Phys 160 Thermodynamics and Statistical

Physics

Lecture 3

Heat, Work and the First Law

• Simple! Temperature is what you measure with a thermometer!

• Temperature is the thing that is the

same for two objects after they have been in contact long enough.

• Temperature is a measure of the ten- dency of an object to absorb or lose energy spontaneously- energy going from a body at a higher temperature to a body at a lower temperature.

• The condition for thermal equilibrium between several assemblies is the equality of a certain single-valued function of the thermodynamic states of the assemblies, which may be called the temperature t

• Zeroeth law of thermodynamics

• We have a wonderful result from this model: The average translati-onal KE of the molecules in a gas is given by a constant time the temperature.

• The temperature of a gas is a direct measure of the average KE of gas molecules. .

Boltzmann's kinetic theory of gases seemed to presuppose the reality of atoms and molecules, but almost all German philosophers and many scientists like Ernst Mach and the physical chemist Wilhelm Ostwald opposed their existence.

Much of the physics establishment did not share his belief in the reality of atoms and molecules — a belief shared, however, by Maxwell in Scotland and Gibbs in the United States; and by most chemists since the discoveries of John Dalton in 1808. He had a long-running dispute with the editor of the preeminent German physics journal of his day, who refused to let Boltzmann refer to atoms and molecules as anything other than convenient

theoretical constructs. Only a couple of years after Boltzmann's death, Perrin's studies of colloidal suspensions (1908–1909), based on Einstein's theoretical studies of 1905, confirmed the values of Avogadro's number and Boltzmann's constant, and convinced the world that the tiny particles really exist.

To quote Planck, "The logarithmic connection between entropy and probability was first stated by L. Boltzmann in his kinetic theory of gases". This famous formula for entropy S is where k = 1.3806505(24) × 10−23 J K−1 is Boltzmann's constant, and the logarithm is taken to the natural base e.

On September 5, 1906, while on a summer vacation in Duino, near Trieste, Boltzmann hanged himself during an attack of depression.

Boltzmann was also one of the founders of quantum mechanics due to his suggestion in 1877 that the energy levels of a physical system could be discrete. The equation for S is engraved on Boltzmann's tombstone at the Vienna Zentralfriedhof — his second grave.

Uranium hexafluoride is a chemical compound consisting of one atom of uranium combined with six atoms of fluorine. It is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas.

Heat and Work Much of thermodynamics deals with three closely -related concepts; temperature, energy, and heat.

• Temperature, fundamentally, is a measure of an object's tendency to spontaneously give up energy. • Energy is the most fundamental

dynamical concept in all of physics

• For this reason, how to express energy in terms of anything more fundamental?

• We can only state what we know definitely. The law of conservation of energy.

• There are many ways to put energy into a system or taken out.

• In Thermodynamics, we classify these mechanisms as Heat and Work

• Heat is defined as spontaneous flow of energy from one object to another due to difference in temperature. • Work is defined as any other

transfer of energy into or out of a system.

• Usually with work an agent is involved in putting in energy; it is not spontaneous

• Both heat and work refers to energy in transit; there is no heat or work in a system. Heat enters a system; work is done on a system. .

• Define symbols: • U for the total energy inside a

system.

• Q and W represent the energy that enters and leave the system as heat and work. Q + W is the total energy that enters the system

• By the conservation law, • U = Q + W . U is the change in energy.

• Can we say similarly change in Q and W? Q and W are infinitesimal quantities.

• This statement U = Q + W is what is know as the first law of thermodynamics

• The SI unit for energy is the Joule kg.m2/s2. Traditional unit is calorie. 1 cal = 4.186 J.

• Three modes of heat transfer. • Conduction, convection and

radiation

• Conduction by molecular contact, convection by bulk motion of gas or liquid, radiation by the emission of EM waves, mostly IR at room temperature, and visible light at highr temperature.

• Estimate how long it would take to bring a cup of water (200g) in a micro-wave oven of 600W capacity to boil. Make any reasonable assumptions necessary. How much heat is involved in this process?

• Let us say the initial temperature is 200C. The final temperature is

• 1000C

• The total energy needed is • m.c. T =200g.80deg. 1cal/g. • = 16000cal =16000cal*4.186J/cal • = 66976J = 67000J

• 600W = 600J/s; • hence the time needed to raise the

temperature is 67000J/(600J/s) • = 110s.

• No heat but electromagnetic work!

• Compression Work –important type of work done on a system (gas)

• W = F. dr = F x =P.A. x (volume change should be very slow-quasistatic)

• What happens if the pressure changes significantly?

.

• Simple! Temperature is what you measure with a thermometer!

• Temperature is the thing that is the

same for two objects after they have been in contact long enough.

• Temperature is a measure of the ten- dency of an object to absorb or lose energy spontaneously- energy going from a body at a higher temperature to a body at a lower temperature.