Thermodynamics AP B

‘its hot enough to fry an egg on the sidewalk’

How can this be done? What is providing the energy to cook the egg? Is energy still being conserved?

Quick review 3 temp scales –Celsius –Fahrenheit –Kelvin Converting between C and K (add ) When doing thermo calculations convert to Kelvin (always)

What happens to solids when they get hot? Most solids expand when heated and contract when cooled –Metals have a formula (linear expansion)  L =  L o  T   C -1 coefficient of thermal expansion These are tabulated on charts Solids will expand equally in all directions –Any holes in the material will also expand because the surrounding material expands

Heat capacity Matter responds to the addition of heat differently –Metals absorb/radiate heat easier than nonmetals –Organic liquids (alcohols) absorb heat easier than inorganic liquids/solutions –Each substance has its own specific heat capacity (c) these are tabulated or solved for c is proportional to the amount of heat and change in temp

Latent heat of fusion, vaporization During phase changes the heat energy added or removed to a system breaks down intermolecular forces –Therefore the temperature of the system does not change There are values for each substance (like c) to describe the energy needed to complete the phase change

Heating cooling curves

Need to describe the matter at each point in the curve Be able to match a process/formula to each point

Describe what is happening

Gases Physicists use gases to do work because gases can do work, they can be heated and expand. This expansion can cause other things to move, like pistons –Review what pressure is –Review the ideal gas law (cause physicists love it)

Kinetic molecular theory 1. large number of gas molecules moving in random directions at varying speeds 2. molecules are far apart, the distances between molecules are much larger than the molecules 3. there are 0 attractive forces between gas molecules 4. all collisions are perfectly elastic, both KE and p are conserved.

Temperature and energy What happens at temperature increases? Ke = 3/2 k b T Kb is boltzmans constant, 1.38x We can calculate the average speed of the molecules V=  (3RT/m)

Doing useful work Thermal equilibrium- no net heat flow between objects –Energy can still be exchanged, but this happens at equal rates –The objects are at the same temperature –This takes time, depends on the types of matter involved in the exchange Warm hand, cold desk….

Laws of thermo 0 th law –Describes thermal equilibrium 1 st law of thermo  U = Uf-Ui = Q-W

1 st law Sign conventions +Q means heat is added to system -Q means heat is removed from system +W means work is done BY the system -W means work is done ON the system +Q or –Q –A gas heats up –A chair cools down –An ice cube melts –A glass of soda cools when ice is added +W or –W –A piston moving outward because of an expanding gas –A piston moving inward by a hand pushing on it

What signs are what You draw –-Q and +W –-Q and -W

Internal energy Internal energy is caused by the vibration of the atoms/molecules making up the object As long as the object is above 0K (absolute zero) there will always be internal energy Internal energy of a gas U = 3/2 nRT Depends on the temperature of the gas and the number of MOLES!!!!

Check for understanding A gas is enclosed in a chamber that is fitted with a frictionless piston. The piston is then pushed in, compressing the gas. Which statement below is consistent with the 1 st law of thermodynamics? a.The internal energy of the gas will increase b.The internal energy of the gas will decrease c.The internal energy of the gas will not change d.The internal energy of the gas may increase, decrease or remain the same depending on the amount of heat that the gas gains or loses.

Check for understanding A gas is enclosed in a chamber that is fitted with a frictionless piston. The piston is then pushed in, compressing the gas. Which statement below is consistent with the 1 st law of thermodynamics? d. The internal energy of the gas may increase, decrease or remain the same depending on the amount of heat that the gas gains or loses.  U = Q-W

processes Isobaric- occurs at constant pressure Work= P  V Which is the area under the red line! Pressure remains constant, volume changes –Expansion of a balloon

processes Isochoric (isovolumic) occurs at constant volume No work is done! So U=Q Pressure increases, but there isn’t any area under the curve –Heating up a can of hairspray

processes Isothermal- occurs at constant temperature For a gas P=nRT/V –Ideal gas law Work= nRTln(Vf/Vi)

processes Adiabatic- occurs without any transfer of heat U= -W W= 3/2RT(Ti-Tf) Internal energy is reduced to provide the necessary energy to DO the work

Reading a cycle graph IB constant p, so work is equal to the area under the curve BF constant V pressure is decreasing, perhaps temp is going down?

2 nd law of thermo Heat flows from high temps to low temps spontaneously –Butt to chair! –This gives us heat engines, pumps, refrigerators and AC! Proviso to 2 nd law –No heat engine can be 100% efficient –Because no heat engine is 100% reversible Energy is lost due to sound, friction, and spontaneous heat flow (engines get hot and they radiate to the air, this can’t be undone)

Heat engines-what is different?

entropy The measure of disorder in the universe. –The universe tends to move spontaneously into more and more disorder  S = Q/T W = T  S Think of heat transfers –Hot air (a gas is very disordered; a hot gas is even more disordered) does work on an ice cube, colliding with it, and causing it to melt –The ice cube, once a very ordered solid, is now a disordered puddle –Entropy has increased naturally