Thermodynamics

Thermodynamics

Chapter 11

Heat, work and internal energy

• Heat can be used to do work– Work can transfer energy to a substance,

which increases the internal energy of a substance.

Fig 11-1

• Work increases the nail’s internal energy at the nail’s surface. This energy is transferred away from the nail’s surface as heat.

Internal Energy• There are two ways to change the internal

energy: with work, and everything else.

Everything else is defined as heat. Heat is the defined as the transfer of energy to a body that does not involve work or those transfers of energy that occur only because of a difference in temperature

Recall…• Balloon over heated flask…

– Energy transferred as heat turns water into steam.

– Energy from the steam does work against the force exerted by air outside the balloon.

Heat and Work Energy

• Both transferred to or from a system

• SYSTEM – a collection of matter within a SYSTEM – a collection of matter within a clearly defined boundary across which no clearly defined boundary across which no matter passesmatter passes

• All parts of a system are in thermal equilibrium with each other before and after a process adds or removes more energy.

SYSTEMS

• Example: Flask, water, balloon, and steam.

• As the hot plate transferred energy as heat to the system, that system’s internal energy increased

• When the expanding steam (a part of the system) did work on the balloon, the system’s internal energy decreased

• WHY??

Heat Work

• The decrease occurs because some of the energy transferred to the system as heat was transferred out of the system as work done on the balloon!

ENVIRONMENT

• Systems are often treated as if they are isolated, but in most cases it will interact with its surrounds

• The surroundings with which the system interacts are referred to as its environment.

WORK in terms of changing volume

• Remember that… W=F x d

and P = F/A

Work = pressure (volume change)

• W = F x d A = F (Ad) = P(V2 – V1)

A A

• Therefore, W = P(V2 – V1)

work = Pressure x Volume

WORK = P(V)

• If the gas volume remains constant, there is no displacement and NO WORK is done on or by the system.

• Work is done ONLY If the

volume changes.

If pressure increases and Volumeremains constant – this is comparable to a force that does not displace mass even if the force is increased. Thus work is not done in either situation.

Thermodynamic ProcessesThermodynamic ProcessesSuppose the car’s windows are closed and

parked inside a hot garage.

Internal energy of system (inside the car) increases as energy is transferred as heat into the car from the hot air in the garage.

Car’s heave steel and sealed windows keep the system’s volume constant

Thus, no work is done by the system.

All changes in the system’s internal energy are due to the transfer of energy as heat.

Isovolumetric Processes

• Last example, car system, was an illustration of an isovolumetric (or constant volume) process.

• Isovolumetric Process = a thermodynamic process that takes place at constant volume so that no work is done on or by the system.

• Another example takes place inside a bomb calorimeter.

• A small container in which a small quantity of a substance A small container in which a small quantity of a substance undergoes a combustion reactionundergoes a combustion reaction

• Energy released by the reaction increases the pressure and temperature of the gaseous reaction products.

• Walls are thick, thus NO CHANGE in volume of the gas; energy transferred only as HEAT

Internal EnergyInternal Energy

• Internal energy remains constant in a Internal energy remains constant in a constant-temperature process.constant-temperature process.

• When you are indoors in a controlled temperature—any temperature change outside the building, will not take place indoors.

• However, buildings are not perfectly However, buildings are not perfectly sealed so changes in the pressure outside sealed so changes in the pressure outside will also take place inside the buildingwill also take place inside the building

• Think about a balloon that has been inflated and sealed off.

• As the atmospheric pressure inside the building slowly decreases, the balloon expands and slowly does work on the air outside the balloon.

• At the same time, energy is slowly transferred into the balloon as heat.

• Net result of these two processes is that the air inside the balloon is at the same temperature as the air outside the balloon

• Thus, internal energy of the balloon’s air does not change

• The energy transferred out of the balloon as work is matched by the energy transferred into the balloon as heat.

ISOTHERMAL PROCESS• This process isothermal process

• ISOTHERMAL PROCESS- a thermodynamic process that takes place at constant temperature and in which the internal energy of a system remains unchanged.

• Transfer of energy as heat can occur in an isothermal process if it is assumed that the process takes place as a large number of very gradual, very small changes as shown above.

• When air inside balloon expandsair inside balloon expands, its internal energy and temperature slightly decrease

• As soon as they decrease, the energyenergy is transferredtransferred as heat from higher temp outside air to the air inside the balloon

FIG 11-6 in textbook

• The temperature and internal energy of the air inside the balloon end up rising to their original values

• Thus, the internal energy of the balloon’s air effectively remains CONSTANTCONSTANT!

FIG 11-6 in textbook

Adiabatic Process

•A thermodynamic process A thermodynamic process during which work is done during which work is done on or by the system but on or by the system but NO energy is transferred NO energy is transferred to or from the system as to or from the system as heat.heat.

Adiabatic Process• In an adiabatic process, the decrease

in internal energy must be equal to the energy transferred from the gas as work.

• Ex: filling up a balloon with air from a compressed air tank

• This work is done by the gas pushing against the inner wall

of the balloon and overcoming pressure exerted by the air outside the balloon.

* As a result the balloon inflates

• Adiabatic expansion and compression of gases is found in many applications…

• Both refrigerators and internal combustion engines require that gases be compressed or expanded rapidly.

HOMEWORK

• Page 405 #1

• Page 408 # 1 and 2

Thermodynamics

Chapter 11

Section 2: Thermodynamic Processes

1st law of thermodynamics

• Considers both a system’s internal energy as well as work and heat.

Change in a system’s internal energy = Change in a system’s internal energy = energy transferred to/from – energy transferred system as heat to/from system as

work

OR

U = Q-WU = Q-W