2.3 Thermodynamics Flashcards
What is temperature?
Measure of average kinetic energy of the particles in a system.
How is thermal equilibrium reached?
When heat flows from a hotter to cooler body and stops when they are the same temp.
What is the difference between heat and temperature?
Temperature is the property of a body and heat is the energy that flows too or from a body.
What are the three types of thermometers?
Bulb
Bimetallic
Thermistor
What is a thermocouple?
They are used to measure high temp, used for applications in the engine.
What is a bulb thermometer?
The classic type you find everywhere.
What is a bimetallic thermometer?
It shows temp as mechanical displacement. They can be used as a heat switch.
What is a thermistor?
A device to measure temp, the electrical resistance changes with temp.
How do you convert Fahrenheit to celsius?
F-30/2
How do you convert Celsius to Fahrenheit?
C x 1.8 + 32
Which measure of temp is the SI units?
Kelvin.
What is absolute zero in degrees celsius?
-273.15
What is the key difference between Calorie and calorie?
The lower case calorie is 1/1000th of a Calorie.
How does heat flow from a lower to higher object?
If work is done.
What is specific heat?
The energy required to raise the temp of 1kg object by 1 degree c.
What is the formula for specific heat?
Heat energy added divided by mass x change in temp
How do you work out the heat given to an object to heat its mass?
mass x specific heat x temp change
What is heat capacity?
Ratio of the amount of heat energy transferred to an object to the change in temp.
What are the three methods of heat transfer?
Convection, conduction and radiation.
What is conduction?
Heat is directly transferred through the material. Only method of heat transfer in solids, sometimes in liquids.
What is convection?
Heat transfer in liquids and gasses.
What is radiation?
Energy coming from source and travelling through space. It is different to conduction and convection in that the objects do not need to be in contact.
What is the coefficient of linear thermal expansion?
The amount that different materials expand/contract when heated or cooled.
What is linear expansion?
The change in length proportional to the original length and change in temp.
What is area expansion?
The change in dimensions due to application of temps, in 2D bodies.
What is volumetric expansion?
The expansion found in 3D bodies.
Because liquids and gasses are free flowing when not in a container the linear coefficient of thermal expansion is not applicable, what is used instead?
The coefficient of volumetric thermal expansion.
What is the first law of thermodynamics?
Energy can neither be created or destroyed but it can be transformed.
What is the formula for the first law of thermodynamics?
Heat energy added= change in internal energy + work done
What is the second law of thermodynamics?
The total energy output is equal to the amount of heat supplied.
Second law: Although the energy transferred to the system by heating is equal to the work done by the system, what is key to make this work?
The heat supplied must be greater than the work done, therefore some heat must be lost during the work.
What is the heat receiver?
The place where heat energy is lost in a machine. Also referred to as heat sink.
What is the formula for thermal efficiency?
Work done / heat energy added
Does thermal efficiency have a unit?
No
What are examples of heat engines designed to minimise thermal losses?
Steam turbines, refrigeration packs and air conditioning units.
Who came up with the carnot engine and what is it?
Sadi Carnot, in theory it is a equal input/output engine.
What is entropy?
The loss of energy to the environment due to an increase of disorder in the system. Higher chaos gives higher entropy.
What is key about entropy?
It can remain the same or increase, it cannot go down.
What is a thermodynamic cycle?
The fluid of a system undergoes a series of processes and finally returns to its initial state.
What are the two types of thermodynamic cycles?
Open and closed.
What are the five basic elements of a thermodynamic cycle?
Working substance
Heat Source
Heat receiver
Pump
Engine
What is the working substance?
A medium that receives, stores and transports the energy.
What is the heat source?
Supplies thermal energy to the working fluid.
What is the heat receiver?
Absorbs the heat from the working substance.
What is the pump?
Moves the working substance from the low pressure side to the high pressure side of the cycle.
What is the engine?
Converts the thermal energy to work in mechanical energy.
What is an open cycle system?
Where an opening in the system boundary allows the fluid to be replaced, as the fluid is allowed to leave. I.e. gas turbine engine.
What is an example of closed cycle system?
Air con unit.
What gasses are considered ideal gasses?
Hydrogen, oxygen, noble gasses and some heavier gasses like carbon dioxide.
What three characteristics are ideal gasses judged on?
Temp
Pressure
Density
What is boyles law also known as?
Isothermal process.
What does boyles law describe?
The direct relationship between the pressure and the volume of a gas.
What is an isothermal process?
Boyles Law
At constant temp, the pressure exerted by a gas varies inversely with the volume.
Why is an understanding of Boyles law important in aviation?
It is essential in calculating the volume and pressure of ICE’s.
What is Charles’s law also known as?
Isobaric process.
What is an isobaric process?
Charles Law
At constant pressure the volume increases or decreases by the same amount as the temp.
What is Gay-Lussacs law also known as?
Isochoric process.
What is isochoric process?
Gay-Lussac
At constant volume, the pressure varies with the absolute temp.
What is the triangle of the three gas laws?
PxV over T
P/T= Gay Lussac
V/T= Charles
PxV= Boyle
What is key about specific heat at constant volume?
No work is done, the only change is in thermal energy.
What is key about specific heat at constant pressure?
There is change in thermal energy and work done to increase pressure.
What is the formula for work?
Force x distance moved (s)
What is an adiabatic expansion?
No heat is transferred to or from the system. It is any process that occurs in a container that is a good insulator.
Strictly speaking do adiabatic processes actually exist?
No
Basic rules: Isochoric
Constant volume
Basic rules: Isobaric
Constant pressure
Basic rules: Isothermal
Constant temperature
Basic rules: Adiabatic
No heat transfer
What is an engine cycle?
Series of thermodynamic processes, heat into work.
What are the two types of thermodynamic process?
Reversible and irreversible.
How can a process be reversible?
It has to be in thermodynamic equilibrium with its surroundings. (hypothetical)
What is a reciprocating piston engines process?
Non flow, heat to mechanical.
What is a Gas turbines process?
Flow, heat to mechanical.
What are the four processes in an Otto cycle (piston engine).
Adiabatic Compression
Reversible Isochoric Heating
Adiabatic Expansion
Reversible Isochoric Cooling
What is important about the Otto cycle?
It is idealistic as it based on a perfect system.
What process is used in gas turbine engines?
Brayton Cycle
What three components are found in a brayton cycle engine?
Gas compressor, mixing chamber and expander.
What is the constant volume process for a perfect gas?
Reversible
What is the refrigeration cycle?
The opposite of the natural heat cycle, transfers thermal energy from a cold place to a warm place.
What three processes can be used to cool gasses?
Convection
Conduction
Radiation
What is latent heat?
The amount of heat that is absorbed or released when a body undergoes a constant-temperature process.
What are the two types of curve when considering change of state?
Heating or cooling curve.
In regards to latent heat what is fusion?
The transition from a solid to a liquid via melting.
What kind of process is solidifcation?
Exothermic.
What is the only exception to the solidification rule?
Solidification of low temp helium.
What is the latent heat of fusion the same as?
The latent heat of solidification
What is the latent heat of vaporisation the same as?
Latent heat of condensation
What is temperature?
A measure of the average kinetic energy of the particles in a substance.
What is Thermal Energy?
The total of energy of the particles in a substance.
What is heat?
The transfer of thermal energy between objects that have different temps.
What is the heat of combustion?
The energy released as heat when a substance undergoes combustion.
What is the key point to understand about latent heat?
Heat gained or lost by a body with no corresponding change in temp.
