Heat Transfer, Conduction, Convection and Radiation

len Alfred Ajibola - Thu, 16th May, 2019 @ 08:12: AM

Topics in Physics

Density and Specific Gravity Force and Motion - Newton's Laws of Motion Motion, Speed, Velocity and Acceleration Heat Transfer, Conduction, Convection and Radiation Concept of Heat and Temperature Gravity explained with some fun facts Zeroth Law of Thermodynamics - Thermal Equilibrium Explanation, Similarities and Differences between Fundamental and Derived Units Physics - Basic and Fundamental Physics Concepts

Academic Questions in Physics

Please check out our Test Your Knowledge page to see all Questions and Answers

The Law of Inertia is also referred to as _____.

  • A. 1st Law of Thermodynamics
  • B. 2nd Law of Thermodynamics
  • C. 3rd Law of Thermodynamics
  • D. Newton's 1st Law of Motion
  • E. Newton's 2nd Law of Motion
  • F. Newton's 3rd Law of Motion

The process whereby the velocity of an object increases and decreases over a period of time is best associated with the term _____.

  • A. Speed
  • B. Velocity
  • C. Momentum
  • D. Constant Velocity
  • E. Acceleration
  • F. Average Velocity

The rate of motion is best referred to as _____.

  • A. Speed
  • B. Velocity
  • C. Momentum
  • D. Inertia
  • E. Acceleration
  • F. Constant Velocity

Consider this expression below:

A body X exerts a force FX on another body Y. In return, body Y exert a force FY back on body X. The forces FX and FY are equal in magnitude but opposite in direction.

Putting the above expression into equation:

FX = -FY

Where FX is the action force

FY is the reaction Force

The above expression relates to _____.

  • A. Theory of Force
  • B. Force Postulate
  • C. Einstein's proposal on Relativity
  • D. Newton's 1st law of motion
  • E. Newton's 2nd law of motion
  • F. Newton's 3rd law of motion

The tendency of a body to remain in a state of rest or in uniform motion is referred to as _____.

  • A. tension
  • B. uniform acceleration
  • C. constant acceleration
  • D. deceleration
  • E. inertia
  • F. velocity

A change in the position of an object with reference to time is best described as _____.

  • A. momentum
  • B. displacement
  • C. velocity
  • D. acceleration
  • E. direction
  • F. motion

The saying "what goes up on earth must come down" can be best attributed to _____.

  • A. force
  • B. motion
  • C. mass
  • D. acceleration
  • E. average velocity
  • F. constant velocity

We have _____ laws of Thermodynamics.

  • A. 1
  • B. 2
  • C. 3
  • D. 4
  • E. 5
  • D. 6

Heat Transfer:

Heat is a form of energy. It cannot be lost according to the First Law of Thermodynamics; rather it will be transferred.

The First and Second Laws of thermodynamics are stated below:

Energy can neither be created nor destroyed but can be transformed or converted from one form to another.

1 st Law of thermodynamics

In the course of energy transfer, some energy is lost in the form of heat to the surroundings.

2 nd Law of thermodynamics

Please read on Food Chain, Food Web and Food Pyramid to better understand both Laws.

Heat transfer indicates how heat flows between two systems of varying temperature.

Note: Heat will always be transferred from the region of higher temperature to the region of lower temperature.

Please read on the Concept of Heat and Temperature here.

To better understand how the processes of heat transfer are achieved, it will be great if we first understood how the particles of matter move.

Below is a quick explanation of the particulate nature of matter:

Matter is composed of particles which can either be:

1. Tightly packed together (as seen in Solids).

2. Loosely packed together (as seen in Liquids).

3. Completely separated (as seen in Gases).

Please read on Matter and the Characteristics of Solids, Liquids and Gases here.

With regards to solids (which have their constituent particles closely packed together), its particles will vibrate in a fixed position.

With regards to liquids (which have their constituent particles loosely packed together), its particles will rotate from one point to another.

With regards to gases, its particles will translate (exhibit a random motion) from one point to another throughout its container.

Note: The movement of the particles of matter is a function of the kinetic energy present in them. (Think of kinetic energy as energy in motion).

Please read on Motion and Newton's Laws of Motion here.

From the above explanations, we can therefore conclude that gaseous particles have got the highest kinetic energy while solid particles have got the least kinetic energy.

An increase in particles' vibrational, rotational or translational motion will imply an increase in temperature while a decrease in the particles motion (vibrational, rotational and translational) will mean a decrease in temperature.

We can thus define temperature as the measure of the average kinetic energy possessed by the particles of matter.

From the above explanations on the movement of particles, we can now properly explain the transfer of heat.


Methods of Heat Transfer

Heat can be transferred through:

  1. conduction
  2. convection
  3. radiation


  • Conduction

Let's explain heat transfer by conduction through this simple experiment which you may carry out at home.

Step 1: Boil 3 cups of water in a pot. When the water is very hot, put out the gas.

Step 2: Get another pot and fill it with 3 cups of cold water.

Step 3: Get an iron rod and make both ends of its edge contact both waters (the hot water and the cold water respectively).

Observation: After we carry out the above procedure, it will be observed that the temperature of the iron rod will increase; and heat will be transferred into the cold water from the iron rod.

Recall that iron is a good conductor of heat and electricity.

Please read on the Physical Properties of Metal here.

The movement of heat energy in the above experiment will follow the path below:

Hot Water >> Iron Rod >> Cold Water

Note: It is the transfer of heat energy that brought about an increased temperature in the iron rod and then, the cold water; nothing more, nothing less.

Below are some notable points concerning the experiment:

  1. Heat is the only factor that escaped from the warm water.
  2. Heat is the only factor that was gained by the iron rod.
  3. Heat is the only factor that was gained by the cold water.
  4. With regards to the iron rod, the heat gain will increase its particles vibrational kinetic energy.
  5. With regards to the cold water, the heat gain will increase its particle rotational kinetic energy.

This heat transfer will continue until equilibrium is achieved; (that is; the water inside both pots and the iron rod will eventually attain the same temperature).

Conduction is therefore the transfer of heat energy from one point to another through a solid material called a conductor


  • Convection

Convection is a process of heat transfer from one point to another through the movement of fluids.

A conductor or metal is NOT involved in convection.

Convection explains the transfer of heat in liquids and gases.

The boiling process of water occurs through convection. Below is an explanation on the processes involved in boiling:

Initially, the water molecules inside the pot (yet to be boiled) possesses equal kinetic energy.

When placed on the gas cooker, the water molecules closest to the bottom and margins of the pot begin to move or rotate as a result of heat transfer by conduction from the metallic pot. (This is so because the pot is a metal and a conductor). As a result, the water molecules (closest to the bottom and margins of the pot) becomes hot.

Due to the hotness of the water molecules (closest to the bottom and margins of the pot), they expand and become lighter, thus gaining an increased ability to move (an increase in their average kinetic energy). They move to the top and center of the pot, displacing the water molecules that were initially there to the bottom.

Note: The displacement of water molecules in the above paragraph is as a result of the difference in density of both water molecules. (Those closest to the bottom and margins of the pot versus those on the top/center)

Please read on Density and Specific Gravity here.

Heat Transfer by Convection - Len Academy
Image Credit: CK12

Note: The water molecules at the top and center of the pot have a higher density since they are farther away from the bottom and margins of the pot. Also, they have a lower kinetic energy when compared to those at the bottom and edges. They are therefore displaced downwards (since they have higher density and lower kinetic energy) by the water molecules at the bottom and margin of the pot which had now moved upwards and inwards.

The above process continues until the entire water molecules attain the same kinetic energy. At this point, equilibrium is said to be achieved.

If the water molecules are still left inside the pot, their increased kinetic energy will further make it's particles move into the atmosphere via translational motion. The pot will end up becoming empty because of the increased kinetic energy of the "now" gaseous particles. (The water had changed its state from liquid to gas by now).

Note: In a liquid, the heat energy produced by the particles of higher kinetic energy stimulates the liquid particles of lower kinetic energy (to an increased kinetic energy or rotational motion) until equilibrium is achieved.

It is important to understand that convection is the main method of heat transfer in water and air.

With regards to convection in air particles; if will put on the heater (assuming) it’s on the floor, the kinetic energy of the air particles around the floor region (closest to the heater) will become increased.

The particles of this "hot air" move upwards and sideways, mixing with the cooler air in these regions.

The net result of this rising "hot air" is the transfer of heat energy from a higher region to a lower region.

Note: Convection is achieved by the rotational and translational movement of matter. (Translational and Rotational movements are involved in gases and liquids respectively); which results in the transfer of heat energy.

Note: Conduction is NOT usually achieved by the vibration movement that is present in solids.


  • Radiation

The term radiate in English mean to “to spread out from a central point”. For instance, the light from the sun radiates to the earth. Also, a lighted bulb will radiate its light in a dark room.

One reason why some people put out their bulbs before they go out is to simply conserve energy. Leaving on the bulb will bring an extra heat into the room where it’s present.

The bulb will possess a large amount of heat energy (becomes hot) if left "switched on" for a long period of time.

Since energy cannot be lost, the heat energy from the bulb will be radiated around the room where it’s present.

The energy transfer goes thus:

Electrical Energy >> Light Energy >> Heat Energy

Note: In radiation, the heat energy is transferred by electromagnetic waves. In fact, the heat energy which the earth receives from the sun is as a result of electromagnetic waves travelling through space.

Please read on the Solar System here.

Heat Transfer by Radiation - Len Academy
Image Credit: Siyavula

All objects (televisions, phones, phones, computer and any other object) radiate energy in the form of electromagnetic waves. The rate at which this energy is released is directly proportional to the Kelvin temperature (T) raised to the fourth power.

Radiation rate = k•T4

The hotter an object is, the more its ability to radiate heat energy.

The radiated energy is a collection of a range of wavelength referred to as emission spectrum.

The wavelengths within the spectra of the emitted radiation will decrease when the temperature of an object increases; and as a result, hotter objects will usually emit a shorter wavelength at a higher frequency.

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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