Download presentation
Presentation is loading. Please wait.
Published byHope Black Modified over 9 years ago
1
Convection - Heat transfer in a gas or liquid by the circulation of currents from one region to another. Can be forced or spontaneous (natural). Hot and cold liquid is brought in a thermal contact; it reduces the distance across which the conduction occurs and increases the contact area.
2
Reducing heat-flow rate for better thermal insulation. Thick (large x ) cavities in house walls filled with insulating (small k ) materials; Reducing the number of walls (small surface area A ).
4
Heat transfer by radiation. Stefan-Boltzmann law Total power, P [J/s=W], emitted by a hot object. ● 4 th power of the absolute temperature, T. ● Surface area, A. ● Emissivity of the material, e. ● Stefan-Boltzmann constant, Emissivity, e, varies between 1 (black body) and 0 (reflecting surface). A good emitter of radiation ( e ≈ 1) is also a good absorber. A radiator and a solar heater should be black. A thermos bottle should be silver-coated. Emissivity, e, depends on the wavelength of the radiation. For a solar heater, what values of e are the best for sunlight and for thermal waves radiated near 100 °C?
5
Gases Gas is matter in a rarefied state. The molecules are moving freely most of the time, and only once in a while undergo short-term collisions. The macroscopic state of a gas in thermodynamic equilibrium is determined completely by its temperature, pressure, and volume. The ideal gas law P is the pressure, V is the volume, T is the absolute temperature… N is the total number of molecules in the gas and k is Boltzmann’s constant, k = 1.38 10 -23 J/K
6
The ideal gas law Gas in a cylinder under a piston Pressure, P, is given by Where m is the total mass of the piston and the lead and A is the area of the piston. We can: add or remove the lead shots to change the pressure of the gas; tune the temperature of the thermal reservoir.
7
The ideal gas law Doubling the temperature, number of molecules, pressure? Keeping the volume and the number of particles constant, but doubling the temperature? N is normally very big, while k is a very small number… N A = 6.022 10 23 – Avogadro number, number of molecules in 1 mol of a substance; n is the number of moles in the gas. R = 8.31 J/mol K universal gas constant
8
Kinetic theory of the ideal gas Kinetic energy is the only form of molecular energy that is important and it is preserved in the collision events.
9
L Collisions of the gas molecules with a wall.. As a result of a collision the momentum changes by Force due to one molecule as a function of time
10
Collisions of the gas molecules with a wall (cont.) Newton’s second law for an instantaneous force: Now t is a long time interval – the time between two consecutive collisions with the wall. For the average force on the wall it becomes: change of momentum in a collision time between collisions
11
Now t is – the time between two consecutive collisions with the wall. L
12
Kinetic theory of the ideal gas. Let’s try to account for all molecules of the gas:
13
L Kinetic theory of the ideal gas. Pressure on the wall with surface area A : Velocity of a molecule: The average velocity – average of a sum is equal to the sum of averages… All the directions of motion (x, y, z) are equally probable! V – volume of the box.
14
L Pressure on the wall with surface area A : The average kinetic energy of a molecule The ideal gas law (experimental fact!) Therefore:
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.