1. Conduction:
Conduction transfers heat via direct molecular collision. An area of greater kinetic energy will
transfer thermal energy to an area with lower kinetic energy. Higher-speed particles will collide
with slower speed particles. The slower-speed particles will increase in kinetic energy as a result.
Conduction is the most common form of heat transfer and occurs via physical contact. Examples
would be to place your hand against a window or place metal into an open flame. The process of
heat conduction depends on the following factors: temperature gradient, cross-section of the
material, length of the travel path, and physical material properties. The temperature gradient is
the physical quantity that describes the direction and rate of heat travel. Temperature flow will
always occur from hottest to coldest or, as stated before, higher to lower kinetic energy. Once
there’s thermal equilibrium between the two temperature differences, the thermal transfer stops.
Cross-section and path of travel both play an important part in conduction. The greater the size
and length of an object, the more energy that’s required to heat it. And the greater the surface
area that’s exposed, the more heat is lost. Smaller objects with small cross-sections have minimal
heat loss.
Physical properties determine which materials transfer heat better than others. Specifically, the
thermal conductivity coefficient dictates that a metal material will conduct heat better than cloth
when it comes to conduction. The following equation calculates the rate of conduction:
Q = [k ∙ A ∙ (Thot – Tcold)]/d
where Q = heat transferred per unit time; k = thermal conductivity of the barrier; A = heat-
transfer area; Thot = temperature of the hot region; Tcold = temperature of the cold region; and d =
thickness of the barrier.

2. A modern of use of conduction is being developed by Dr. Gyung-Min Choi at the University of
Illinois. Dr. Choi uses spin current to generate spin transfer torque. Spin transfer torque is the
transfer of the spin angular momentum generated by the conduction electrons to the
magnetization of a ferromagnet. Instead of using magnetic fields, this allows the manipulation of
nanomagnets with spin currents. (Courtesy of Alex Jerez, Imaging Technology Group, The
Beckman Institute).
Types of Conduction:
1. Electric Conduction:
Electric conduction refers to the ability of a material to transfer an electric current. Conductivity
is determined by how dense an object is compared to the strength of the electric field it can
maintain. Metals are substances with a high level of conductivity (also known as a conductor)
since they display minimal resistance to an electrical charge. Insulators, such as glass, are
materials that are resistant to electrical charges. Televisions, radios and computers are examples
of inventions that rely on the current provided by electric conduction.

3. 2. Heat Conduction:
Where electric conduction refers to a transfer or electric current, heat conduction refers to a
transfer of energy, specifically thermal energy. Heat conduction is sometimes called thermal
conduction. The energy is transferred within a stationary object as a result of a change in
temperature in parts of a material adjacent to one another. The energy will move quickly or
slowly depending on what the object is made of, how large it is and, most importantly, the
temperature gradient. Temperature gradient refers to the rate and direction in which the
temperature changes from a specific point to another point. Diamonds and copper are materials
with high thermal conductivity.
3. Photoconductivity:
Photoconductivity occurs when a material absorbs electromagnetic radiation, resulting in a
change in the substance's electrical conductivity. The electromagnetic radiation can be caused by
something as simple as a light shining on a semiconductor or something as complex as a material
being exposed to gamma radiation. When the electromagnetic event occurs, the number of free
electrons increases, as does the number of electron holes, thus increasing the object's electrical
conductivity. Common applications of photoconductivity include copy machines, solar panels
and infrared detection equipment.

4. Convection:
When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries
the thermal energy along. This type of heat transfer is called convection. The fluid above a hot
surface expands, becomes less dense, and rises.At the molecular level, the molecules expand
upon introduction of thermal energy. As temperature of the given fluid mass increases, the
volume of the fluid must increase by same factor. This effect on the fluid causes displacement.
As the immediate hot air rises, it pushes denser, colder air down. This series of events represents
how convection currents are formed. The equation for convection rates is calculated as follows:
Q = hc ∙ A ∙ (Ts – Tf)
where Q = heat transferred per unit time; hc = convective heat transfer coefficient; A = heat-
transfer area of the surface; Ts = temperature of the surface; and Tf = temperature of the fluid.A
space heater is a classic convection example. As the space heater heats the air surrounding it near
the floor, the air will increase in temperature, expand, and rise to the top of the room. This forces
down the cooler air so that it becomes heated, thus creating a convection current.

5. Types of Convection
 Natural Convection:
Natural convection is a method of heat transfer in which natural means influence the motion of
the fluid. There is no influence from external facts. This movement of molecules in the fluid is
due to the differences between densities of different regions of the same fluid. The density of a
fluid decreases when it heats and vice versa. That is because of the thermal expansion of the fluid
(the speed of molecules increase with the temperature increase, which results in the increase of
the volume of the fluid. Although the volume increases, the mass remains constant. Therefore the
density decreases).When we heat a fluid in a container from its bottom, the density of the bottom
layer of the fluid decreases. Then the lower density region tends to move to the top of the
container. Then the cooler fluid at the top of the container replaces the bottom region. This
continues, as a result, convection occurs.
Examples of natural convection include cooling down a boiled egg when kept in the normal air,
loss of cool of a cool drink can, etc. When considering the mechanism of natural convection,
first, the temperature of the outside of a hot object (kept in cold air) drops down. At the same
time, the temperature of the air adjacent to the object will rise due to heat transfer. Then the
density of this adjacent layer of air decreases. As a result, the air rises upward. Cold air will
replace this region. Then the convection continues. In the end, the object will cool down.
 Forced Convection:
Forced convection is a method of heat transfer in which external means influence the motion of
the fluid. There, external sources such as pumping, fans, suction devices, etc. are useful in
generating the fluid motion. This method is very valuable because it can efficiently transfer heat
from a heated object. Some common examples of this mechanism include air conditioning, steam
turbines, etc.When considering the mechanism of forced convection, it is has a complicated
mechanism than the natural way. That is because, in this method, we have to regulate two
factors; fluid motion and heat conduction. These two factors have a strong connection since the

6. fluid motion can enhance the heat transfer. Ex: higher the rate of motion of the fluid, higher the
heat transfers

7. Radiation
Thermal radiation generates from the emission of electromagnetic waves. These waves carry the
energy away from the emitting object. Radiation occurs through a vacuum or any transparent
medium (either solid or fluid). Thermal radiation is the direct result of random movements of
atoms and molecules in matter. Movement of the charged protons and electrons results in the
emission of electromagnetic radiation.
All materials radiate thermal energy based on their temperature. The hotter an object, the more it
will radiate. The sun is a clear example of heat radiation that transfers heat across the solar
system. At normal room temperatures, objects radiate as infrared waves. The temperature of the
object affects the wavelength and frequency of the radiated waves. As temperature increases, the
wavelengths within the spectra of the emitted radiation decrease and emit shorter wavelengths
with higher-frequency radiation. Thermal radiation is calculated by using the Stefan-Boltzmann
law:
P = e ∙ σ ∙ A· (Tr
4 – Tc
4)
where P = net radiated power; A = radiating area; Tr = temperature of the radiator; Tc =
temperature of surroundings; e = emissivity; and σ = Stefan’s constant.Emissivity for an ideal
radiator has a value of 1. Common materials have lower emissivity values. Anodized aluminum
has an emissivity value of 0.9 while copper’s is 0.04.

8. Solar cell or photovoltaic cell, converts the energy of light into electricity via the photovoltaic
effect. Light is absorbed and excites the electrcon to a higher energy state and the electric
potential is produced by the separation of charges. Efficiency of solar panels has risen in recent
years. In fact, those currently being produced by SolarCity, a company co-founded by Elon
Musk, are at 22%.
 Black Body Radiation
A black body or blackbody is an idealized physical body that absorbs all incidentelectromagnetic
radiation, regardless of frequency or angle of incidence. (It does not only absorb radiation; It can
also emit radiation.
 Grey Bodies Radiation
Grey Bodies Heat Exchange. The easiest method to calculate radiative heat transfer between
two bodiesis when they are assumed to be black bodies. ... While J represents the radiosity which
is the total amount of radiation that is reflected off a surface per unit time and unit area.