Conduction Flashcards
What is meant by temperature
The average kinetic energy possessed by an atom or molecule
For non atomic gases why is not all of the converted to the velocity of the atom
Some is converted into making the molecule vibrate or rotate
What is heat defined as
The flow of energy as a result of a temperature gradient
What does Fourier’s law state
The rate of heat transfer is proportional to the the temperature gradient across a distance and the area of heat transfer
What is the most effective way of transferring heat
Physically moving the heat
What is the dominant heat transfer mechanism in solids
Conduction
State the 1st law of thermodynamics
Energy can be transformed (one from to another) but cannot be created or destroyed
State the zeroth law of thermodynamics
If 2 systems are in thermal equilibrium with a third system the they must be in thermal equilibrium with each other
What does the 2nd law of thermodynamics describe
The tendency that overtime isolated physical systems will reach thermal equilibrium, so the law deduces the principle of irreversible increase in entropy
What is the 3rd law of thermodynamics
Entropy of a perfect crystal will approach 0 as the absolute temperature reaches 0, this provided an absolute reference point for the determination of entropy
What assumptions have to be made to use kinetic theory of gases
Very small particles Same mass Can use statistical treatment Constant random movement Constant collisions Interactions other than collisions are negligible Ek only depends on temperature
What is the mean free path length
The average distance travelled by a particle between collisions
Will a large or small mean free path length transfer heat better between to surfaces
A large
In the pressure v conduction chart why is radiation dominant in the 1st region
Very few particles for conduction
What are the waves in which energy is transferred in solids called
Phonons
Explain how electrons can transfer heat
in good electrical conductors electrons move freely so can rapidly transfer thermal energy
In electrical conductors what conducts nearly all the heat
The Electron flow
In lumped system analysis what represents current, voltage and resistance
Current- heat flow
Voltage- Temperature difference
Resistance- Thermal resistance
What is the equation for the thermal resistance of an object
Rth=dx/KA
What must be assumed with regards to the material in lumped system analysis
It is homogeneous
Is lumped system analysis acceptable for micro or nano scale thermal behaviour
No
Why do interfaces between to surfaces have a thermal resistance
Surface roughness causes a non perfect contact between the 2 surfaces, trapped air will reduce the overall thermal conductivity and heat transfer area
What is the biot number used for
To determine if we can use lumped system analysis if bi<0.1 then we can
What is the definition of the biot number
The biot number is the ratio of Surface conduction (to outside world) to internal conduction
