Ch. 15: 1st Law of Thermodynamics

“the state of a gas” means that

the gas has a specific P, V, n, T (=ideal gas law)

-We assumed that ideal gas particles do not……, thus the system has no…

• In an ideal gas, the total internal energy of the gas particles = ___

• The thermal energy depends only on…

-interact at a distance…. potential energy due to particle interactions

• its thermal energy

• its absolute temperature and on the number of moles of gas

*N=#molecules; n=#moles

Work (W)

-the negative of the area under the pV curve between Vi and Vf

-is an isobaric process, when pressure is a constant and the volume changes, the work done during the process is: W=-pchangeV

-if a gas expands…

-if a gas is compressed…

-changeV>0; W<0

*W<0= energy out of the gas

-changeV<0; W>0

*W>0= energy into gas

-isobaric=

-isochoric=

-same pressure

-same volume

for an isochoric process, W=?

-for an isochoric process, the area under the pV curve is zero, and no work is done (W=0)

*vertical line

Cyclic process

=begins and ends at the same P and V

-the total work done is the area enclosed

-arrows tell you direction; CCW=+

-area of a triangle

-area of a cricle

-A=1/2*base*height

-A=pie*radius²

Heating

-Q

-a physical quantity that characterizes a process transferring energy from the envionrment to a system, which is at a different temperature

-SI unit is a Joule (J)

-+=heat in; - = heat out

in a sealed container, change in temperature is proportional to change in __

U

*for example: if temperature goes up, U goes up; if temp constant, U=0

*look at PV=nRT and then changeU=3/2nRchangeT to decide sign of U

-Thermal Energy

-Heat

-Temperature

-Thermal Energy (Uth): an energy of the system due to the motion of its atoms and molecules; J

*total energy in a gas; proportional to temp

-Heat (Q): energy transferred between the system and the environment as they interact; J

-Temperature (T): a state variable that quantifies the “hotness” or “coldness” of a system; degrees K

___________________________________

-be careful not to confuse heat and temperature;

Adiabatic process

-Q=0

-a process in which no heat is transferred; perhaps bc the system is extremely well insulated

Review Types of Gas Processes (if N or n=constant)

-General: P, V, T all change

-Adiabatic: Q=0 (ex: insulated thermos)

-Isochoric: V=constant

-Isobaric: P= constant

-Isothermal: T=constant

-Cyclic: combination of the above; work net=area enclosed (“+'“ for CCW)

-if V is compressed, W=

-if V expands, W=

-if no change in V, W=

=positive

=negative

=zero

specific heat (c)

-the physical quantity equal to the amount of energy that needs to be added to 1kg of a substance to increase its temperature by 1 degree C; energy is added through heating or work or both

-#J’s needed to raise 1kg substance 1C

-units: J/kg*C

*C or K

-Q=mc(deltaT)

Phase changes happen at a constant ____

temperature

phase changes

-can occur for an object with mass in contanct with a reservoir:…

Q= -mL (condensing, freezing;=heat out) or +mL (melting, boiling;=heat in)

*L=latent heat of the object

-phase changes occur at a constant temperature

-the regions with a non-zero slope are regions where heat is used to warm the substance, obeying the equation Q=mcdeltaT

when use Q=mcdeltaT vs Q=mL

-mcdeltaT: slope (where heat is used to warm the substance)

-mL: zero slope (/horizontal line/phase change)

true or false: there is more energy for phase changes than increase in temperature

true