Danielle Scheidl Cooper:)

Heat transfer is the transition of thermal energy from a hotter object to a cooler object. When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium. Heat transfer always occurs from a higher-temperature object to a cooler temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed.

Heat transfer mechanisms can be grouped into 3 broad categories:

Conduction:

Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions, a process known as conduction. In metals, a significant portion of the transported thermal energy is also carried by conduction-band electrons. During conduction, energy is transferred by collisions between adjacent molecules. No matter is transported. The materials must be in contact with each other. Different materials conduct thermal energy at different rates. This property is called thermal conductivity. 

Convection:

When heat conducts into a static fluid it leads to a local volumetric expansion. As a result of gravity-induced pressure gradients, the expanded fluid parcel becomes buoyant and displaces, thereby transporting heat by fluid motion (i.e. convection) in addition to conduction. Such heat-induced fluid motion in initially static fluids is known as free convection. Thermal energy is transferred due to the motion of the substance containing thermal energy (examples:  water or air) Warmer matter rises because it is less dense. This motion can set up convection currents. Convection has more effect in gasses and liquid than in solids. 

Radiation:

All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. When temperatures are uniform, the radiative flux between objects is in equilibrium and no net thermal energy is exchanged. The balance is upset when temperatures are not uniform, and thermal energy is transported from surfaces of higher to surfaces of lower temperature. Electromagnetic radiation can transfer thermal energy – usually as infrared radiation. No medium such as air or water is required. Radiation can be reflected to reduce energy transfer. 
 

Thermal conductivity K indicates how easily heat is transferred through a substance. 

 
K  =  the thermal conductivity 

 

(J/s m oC or BTU inch/hour foot2 oF) 

A  =  the cross sectional area (meters2 or feet2) 

T  =  temperature (oC or oF) 

L  =  thickness (meters or inches) 

 

 

Using Insulation to Reduce Heat Flow.The effectiveness of insulation is called its R-value. The R-value is based on the insulation thickness and its thermal conductivity.

 

 
By substituting the above equation, we will get a new equation from the first equation,

The end. Thank you for reading. Sorry I'm late Mr Lim!!!!!