Honors PhysicsChapter 10Thermodynamics

Some background information.

Remember the Law of Conservation of Energy. Remember the difference between Potential and Kinetic Energies. Remember the difference between temperature and heat. Remember Work.

Exothermic/Endothermic

  

SYSTEM

SURROUNDINGS/ENVIRONMENT

HEAT LEAVES SYSTEM EXOTHERMIC  

HEAT ENTERS SYSTEM ENDOTHERMIC

Work by a gas.WORK BY A GAS = P V see p 337(P = External Pressure) Expanding gas, the work done by the gas on the piston is +.Compressing gas, the work done by the gas on the piston is -.

Work on a gas, cont’d.

area

d

Volume change is d x area

Concept Check

Gas in a container is at a pressure of 1.6 x 105 Pa and a volume of 4.0 m3. What is the work done by the gas if a. it expands at constant pressure to twice its initial volume?b. it is compressed at constant pressure to one-quarter of its initial volume?

Concept Check

a. 6.4 x 105 Jb. -4.8 x 105 J

Concept Check

A gas is enclosed in a container fitted with a piston. The applied pressure is maintained at 599.5 kPa as the piston moves inward, which changes the volume of the gas from 5.317 x 10-4 m3 to 2.523 x 10-4 m3. How much work is done? Is the work done on or by the gas? Explain your answer.

Concept Check

-164.5 J; work is done on the gas because the work and volume change are negative.

Concept Check

A balloon is inflated with helium at a constant pressure that is 4.3 x 105 Pa in excess of atmospheric pressure. If the balloon inflates from a volume of 1.8 x 10-4 m3 to 9.5 x 10-4 m3, how much work is done on the surrounding air by the helium-filled balloon during this expansion?

Concept Check

3.3 x 102 J

Concept Check

Steam moves into the cylinder of a steam engine at a constant pressure and does 0.84 J of work on a piston. The diameter of the piston is 1.6 cm, and the piston travels 2.1 cm. What is the pressure of the steam?

Concept Check

2.0 x 105 Pa

CalorimetryQp = m c t at constant pressure, the change in energy of a solution is equal to the mass of the solution times the specific heat capacity of the solution times the change in temperature.

Bomb CalorimetryFor constant volume, (bomb calorimeter) -P V =0, so U = Q - W, but W = 0, So U = Qv

Qv = c T,

c= heat capacity, the energy required to change the temperature of the bomb 1C

p 339

1ST LAW OF THERMODYNAMICSW

WHEN ENERGY IS TRANSFERRED TO HEAT E

ENERGY OR VICE-VERSA, THE HEAT ENERGY I

IS EXACTLY EQUAL TO THE AMOUNT OF T

TRANSFORMED ENERGY. E

ENERGY IS CONSERVED FOR ANY SYSTEM AND

ITS ENVIRNOMENT.

Cont’d ** U = Q - W or Q = U + W**    Q = AMOUNT OF ADDED HEAT    W = EXTERNAL WORK DONE.    U = CHANGE IN INTERNAL

ENERGY

Sign for energy flowSign indicates direction of flow exothermic = -Q, endothermic = +Q work done on the system = -W system does work on surrounding = +W

Process TermsIsovolumetric means that the volume doesn’t change.Isothermal means that the temperature is constant.Adiabatic means that energy isn’t transferred as heat. Usually occurs quickly.Isobaric means that the pressure is constant.Isolated has no interactions with surroundings/environment.

Specific Processes for 1st Law

See Table 2 for isovolumetric, isothermal, adiabatic, and isolated system.Isovolumetric: W=0, Q=U = mcvtIsothermal: U=0, Q = WAdiabatic: Q= 0 =U+W or U = -WIsolated: Q = W= 0, U = 0

Concept Check

Heat is added to a system, and the system does 26 J of work. If the internal energy increases by 7 J, how much heat was added to the system?

Concept Check

33 J

Concept Check

The internal energy of the gas in a gasoline engine's cylinder decreases by 195 J. If 52.0 J of work is done by the gas, how much energy is transferred as heat? Is this energy added to or removed from the gas?

Concept Check

-143 J; removed as heat

Concept CheckA 2.0 kg quantity of water is held at constant volume in a pressure cooker and heated by a range element. The system's internal energy increases by 8.0 x 103 J. However, the pressure cooker is not well insulated, and as a result, 2.0 x 103 J of energy is transferred to the surrounding air. How much energy is transferred from the range element to the pressure cooker as heat?

Concept Check

1.00 x 104 J

Concept Check

The internal energy of a gas decreases by 344 J. If the process is adiabatic, how much energy is transferred as heat? How much work is done on or by the gas?

Concept Check

0 J; 344 J done by gas

Concept Check

A steam engine's boiler completely converts 155 kg of water to steam. This process involves the transfer of 3.50 x 108 J as heat. If steam escaping through a safety valve does 1.76 x 108 J of work expanding against the outside atmosphere, what is the net change in the internal energy of the water-steam system?

Concept Check

1.74 x 108 J

“Heat Engine”

QH

QH=W + Qc

Hot Temp Reservoir

Cold Temp Reservoir

Qc

Work Out

Cyclic ProcessesThe change in internal energy of a system is zero in a cyclic process. Unet= 0 and Qnet = Wnet

Wnet = Qh – Qc, where Qh > Qc

(hot, cold)

“Refrigerator”

QH

QH=W + Qc

Hot Temp Reservoir

Cold Temp Reservoir

Qc

Work In

2nd Law of Thermodynamics

No cyclic process that converts heat entirely into work is possible.Without “help” heat flows from higher heat to lower heat.Systems go in the direction that increases the disorder of a system.“The universe tends to be lazy and messy.”

EfficiencyThere are no 100 % efficient

engines.Efficiency = net work done by engine

energy added to engine as heat

eff = W net = Qh – Qc = 1 – Qc

Qh Qh Qh

Concept Check

If a steam engine takes in 2.254 x 104 kJ from the boiler and gives up 1.915 x 104 kJ in exhaust during one cycle, what is the engine's efficiency?

Concept Check

0.1504

Concept Check

A test model for an experimental gasoline engine does 45 J of work in one cycle and gives up 31 J as heat. What is the engine's efficiency?

Concept Check

0.59

Concept Check

A steam engine absorbs 1.98 x 105 J and expels 1.49 x 105 J in each cycle. Assume that all of the remaining energy is used to do work. a. What is the engine's efficiency?b. How much work is done in each cycle?

Concept Check

a. 0.247b. 4.9 x 104 J

Concept Check

If a gasoline engine has an efficiency of 21 percent and loses 780 J to the cooling system and exhaust during each cycle, how much work is done by the engine?

Concept Check

210 J

Concept Check

A certain diesel engine performs 372 J of work in each cycle with an efficiency of 33.0 percent. How much energy is transferred from the engine to the exhaust and cooling system as heat?

Concept Check

755 J

Concept Check

If the energy removed from an engine as heat during one cycle is 6.0 x 102 J, how much energy must be added to the engine during one cycle in order for it to operate at 31 percent efficiency?

Concept Check

8.7 x 102 J

EntropyEntropy (S) is a measure of the disorder of a system. Higher the entropy, the less energy that is available to do workSystems with maximum entropy are favored.