Thermodynamics Flashcards
What is temperature?
A measurement of the average kinetic energy of the particles in a system
When an object warms up what happens to the particles?
They move faster
What happens if you had two objects at different temperature?
Heat will flow to the colder until their temperatures are identical and thermal equilibrium is reached
What is a thermometer?
A temperature measuring instrument
What are the three main types of thermometers?
-bulb thermometer
-bimetallic thermometer
-thermistor
What are thermocouples used for?
Measurement of high temperature
Why is mercury used in thermometers?
They have a linear rate of expansion with rising temperature
In a bulb thermometer what happens when temperature rises?
The fluid expands and moves up the tube
Why is water not used in thermometers?
Because it would freeze or boil
How does a bimetallic thermometer work?
Two different metals are bonded together and when the temperatures rises or falls the bonded metal will bend in a certain direction
A thermistor is a combination of two words, what are these words?
Thermal and resistor
How does a thermistor work?
A wire that changes its electrical resistance with change in temperature
What is the most commonly used sensor to measure temperature?
A thermistor
What temperature range is used in the metric system?
Celsius or centigrade
Where is Fahrenheit used?
USA and some Caribbean countries
In Fahrenheit what is the freezing point of water?
32˚F
In Fahrenheit what is the boiling point of water?
212˚F
What is the only temperature that is the same in Celsius and Fahrenheit?
-40˚
When is kelvin used as the base unit for temperature?
In the SI units
What is the lowest possible temperature?
Absolute zero
What would happen to ordinary gases like air if they reached absolute zero?
They’d become rock solid
Is it possible to get to absolute zero?
No but we can get to a fraction of ˚C from it
What is the equation to work out kelvin?
K = C + 273.15
What is heat?
A measurement of energy transferred from one substance to another
What is the units for heat energy?
joules or calories
When the unit calories is written down it can come in two forms, lower c and upper C, what does this mean?
Lower c is one thousandths of a upper C
Does heat transfer from a lower temperature to a higher?
No unless work is done
What would happen if absolute zero is reached?
All molecular movement would stop
What is specific heat capacity?
The amount of energy that is needed to raise the temperature of 1 kg of that substance by 1Kelvin
If you added heat to two different substances would they increase at the same rate?
No as they have different specific heat capacity
What is the equation for specific heat of a substance?
Heat energy added / (mass x change in temperature)
What is heat capacity?
The ratio of the amount of heat energy transferred to an object to the resulting increase in its temperature
What is another way to describe heat capacity?
Thermal mass
What is the heat capacity equation?
Heat energy added / change in temperature
What is heat transfer?
The flow of heat due to temperature differences
What are the three methods by which heat is transferred?
-convection
-conduction
-radiation
In relation to heat transfer what is conduction?
The process by which heat is directly transferred through the material
What is the only heat transfer method in solids?
Conduction
In relation to heat transfer what is convection?
Heat transfer from one part of a fluid to another
What is thermal radiation?
Energy that comes from a source and travels through air or a vacuum
In relation to heat transfer why is radiation different to conduction and convection?
Because the substances don’t have to be in contact with eachother
When a material is heated up and it expands what happens to the molecules for this to happen?
They vibrate about their positions and space between them increases
What is linear expansion?
It’s the comparison between length change to the original and the change in temperature
What type of solids does area expansion affect?
Two dimensional solids
What type of solids get affected by volumetric expansion?
Three dimensional solids
From the three basic states of substances, what order do they expand the most?
1st: gases
2nd: liquid
3rd: solids
Why cant the expression for the linear thermal expansion of solids be used on liquids and gases?
Because they have no fixed shape
What is the first law of thermodynamics?
Energy can neither be created nor destroyed but can only be transformed from one to another
What is the second law of thermodynamics?
The total energy output (as produced by a machine) is equal to the amount of heat supplied
What is a heat receiver?
The place where heat energy is lost
What is another way a heat receiver is described?
A heat sink
What is a thermodynamic cycle?
Where the. Working fluid of the system undergoes a series of processes and finally returns to its initial state i.e a refrigerator
What are the two types o thermodynamic cycles?
Open and closed
What are the five basic elements of a thermodynamic cycle?
-working substance
-heat source
-heat receiver
-pump
-engine
In thermodynamic cycles what is a working substance?
A medium that receives, stores and transports the energy
In thermodynamic cycles what is a heat source?
Supplies thermal energy to the working fluid
In thermodynamic cycles what is a heat receiver?
It absorbs the heat (or thermal energy) from the working substance
In thermodynamic cycles what is a pump?
Moves the working substance from low pressure to high pressure
In thermodynamic cycles what is a engine?
It converts the thermal energy to work in mechanical energy
What is the factor in a thermodynamic open cycle?
An opening in the system allows the fluid to be regularly replaced i.e aircraft air conditioning
What is the main factor in a thermodynamic closed cycle?
The working substance never leaves the cycle
What are the three main characteristics that affect gases?
-temperature
-pressure
-density
When one of the main characteristics that affects gases what happens to the other 2?
They will also change
When a gas compresses or expands, what happens to the temperature?
It will change
What is boules law?
The direct relationship between pressure and the volume of a gas
What is an isothermal process?
The pressure exerted by a gas varies inversely with the volume of the gas (temperature remains constant)
What is Charles law?
Describes the direct relationship between the temperature and the volume of a gas
What is an isobaric process?
The volume of a given mass of an ideal gas increases or decreases by the factor as its temperature increases or decreases (constant pressure)
What is gay-lussac’s law?
Describes the direct relationship between the pressure an the temperature of a gas
What is a isochoric process?
The pressure of gas varies directly with the temperature (constant volume)
What is combined gas law?
It puts all three gas laws together to show that pressure, temperature, and volume are interrelated
How do you calculate work in a thermodynamic process?
Work = pressure x change in volume
What is an adiabatic process?
Where no heat is transferred to or from the system
Why is an adiabatic process an idealisation?
Because it is impossible to get absolute thermal insulation
What is an engine cycle in thermal dynamics?
Any thermodynamic process constituting a cycle for the conversion of heat into work
What is meant when a system is said to have undergone a process?
One of the properties of the system change (temperature,volume, etc)
What is a reversible process?
Is a process when the system can be restored to the initial state from the final state without causing any changes in the surroundings
In practice why does a reversible process never occurs?
Because you will always affect the surroundings as you can’t achieve thermodynamic equilibrium
What is a irreversible process?
A process that cannot be completely reversed without causing any changes in the surroundings
Why is a irreversible process also called the natural process?
Because all the processes occurring in nature are irreversible processes
What is an internal combustion engine?
A heat engine in which the combustion of a fuel occurs with an oxidiser in a combustion chamber
What are the two types of theoretical and practical cycles for the internal combustion engine?
-reciprocating piston engines
-gas turbine engines
What is the difference between a reciprocating piston engine (car engine) and a gas turbine engine (plane engine)?
The piston engine use non-flow processes that convert heat energy into mechanical energy however a gas turbine engine use flow processes to convert heat energy into mechanical energy
What happens with pressure and volume in a gas turbine engine?
Combustion occurs at a constant pressure with an increase in volume
What happens with pressure and volume in a piston engine?
Combustion occurs at a constant volume with an increase in pressure
In both piston and gas turbine engines what are the four stages?
Induction , compression, combustion, and exhaust
What is the Otto cycle?
An idealised thermodynamic cycle describing the processes in a spark ignition piston engine
What is the Brayton cycle?
A thermodynamic cycle describing the process in a constant-pressure heat engine
What are the three main components to a brayton-type engine?
A gas compressor, a mixing chamber, and an expander
What does a heat pump do?
Transfers thermal energy from a cold place to a warm place (opposite to natural flow of heat)
What is refrigeration?
The removal and relocation of heat
Cooling can be achieved by what three processes?
-conduction
-convection
-radiation
What are the two main processes used in the design of refrigeration equipment?
Conduction and convection
What principle is used for an aircraft refrigeration system?
That a liquid can be made to vaporise at any temperature by altering the pressure acting on it
What is latent heat?
The amount of heat that is absorbed or released when a body undergoes a constant temperature process
Why is latent heat said to be latent?
There is a change of heat but no change of temperature
When is there a change in heat energy but no change in temperature of a substance?
When that substance is changing its state i.e ice to water or water to steam
What is latent heat Of fusion?
The heat added causing a change of state from solid to a liquid
What is latent heat of evaporation?
The heat added causing a change of state from liquid to a vapour
What are the two types of curves when considering the changes of states?
A cooling curve and a heating curve
In relation to cooling curves and heating curves what is fusion?
The phase transition from a solid to a liquid
In relation to cooling and heating curves what is solidification?
A phase change of matter that results in the production of a solid
Why is solidification an exothermic process?
Because heat is released during the change
What is thermal energy?
The kinetic energy possessed by an object due to the movement of particles within it
The faster the particles move within an object or system results in?
A higher temperature
What is the heat of combustion?
The energy released as heat when a substance undergoes complete combustion with oxygen
