work and flow of into consideration both work and flow of thermal energy in a systemthermal energy in a system

11stst law, mathematically… law, mathematically…

Quantitatively, we can write the first law of Quantitatively, we can write the first law of thermodynamics as follows:thermodynamics as follows:

Q = thermal energyQ = thermal energy

W = workW = work

U = change in internal energyU = change in internal energy

WQU

or

PVUWUQ

11stst law, continued… law, continued…Thermal energy:Thermal energy:a positive Q means energy is a positive Q means energy is addedadded toto

the systemthe systema negative Q means energy was a negative Q means energy was

removedremoved fromfrom the system the systemWork:Work:a positive W means that work was done a positive W means that work was done

BYBY the system the systema negative W means that work was done a negative W means that work was done

ONON the system the system

For an isolated system:For an isolated system:

When a system is isolated, then the work When a system is isolated, then the work done by or on a system is equal to the done by or on a system is equal to the thermal energy that has been added or thermal energy that has been added or removed from the systemremoved from the system

Therefore,Therefore,

W = Q = 0W = Q = 0 andand U = 0U = 0

Example problem:Example problem:

If 22 J of work is done on a system, and If 22 J of work is done on a system, and 340 J of heat is added to the system, 340 J of heat is added to the system, what is the change of internal energy of what is the change of internal energy of the system?the system?