Laws of Thermodynamics

• The first law states that the change in the energy of a system is the amount of energy added to the system minus the energy spent doing work.

• ∆U = q – w

Laws of Thermodynamics

• ∆U = q – w• ∆U is the change in internal energy of

the system.• q is the energy transferred into the

system. If heat flows out of the system, then q is negative.

• w is the work done by the system. If the surroundings do work on the system, w is negative.

Laws of Thermodynamics

• Copy the illustration on white board and use to explain the behavior of the piston, the mass, and the internal energy of the piston if we change the following:

• Increase gas temperature?• Increase size of mass?• Decrease size of mass?• Decrease gas temperature?• Put more gas in the piston?

Laws of Thermodynamics

• Work is often calculated as pressure times volume and is called PV work.

• Pressure is N/m2 or the pascal Pa• Prove that pressure multiplied by volume

is equal to work using their appropriate units.

Thermodynamic processes

• Isochoric processes• Suppose we have a sealed can on a

stove. • Can any work be done by it? • Then w is 0 and the equation becomes

∆U = q.

Thermodynamic processes

• Adiabatic processes• Suppose we keep the system well

insulated or the process occurs very quickly so that no heat can enter or leave the system. Then q is 0 and the equation becomes ∆U = -w.

Thermodynamic processes

Adiabatic processes occur in nature and result in events known as rain shadows.

Thermodynamic processes

• Isobaric processes occur with no pressure change and apply to the piston diagram looked at earlier. You get ∆U = q – w and the work is calculated as P ∆V ∙at constant pressure.

• ∆U = q – P ∆V ∙

Thermodynamic processes

• Isothermal processes occur at very slow speeds and have constant temperature. An example is melting ice or boiling water.

• The equation remains ∆U = q – w

Laws of Thermodynamics• The second law can be stated in over 100

different ways • 3 most common are:• Heat flows from a hot body to a cold one.• Heat cannot be completely converted into

work; there are always losses.• Every system becomes disordered over

time.

Laws of Thermodynamics• The third law of thermodynamics generally

means that you will never be able to reach absolute zero and the entropy will never reach zero.

• This is referred to as heat death of the universe.

Laws of Thermodynamics• The zeroth law of thermodynamics states

that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

• Says if A = B and B = C, then A = C.

Law of entropy• Entropy is a measure of the degree of

disorder of a system. • The law of entropy states that everything

in the universe tends to become more disordered over time unless energy is used to prevent this.

Examples of entropy• A deck of playing cards thrown up in the

air come down disordered. It doesn’t work in the opposite way.

• Ice melts.• Desks become messy.• Weather reduces rocks to sand and gravel.• Tearing paper increases entropy.• Can you reverse these? Does it take

energy?

Entropy in nature• Water naturally evaporates due to heat

from Sun.• Disordered water molecules in clouds lose

heat, form droplets and fall as rain.• Photosynthesis combines CO2 and H2O to

form sugars and allows plants to grow.• Dead plants naturally decay back to CO2

and H2O.• Which steps require energy?

Entropy in nature• Water naturally evaporates due to heat

from Sun.• Disordered water molecules in clouds lose

heat, form droplets and fall as rain.• Photosynthesis combines CO2 and H2O to

form sugars and allows plants to grow.• Dead plants naturally decay back to CO2

and H2O.• Which steps require energy?

Heat engines• Heat engines are devices that turn internal

energy into mechanical work.• These include steam, internal combustion,

and jet engines.• All of these have a hot reservoir and a cold

reservoir. Heat flowing between them creates work.

Heat engines• The maximum efficiency of a heat engine is

calculated as • Thot – Tcold/ Thot

• Be sure temperature is in Kelvin.• Calculate the efficiency of a heat engine

with a hot reservoir temperature of 1500 0C and a cold reservoir temperature of 350 0C

Heat engines• Automobiles use 4 stroke or 4 cycle

engines:• Intake, Compression, Power, Exhaust.• Refrigeration uses a compressor and two

different tubing sizes to achieve cooling.