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AP Physics. Thermodynamics II. Could This Happen?. Could all the air in a box rush to one side of the box leaving a vacuum in the other side? Will heat flow spontaneously from a cold object to a hot object? Nature’s processes have preferred directions

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could this happen
Could This Happen?
  • Could all the air in a box rush to one side of the box leaving a vacuum in the other side?
  • Will heat flow spontaneously from a cold object to a hot object?

Nature’s processes have preferred directions

Direction is important in Thermodynamic processes

reversible vs irreversible processes
Reversible vs. Irreversible Processes



A process that despite any small change in conditions cannot be reversed.

Ex. Conversion of work to heat by friction

A reversible process is a transition from one state of a thermodynamic system to another, during which the system is always very close to a state of mechanical and thermal equilibrium (including uniform temperature and pressure)

In other words…a small change in the system that can be reversed.

Ex. a gas that expands slowly and adiabatically can be compressed slowly and adiabatically by a small increase in pressure

heat engines
Heat Engines

A device that converts heat partly into work or mechanical energy.

In a heat engine a quantity of matter, called the working substance, inside the engine undergoes addition and subtraction of heat, expansion and compression, and sometimes, a phase change.

heat engines1
Heat Engines

The simplest engines to analyze are those that have a working substance which undergoes a cyclic process.

Cyclic process

  • a sequence of processes that eventually leaves the substance in the same state as it started
  • when a cycle is complete the system starts and ends with the same internal energy


Absorb Heat at High Temp.  Do Work  Discard Heat at Low Temp.

heat engines2
Heat Engines

Internal Energy doesn’t change after a cycle.

fuel consumption
Fuel Consumption

A gasoline engine in a large truck takes in 2500 J of heat and delivers 500 J of mechanical work per cycle. The heat is obtained by burning gasoline with heat of combustion Lc = 5.0 x 104 J/g.

  • What is the thermal efficiencyof this engine?
  • How much heat is discarded in each cycle?
  • How much gasoline is burned during each cycle?
  • If the engine goes through 100 cycles per second, what is its power output in watts? In horsepower? [1hp = 746 W]
  • How much gasoline is burned per second? per hour?
the otto cycle
The Otto Cycle

idealized model of the thermodynamic processes in a gasoline engine

the otto cycle1
The Otto Cycle

idealized model of the thermodynamic processes in a gasoline engine

the diesel cycle
The Diesel Cycle
  • idealized model of the thermodynamic processes in a diesel engine
  • there are no spark plugs for ignition in a diesel engine
  • the fuel is injected just before the power stroke
  • the fuel ignites spontaneously from the high temperature developed during the adiabatic compression

heat engine operating in reverse

  • take heat from a cold place (inside the fridge) and expel it to a warmer place (outside the fridge)

I thought heat didn’t flow from cold to hot?!?!...well it doesn’t…to move heat from cold to hot requires some work!

refrigeration process
Refrigeration Process

Don’t Write This

  • The compressor takes in the refrigerant fluid (working substance) and compresses it adiabatically (this requires work)
  • The refrigerant is then sent to the condenser coil at high temperature
  • Heat transfers from the hot refrigerant to the surroundings (outside the fridge) which partially condenses the fluid
  • Refrigerant fluid expands adiabatically into the evaporator (the expansion valve controls this). The expansion cools the fluid enough so that it is colder than the inside of the fridge.
  • The fluid is then transferred into the fridge where it absorbs the heat from the inside of the fridge and partially vaporizes






air conditioners heat pumps
Air Conditioners & Heat Pumps
  • Air conditioners operate the same as a refrigerator
  • Difference – inside of room/building is like the inside of fridge;

outside of fridge is now outside of room/building

  • Heat Pumps are similar to an

air conditioner except

they’re backwards.

2 nd law of thermodynamics 2lt
2nd Law of Thermodynamics (2LT)

2LT for Engines

It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work with the system ending in the same state in which it began.

A 100% efficient engine does not exist.

2 nd law of thermodynamics 2lt1
2nd Law of Thermodynamics (2LT)

2LT for Refrigerators

It is impossible for any process to have as its sole result the transfer of heat from a cooler object to a hotter object.

It is impossible to have a workless refrigerator.

how efficient can an engine be
How Efficient Can An Engine Be?

2LT – No engine is 100% efficient

In 1824 Sadi Carnot (a French engineer) determined the hypothetical maximum possible efficiency for an engine based on the 2LT.

Carnot Cycle

To maximize efficiency we must avoid irreversible processes.

Heat flow as a result of a temperature difference is an irreversible process.

To avoid this irreversible process the only time that heat can flow must be during an isothermal process.

carnot cycle
Carnot Cycle

W is the work done by the system

Isothermal Expansion (a b) (Heat Absorbed)

Adiabatic Expansion

(b  c)

Isothermal Compression

(c  d)

(Heat Released)

4. Adiabatic Compression

(d  a)

thermal efficiency of carnot cycle
Thermal Efficiency of Carnot Cycle

The thermal efficiency of a Carnot Engine is dependent only on the absolute temperature (temperature in Kelvin) of the heat reservoirs.

a carnot engine
A Carnot Engine

A Carnot engine takes 200o J of heat from a reservoir at 500 K, does some work, and discards some heat to a reservoir at 350 K.

What is the engine’s efficiency?

How much work does it do?

How much heat is discarded by the engine?


A quantitative measure of disorder.

  • Nature (irreversible processes) tends towards randomness (disorder).
  • No process is possible in which the total entropy decreases when all systems taking part in the process are included.

Reversible Processes

Change in Entropy is the heat transferred divided by the absolute temperature (Kelvin).