Chapter 11Laws of Thermodynamics

Chapter 11 Objectives• Internal energy vs heat• Work done on or by a system• Adiabatic process• 1st Law of Thermodynamics• 2nd Law of Thermodynamics• Isobaric, Isovolumetric,

Isothermal• Heat engines• Efficiency of a heat engine• Carnot engine• Entropy

Internal Energy• Internal energy can be thought of as all the energy

in a system that is not being transferred as heat.• This could include nuclear energy, chemical energy,

elastic energy as well as heat that has not been transferred yet.

• Temperature can often be thought of as a measure of internal energy.

• This is any amount of energy that cannot be included as mechanical energy.• Potential• Kinetic

Work• Internal energy can be transferred between systems without

transferring heat.• That would mean that the temperature would not change.

• So the internal energy could be transferred as mechanical energy in the form of work.

• Recall that work required some displacement to exist, we also need that fluid to create a displacement.

• So work can only be done when there is a change in volume.• The pressure should remain constant.• If not, then the equation above should be broken down parts of

constant pressure.

W = PV

Work On or By the System• Work can be positive or negative, depending “who”

is doing the work.• The gas does work on the system when the volume

is expanding.• That means that V is positive, so work is positive.

• When work is being done by the system, the volume is decreasing.• So V should be negative, so work will be negative.

Isobaric,Isovolumetric,

Isothermal• A system can be isobaric when the pressure

is held constant in that system.• So cross out P in the equation

• A system can be isovolumetric when the volume is held constant in that system.• So cross out V in the equation

• A system can be isothermal when the temperature is held constant in that system.• So cross out T in the equation

Adiabatic• An adiabatic process is one in which no heat is

transferred between the system and the environment while work is being done.

• Which means the gas has the ability to freely expand in the container while the container is completely insulated from its environment.

• Usually involves filling a container with more gas molecules.• Such as filling a balloon with air.

1st Law of Thermodynamics• This is generally known as the Law of

Conservation of Energy.• So the internal energy of the system cannot be

created or destroyed.• So the change in internal energy needs to

account for the heat in the gas and whatever work is done by the gas.

U = Q - W

Change in Internal Energy

Work done by gas

Heat released or absorbed by gas

Isolated System• An isolated system is one in which the system does

not interact with its surroundings.• No interaction means

• No pressure to change the gas pressure• No volume change of the container• No temperature due to no transfer of energy.

• No pressure = no work!• No temperature difference = no heat!

Cyclic Process• A cyclic process is a process that starts and

finishes at the same state.• Heat engines are a good example of a cyclic process.

• Air conditioner• Since the initial and final state of the system is

constant, the internal energy remains the same.• So Q = W

U = Q - W

Isobaric Process• An isobaric process remember maintains

constant pressure.• Since pressure is constant, that allows work to

be done whenever there is a volume change.• Temperature can also change, so that means

heat can be transferred.

U = Q - WW = PV

Isovolumetric Process• An isovolumetric system is one in which the

volume does not change.• No change in volume means that there is no

work being done.• So any change in the internal energy is directly

due to the heat being released or absorbed by the gas.

U = Q - W W = PV

Isothermal Process• An isothermal process is one in which the

temperature is kept constant.• This would mean that the internal energy of the gas

must be kept constant.• So Uf = Ui

U = 0• So any heat released or absorbed by the gas is

a result of work being done.

U = Q - WQ = W

Adiabatic Process• Recall that an adiabatic process is one in which there is

no heat transfer and yet there is work being done.• This process is one in which the number of particles are

being increased.• Like blowing up a balloon.

• An increase of particles would require the system to do work to bring those particles in.• That would use up internal energy to do that work.

• The opposite would be true also when the particles are released.• Here the gas would do work on the system by adding gas

molecules to it.

U = Q - W

U = - W

Heat Engine• A heat engine is any device that converts heat energy

into useful forms of energy.• Mechanical energy• Electrical energy

• A heat engine carries some working material (fluid) that transfers energy from a cold to hot reservoir.• Steam engine• Internal combustion engine• Refrigerator• Air Conditioner/Furnace

• The net work done by a heat engine is equal to the difference of the hot and cold reservoirs.• Hot reservoir can also be thought of as input energy.• Cold reservoir can also be thought of as wasted energy.

W = Qh - Qc

2nd Law of Thermodynamics

• It is impossible to construct a heat engine that is 100% efficient.

• Efficiency is found by the ratio of net work done to the heat absorbed by the hot reservoir (input energy).

We =Qh

Qh - Qc

Qh

= = 1 -Qc

Qh

Entropy and Disorder

• Entropy is a measure of the disorder found in a thermodynamic system.• Larger the entropy, the more disorder of the

molecules and their behavior.• Based on probability, systems with high

disorder are much more likely to happen in nature.• With that said, the entropy of the Universe is always

increasing.