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
Nature’s processes have preferred directions
Direction is important in Thermodynamic 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
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.
The simplest engines to analyze are those that have a working substance which undergoes a cyclic process.
Absorb Heat at High Temp. Do Work Discard Heat at Low Temp.
Internal Energy doesn’t change after a cycle.
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.
heat engine operating in reverse
I thought heat didn’t flow from cold to hot?!?!...well it doesn’t…to move heat from cold to hot requires some work!
Don’t Write This
outside of fridge is now outside of room/building
air conditioner except
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.
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.
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.
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.
W is the work done by the system
Isothermal Expansion (a b) (Heat Absorbed)
4. Adiabatic Compression
The thermal efficiency of a Carnot Engine is dependent only on the absolute temperature (temperature in Kelvin) of the heat reservoirs.
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.
Change in Entropy is the heat transferred divided by the absolute temperature (Kelvin).