1 What are the two mechanisms of conduction heat transfer in

1) What are the two mechanisms of conduction heat transfer in metals ?

2) Explain how conduction of heat through gases occurs ??? and what system properties and conditions lead to higher or lower conductivities ???

please answer the questions not just putting whatever. Thanks

Solution

Heat is a transfer of energy from one object to another due to a difference in temperature. Temperature is a measure of the molecular energy in an object. Heat always flows from an object of higher temp (TH) to one of lower temp (TL). Molecules are in constant motion, their speed is proportionate to the temperature of the object. When two objects come in contact, their surface molecules will transfer momentum. An aluminum pot will conduct heat from a glass stove-top. Metals are good heat conductors due to the free electrons available.Heat transfer is energy per unit time = power.

For two objects at TH and the other at TL, connected by a rod of uniform material

P = kA(TH – TL)/L

Where k is the thermal conductivity of the rod, A is the cross-sectional area, and L is the length of the rod

Home owners are concerned with the “R-value” of their insulation

R = L/k

If left alone for sufficient time, both objects will come to thermal equilibrium. The smaller the value of k, the slower the heat transfer. Home insulation strives to maximize this transfer time (high R-value), allowing for a temperature gradient to exist longer.

Flow of heat through currents within a fluid (liquid or gas). Convection is the displacement of volumes of a substance in a liquid or gaseous phase. When a mass of a fluid is heated up, for example when it is in contact with a warmer surface, its molecules are carried away and scattered causing that the mass of that fluid becomes less dense. For this reason, the warmed mass will be displaced vertically and/or horizontally, while the colder and denser mass of fluid goes down the low-kinetic-energy molecules displace the molecules in high-kinetic-energy states. Through this process, the molecules of the hot fluid transfer heat continuously toward the volumes of the colder fluid.

For example, when heating up water on a stove, the volume of water at the bottom of the pot will be warmed up by conduction from the metallic bottom of the pot and its density decreases. Given that it gets lesser dense, it shifts upwards up to the surface of the volume of water and displaces the upper -colder and denser- mass of water downwards, to the bottom of the pot.

Formula of Convection:

q = hA (Ts - T )

Where h is for convective heat transfer coefficient, A is the area implied in the heat transfer process, Ts is for the temperature of the system and T is a reference temperature.