Energy and thermodynamics System: the object we are studying. Environment: what it interacts with Work done by expansion

Work during expansion or contraction 1m3 of a gas at 300 K and one atmosphere are expanded at constant pressure to 3 m3 . How much work was done?

Work done when P is not constant In which process (red or blue) is the most work done?

Sign of Work If system expands, we say Wdone by system is ______ If system contracts we say Wdone by system is ______. The system has to push the atmosphere away to grow in size.

P1. The process in which the work is the greatest (magnitude) a) path 1 (AB) b) path 2 (ACB) c) neither; it’s the same P2. The work done in both processes a) puts energy into the system b) takes energy out of the system c) has no effect on the energy of the system

Heat flow Q into/out of a system Always due to temperature differences (which might be very small) Sign of Q: If heat flows into system, Q is ______ If heat flows out of system, Q is ______

Internal energy U of a system Total energy content of a system: molecular KE (thermal), PE, chemical bonds, etc. W and Q are energy exchanges . U is the energy in the system.

1st Law of Thermodynamics ΔU = Q - W or Q= ΔU + W  (if we don’t let any particles in or out) The “–” for W is because when W is +, energy goes out of the system

U of an ideal gas ideal gas: monatomic: U = 3/2 NkT = 3/2 nRT more complicated molecules: add vibration, rotation (Most substances have complicated U(T))

P-V diagram A point on the diagram gives the state of the system Isothermal lines: contours of constant T show which states have the same T.

Constant volume process What happens to T as we follow this path? What sign is U? W =   But also so which can be used for any process if is known (and it works for any material)

Constant pressure process (isobaric) What happens to T as we follow this path? What sign is U? W =   But also For all materials, which is greater, or ?

P3. Along path AC, temperature is _______________ a) rising b) falling c) constant P4. The process in which U is the greatest (magnitude) is a) path 1 (AB) b) path 2 (ACB) c) neither; it’s the same

For 300 moles of some ideal gas, suppose we know Cv = 2 J/mol/K For 300 moles of some ideal gas, suppose we know Cv = 2 J/mol/K. find Q, W, DU, along various paths   First find T’s Path AC: find find Q, W, DU Path ADC: find find Q, W, DU Path ABC: find find Q, W, DU

Constant T process (isothermal) ideal gas U = W = area under curve = Q =

Adiabatic expansion or compression Adiabatic: system is insulated or DV is too fast for heat to flow, so Q=0 Applications: Very fast engine piston compression Air conditioning!

Adiabatic expansion or compression Adiabatic: system is insulated or DV is too fast for heat to flow, so Q=0 W = U = Ideal gas adiabatic curves are steeper than isothermal curves because T’s change! use with to find T’s:

P5. You boil water on a stove. System = water P5. You boil water on a stove. System = water. The steam must displace the atmosphere. U is higher for steam than water because you pull molecules apart (PE) to make steam. Choose A, B, C, D or E below for the correct signs   Q added W by sys DU A: + B: C: - D: E:

P6. You compress air very quickly in an engine cylinder (or in a perfectly insulated one). System = air Choose A, B, C, D or E below for the correct signs   Q added W by sys DU A: + B: C: - D: E:

P7. You heat a spray can in a fire, and it doesn’t explode, and volume stays essentially the same. System = gas in can Choose A, B, C, D or E below for the correct signs   Q added W by sys DU A: + B: C: - D: E: