Heat Transfer
There are 3 methods of transferring heat to or from a system.
Convection is heat transfer due to mass transfer. Warmer and less dense liquid or gas rises while denser liquid or gas replaces the rising liquid or gas. Convection can be natural or forced.
Why do we use baseboard heaters?
Why is the cooling coil in a refrigerator placed at the top of the refrigerator?
Describe boiling water.
Conduction is the process of heat transfer Conduction is the result of molecular interactions. Higher energy molecules (higher temperature & faster vibration) collide with and transfer energy to low energy molecules (lower temperature & less vibration).
Thermal Conductors are good conductors of heat. Metals typically have large numbers of free electrons that are responsible for good transfer of heat.
Thermal Insulators are poor conductors of heat. Gas molecules are far apart thus making collisions less frequent resulting in poor transfer of heat.
How much heat, Q, is transferred due to conduction in a material depends on the following factors.
Thermal Conductivity. Some materials are better conductors than others.
Elapsed Time. More heat is transferred during longer periods of time. Q µ t
Temperature Difference in the material. If one end of a metal bar has a high temperature and the other end has a low temperature then conduction will occur in the bar. More heat will be transferred if the temperature difference is greater. Q µ ΔT
Cross Sectional Area. Twice as much heat would be able to flow through 2 metal bars than through 1 metal bar. Q µ A
Length. If the metal bar is longer the heat has to travel a much greater distance to go from the high temperature end to the low temperature end. Q µ 1 / L
ΔEInternal = Heat = Q
Heat = (Thermal Conductivity) · Cross Sectional Area · ΔTemp· Time / Length
Q = ( k A ΔT t ) / L
Why dress in layers?
If air is such a good insulator why not insulate the walls in our house with air?
Radiation is the transfer of heat by electromagnetic waves. All objects radiate energy in the form of electromagnetic waves. While all objects emit electromagnetic waves, objects don't typically emit sufficient numbers of visible electromagnetic waves to be seen until they reach high temperatures. How good an object is at emitting or absorbing radiation is a characteristic of the material. Typically a good emitter is also a good absorber.
A Perfect Blackbody is an object that absorbs all the electromagnetic waves that fall on it.
Emissivity is a unitless number between 0 and 1 that is a characteristic of the material. Dark surfaces have emmisivities close to 1 and shiny surfaces have emmisivities close to 0. A perfect blackbody has an emissivity of 1.
If an object is in thermal equilibrium with its surroundings the object must be emitting the same amount of radiant energy that it is absorbing.
How much heat, Q, is transferred due to radiation depends on the following factors.
Emissivity: Q µ е
Elapsed Time. More heat is transferred during longer periods of time. Q µ t
Surface Area. An object with twice as surface area would radiate twice as much heat. Q µ A
Temperature. An object with a higher temperature emits more radiant energy. Q µ T4
ΔEInternal = Heat = Q
Stefan-Boltzmann Law of Radiation
Heat = Emissivity · (Stefan-Boltzmann Constant) · Temp4 · Surface Area · Time
Q = е σ T4 A t