What is Heat or Energy Transfer? 

When energy travels from one object to another it is said to have undergone heat transfer. Heat transfer is nothing but the transfer of thermal energy or internal energy from one thing to another, like e.g., when a vessel with water, kept on top of a burner, heats up because of the flame underneath it.  

The one unwavering fact about heat is that it always flows from the hot to the cold and never in reverse. There is a minute but important difference between the concepts of temperature, internal energy and heat. Temperature is a measure of how much kinetic energy an object holds on average. It can also be used to determine the direction of heat transfer as hot always flows into the cold. Internal Energy is the total energy of all the particles inside an object, a total of both kinetic and potential energies. Heat is the quantity of energy that is transferred from one object to another due to the differences in their temperatures. 

Heat transfer between objects will only occur as long as there is a difference between their temperatures. When the temperatures become the same for both objects and no more heat flows from one to the other, they will be said to have reached Thermal Equilibrium. 

This transfer of heat between objects or bodies can happen in three different ways — Conduction, Convection & Radiation. Each is different and has its characteristics depending upon the nature of objects involved in the heat dissipation or transfer. 

Conduction 

Conduction is a form of energy transfer that occurs when the mechanical energy of molecules gets transferred from higher speed to lower speed molecules due to their motion & vibration. This type of heat transfer takes place by physical contact between the objects involved. Various factors determine the rate at which this transfer takes place including but not limited to, the physical properties of the material, size and shape, gradient, etc.  

Think of a cold day in winter with the heater on, keeping your house and body very warm. Now, if you step outside and keep your hand on the railing, heat will flow from your hand to the railing via conduction as long as there is surface to surface contact. It is also conduction due to which handles of your cooking vessels heat up even when it is the bottoms of the pans that are in contact with the flame of the stove.  

Conducting Materials 

Theoretically, any material can transfer energy via conduction but only materials that have molecules closely packed together can effectively conduct heat or energy. This is because if the molecules are far apart then there is a much lesser chance of them colliding or transferring vibrations and hence enabling conduction. However, if the molecules are very densely boxed in then conduction occurs much faster. This is why solids are much better than liquids when it comes to conduction and gases can very rarely conduct, if at all. Those objects which allow easy passage of heat through them are called conductors. 

How does Conduction take place? 

There are two ways by which conduction of heat takes place: 

1. The vibration of the atoms—due to increased thermal agitation of molecules this energy is transferred to nearby atoms via inter-atomic bonds. 
2. Movement of electrons—when electrons collide with each other in their agitated state, energy is transferred from higher speed ones to ones with lower speed. 

Please note that all these vibrations and movements that happen during conduction are only on a microscopic level, the actual objects undergoing heat transfer are stationary and not moving. 

Convection 

Convection is what happens when a fluid (like water or air) undergoes heat transfer. Fluids, when they are heated undergo expansion at the molecular level, thus causing the molecules nearest the heat source to become less dense than their surroundings. This then causes the colder molecules around to drop down (due to gravity) while allowing the hotter molecules to rise. The continuous movement of the particles causes heat to be transferred via the actual movement of the fluid as it expands and rises.  

Convection can be both very slow and extremely fast. Water currents in an ocean can be observed as an example of slow convection while a hurricane or a microwave oven is a good example of rapid or fast convection. Gases and liquids both have low thermal conductivity but higher convection as fluids, in general, have molecules that can move quite freely and rise to the surface unlike in solids.  

Free vs. Forced Convection 

Natural convection or free convection as it is sometimes called occurs without the involvement of external factors. The movement of the fluid is due to the density difference between regions in the liquid itself. When heat is applied to a fluid, the increased speed of molecules causes the volume of the fluid to expand, thereby reducing the density of the fluid and causing the buoyancy effect.  

Forced convection is when the fluid is influenced by external factors like a pump or fan to flow over surfaces or through tubes for specific results.  

Radiation 

There is another way by which energy is transferred between objects which happens even without the presence of any medium through which the heat transfer takes place. Radiation is called so because it happens by way of electromagnetic waves (mostly in the infrared spectrum) which are radiated by an object that is at a higher temperature than its surroundings. The most prominent example of radiation is the energy from the sun, which reaches across space and vacuum in the form of electromagnetic radiation and heats the objects that come into contact with it. 

Radiation is unique as it is the only type of energy transfer that obeys the Laws of Reflection & Refraction. Surface area, color, shape, and size are big factors in the rate of transmission of energy in the process of heat transfer by radiation. Radiation is also the release of energy in the form of high-energy sub-atomic particles which cause ionization.  

The Stefan Boltzmann law draws a relation between the rate of emission of energy of a black body and its temperature, per unit surface area and per unit time. This also explains why dark objects absorb and emit radiation faster as compared to light-colored objects. 

Table showing differences between the three major forms of energy transmission: 

Common Mistakes

• Since nature has objects in varied and mixed states, there may be more than one form of heat transfer that takes place between a set of objects. 
• The rate of transmission of energy is rarely ever constant and shouldn’t be assumed to be so. 

Related Concepts 

• Thermodynamics 
• Heat Transfer 
• Molecular Energy 
• Mass and Density 

Context and Applications 

This topic is mostly studied in the 11^th, 12^th grades as well as in 

• Bachelors in Science (Physics) 
• Bachelors in Technology (Mechanical Engineering)