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Heat Transfer is the movement of heat from a body at a higher temperature to a body at a lower temperature. Every matter around us consists of subatomic particles whose movement produces thermal energy. This movement causes the transfer of heat to occur from a high-temperature region to a lower-temperature region.
This thermal energy is heat transfer and the study of heat transfer and related properties like energy, and work is called Thermodynamics. Heat can travel from one place to another in many different ways. The several modes of heat transfer are:
- Conduction
- Convection
- Radiation
In case the temperature difference exists between any two given systems, heat can then transfer from the higher to the lower system. Heat is known to derive its origins at the molecular scale. The molecules of a substance can be observed to vibrate in its definite positions, regardless of being fixed or not, when energy is supplied. When the molecules vibrate, the energy is transferred to the surrounding molecules, causing their vibration.
Read More: Thermodynamics
| Table of Content |
Key Terms: Heat Transfer, Conduction, Convection, Radiation, Thermodynamics, Temperature, Joule, Calorie
What is Heat Transfer?
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Heat Transfer occurs when there is a:
| “Movement of heat energy across a well-defined boundary due to a difference in temperature between the system and its surroundings.” |
The heat flow rate intensity is also called Flux. Heat Transfer is classified into three mechanisms:
- Conduction
- Convection
- Radiation
Conduction usually takes place in solids, convection in liquids and gases and radiation does not require any kind of medium. Heat will continue to transfer from a higher system to a lower system until a temperature difference exists or until the thermal balance is restored.
What is Heat?
Heat is defined as a form of energy. Heat typically derives its origins at the molecular scale.
- A substance’s molecules vibrate at their positions, either when fixed or not, after energy is supplied to them.
- When they begin to vibrate, the energy is transferred to the surrounding molecules, which causes it vibration as well.
- This kinetic energy starts to build at the macro level as more and more energy starts to be supplied to the molecules of the substance.
- Thus, as a result, when it reaches the threshold (examples include the Melting point, Boiling point), the atoms or molecules release themselves from interatomic forces of attraction and conversion of a state, which is, a phase change occurs.
- The heat energy of a body is simply a form of energy which is transferred from one body to another, or within the body itself, alongside a difference in temperature.
- The SI unit of heat energy is Joule.
- As per the CGS system, heat can be measured in ‘Calorie’ (Cal.) wherein, 1 Calorie = 4.186 J.
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What is Conduction?
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Conduction is defined as the transfer of energy between objects that are physically connected.
- Heat flow in conduction occurs from objects with higher temperatures to objects with lower temperatures due to molecular collisions.
- The molecules nearer to the heat source gain heat and their average kinetic energy increases.
- As these molecules collide with neighbouring molecules on the colder side having less kinetic energy, the energy is shared between these two groups, and thus the kinetic energy of the latter increases resulting in the transfer of heat.
Conduction Equation
Rate of Thermal Conduction:
- The high-speed particles clash with particles moving at a slow speed increasing the kinetic energies of slow-speed particles.
- It is shown through the coefficient of thermal conductivity that a metal body conducts heat better.
- The rate of thermal conduction can be calculated by the following equation:
| \(\begin{array}{l}Q = \frac{[K.A.(T_{hot}-T_{cold})]}{d} \end{array}\) |
Where,
- Q = the transfer of heat per unit time
- K = the thermal conductivity of the body
- A = the area of heat transfer
- Thot = the temperature of the hot region
- Tcold = the temperature of the cold region
- d = the thickness of the body
Examples of Conduction
Some of the Conduction examples include:
- The melting of an ice cube when held in the hand due to heat transfer from our hands to the ice cube.
- The ironing of clothes causes the physical transfer of heat from the iron to the clothes.
- When an iron rod is held over a stove or fire, one of the ends gets hotter first and subsequently, the whole rod becomes hot by the conduction process.
Read More: Thermodynamics Laws
What is Convection?
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Convection is defined as the transfer of energy between an object and its environment, due to fluid motion.
- The movement of fluid molecules i.e. the actual motion of heated materials from higher temperature regions to lower temperature regions causes the gradual transfer of heat.
- If the heated material is forced to move, it is called forced convection.
- If the material moves due to a difference in density, it is called natural or free convection.
Convection Equation
Rate of Thermal Convection:
- The increase in temperature of any kind of matter causes the volume of that particular matter to increase by the same factor and this effect is known as displacement.
- The rate of thermal convection can be measured by the following equation:
| Q = hc A(Ts – Tf) |
Where,
- Q is the heat transferred per unit time
- hc is the coefficient of convective heat transfer
- A is the area of heat transfer
- Ts is the surface temperature
- Tf is the fluid temperature
Examples of Convection
Some examples of Convection include:
- Boiling of water: The heated water at the bottom becomes lighter and moves upward. The denser colder water moves to the bottom resulting in the circular motion of the molecules so that water gets heated.
- Hot air blower: The air is heated by a heating machine and is blown by the fan. The hot air carries the heat with it when it moves and subsequently heats the surrounding with it.
- Body temperature in Warm-Blooded Animals: The body temperature is maintained by blood circulation which in turn takes place with the help of convection.
Read More: Thermodynamics Important Questions
What is Radiation?
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Radiation is defined as the transfer of energy by the emission of electromagnetic radiation.
- In radiation, the waves carry away the energy from the heat-emitting body.
- It takes place through a vacuum or transparent medium which can be either solid or liquid.
- The random movement of molecules in a matter causes thermal radiation.
Radiation Equation
Rate of Thermal Radiation:
- All bodies constantly emit energy in the form of electromagnetic waves, called radiant energy.
- If a body is colder than the surrounding environment then it would absorb part of the thermal radiation falling on them and vice versa.
- If that body radiates more energy than it absorbs then the temperature decreases.
- With the rise of temperature, the wavelengths in the spectra of the radiation emitted decrease, and shorter wavelengths of radiation are emitted.
- The rate of thermal radiation can be calculated by the following equation:
| P = e σ A(Tr – Tc)4 |
Where,
- P is the net power of radiation
- A is the area of radiation
- Tr is the radiator temperature
- Tc is the surrounding temperature
- e is emissivity
- σ is Stefan’s constant
Examples of Radiation
Some examples of radiation include:
- Microwave radiation emitted in an oven
- UV rays coming from the sun
- Decay of Uranium-238 into Thorium-234- the release of alpha particles
Heat Transfer Infographic
Newton’s Law of CoolingNewton’s law of Cooling describes a direct correlation between a body’s rate of change of temperature and the temperature difference between the object and its surroundings.
Newton's Law of Cooling formula can be denoted by, T(t) = Ts + (To – Ts) e-kt Here,
|
Read Also: Blackbody Radiation
Unit of Heat Transfer
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There are different units of heat transfer used based on their applications. Listed below are the most commonly used units of measurement.
| System Name | Unit Name |
|---|---|
| SI system | Joule |
| FPS system | Cal(calorie) / Kcal |
| MKS/CGS system | Btu (British thermal unit) |
| Rate of transfer of heat | kW (kilowatt) |
Read More: Coefficient of Linear Expansion
Things to Remember
- Heat Transfer occurs from a body at higher temperature to a body at lower temperature.
- Heat Transfer follows three different mechanisms: Conduction, Convection and Radiation.
- Conduction heat transfer occurs when the objects are physically connected.
- Convection heat transfer occurs by fluid motion.
- Radiation heat transfer occurs through electromagnetic radiation.
- Joules or Calories are the most commonly used units of heat transfer measurement.
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Previous Year Questions
- Calculate the rate of increase in thickness of the ice layer. [NEET 2019]
- Calculate the temperature of the sink of the Carnot engine. [UPSEE 2019]
- Calculate the wavelength of the blackbody at 3000 K. [NEET 1989]
- Which is the right correlation between the radiative emission and temperature? [KEAM 2004]
- Calculate the thermal conductivity of the metal rod. [AP EAPCET]
- Calculate the radius ratios of two spherical black bodies. [MH CET 2017]
- What is the rate of heat radiated by the black body? [NEET]
- Calculate the heat transfer through the rod at a steady state. [NEET 2009]
- Calculate the wavelength corresponding to maximum energy at 3000K. [NEET 2001]
- The ratio of heat loss from Body A to Body B is? [BITSAT 2019]
- The SI unit and dimensions of Stefan’s Constant are? [MH CET 2019]
- What is the rate of heat flow through two cylindrical rods? [NEET 1995]
- What is the rate of heat flow through the metal rod? [NEET 2015]
- A sphere, cube, and a thin circular plate, give the order of their cooling. [KCET 2020]
- What fraction of heat energy is converted to work? [KCET 2020]
Sample Questions
Ques. How is electromagnetic radiation emitted? (1 mark)
Ans. Due to the continuous movement of electrons and protons in an atom, electromagnetic radiation is emitted.
Ques. List one example of radiation. (1 mark)
Ans. One example of Radiation includes Microwave radiation emitted in an oven.
Ques. What is an application of a cross-flow heat exchanger? (1 mark)
Ans. It is specially used for a gas-liquid system to heat or cool the gas.
Ques. What is the driving factor for heat transfer? (1 mark)
Ans. The temperature difference is the primary factor for the exchange of heat naturally.
Ques. What is meant by the heat duty of a heat exchanger? (1 mark)
Ans. It is defined as the heat that is transferred from hot fluid to cold fluid per unit of time.
Ques. What is Newton’s law of cooling? (2 marks)
Ans. According to this law, the heat flux (i.e. heat flow per unit area) is equal to the temperature difference between wall temperature and fluid temperature.
Ques. What is the difference between an equilibrium state and a steady state? (2 marks)
Ans. Where properties are uniform throughout and they do not vary with time is called an equilibrium state. And, where properties are not changing at a point or a specified location and do not vary with time is called a steady state.
Ques. A house is cooled by an electric heat pump by using the outside as the high-temperature reservoir. Calculate the present savings in electricity for several different outdoor temperatures if the house is kept at 77 F instead of 68 F. Assume that the house is gaining energy from the outside which is directly proportional to the temperature difference. (3 marks)
Ans. Air conditioner (Refrigerator)
Ques: A house is cooled by an electric heat pump with the help of a high-temperature reservoir. For several different outdoor temperatures, find out the percent savings in electricity if the house is kept at 25. instead of 20. Assume that from the outside which is directly proportional to the temperature difference. (5 marks)
Ans. Air conditioner (Refrigerator)
A: TLA = 20°C= 293. 2 KB: TLB = 25°C = 298. 2 K
Ques: A car engine operates with a thermal efficiency of 35%. Assume that the air conditioner has a coefficient of performance that is one third of the theoretical maximum and it is mechanically pulled by the engine. How much fuel energy should you spend extra to remove 1 Btu at 60 F when the ambient is at 95 F? (5 marks)
Ans: Air conditioner
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