Unit 5: Thermodynamics and engines Flashcards
What is thermodynamics?
The study of the relationship between heat and other forms of energy
What do equations of state involve i.e. the three gas laws?
Pressure, volume and temperature
What is Boyle’s law in words?
For a fixed mass of gas at a constant temperature, the product of the pressure and volume is constant
What is the equation for Boyle’s law?
pV = constant or p is proportional to 1/V at constant T
What is Charles’ law in words?
For a fixed mass of gas at constant pressure, the volume is directly proportional to the temperature measured in kelvin
What do gas laws apply only to?
A fixed amount of gas
What is the equation for Charles’ law?
V/T = constant or V is proportional to T at constant p
What is the definition of 1 kelvin?
A temperature change of 1 kelvin is identical to a temperature change of 1°C, the kelvin and celsius scales have equal increments but they start at different places: 0K = -273.15°C
What is the pressure-temperature law in words?
For a fixed mass of gas at constant volume, the pressure is directly proportional to the temperature measured in kelvin
What is the equation for the pressure-temperature law?
p/T = constant or p is proportional to T at constant V
Do real gases obey the gas laws exactly?
No
What do the gas laws give an accurate description of?
The way gases behave at low temperatures and temperatures that are well above their liquefaction temperature
What is the equation for the ideal gas law and what do the terms in it represent?
pV = nRT where V is the volume in m^3, T is the temperature in kelvin, p is gas pressure in Nm^-2 or Pa, R is the universal molar gas constant = 8.31 JK^(-1)mol^(-1) and n is the number of moles
What can be said about a gas that obeys the ideal gas law exactly?
This gas has exactly no forces acting between the molecules and hence the internal energy (energy of molecules) is entirely kinetic and depends only on its temperature
What does the first law of thermodynamics relate?
Heat and work and it is essentially the principle of the conservation of energy
What is the equation for the first law of thermodynamics and what do the terms represent?
Q = ΔU + W where Q is the heat entering the system, ΔU is the increase in internal energy and W is the work done by the system
How can internal energy be increased?
By doing work on the system i.e. compressing it or putting energy into the system
What does the change in internal energy depend on?
The change depends on the initial and final states of the system and not how the change was brought about
What is an isolated system?
A system cut off from any external influence
What are the conditions of an isolated system?
No work can be done hence W = 0, no heat can enter or leave the system hence Q = 0 therefore ΔU = 0 and the internal energy of an isolated system is constant
When do adiabatic process occur?
They occur very rapidly or in a system that is so well insulated that no transfer of energy as heat can occur between the system and its external environment
What is the condition for a adiabatic process?
Q = 0
What does the first law of thermodynamics change to in an adiabatic process?
ΔU = -W
What is the definition of internal energy?
The internal energy of a system is the sum of the kinetic and potential energies of the atoms and molecules comprising the system, the value of the absolute internal energy of a system cannot be measured, only its change from one state into another