Heat Transfer

By GreatQuick Team

Through this article, you are able to get a wide knowledge about the following points.

  • What is heat, differences between heat and temperature, thermal expansion, heat and state of matter
  • What is heat transfer, conductors of heat and insulations
  • Heat transfer and laws of thermodynamics
  • Why does ice melt in water, heat absorption and heat pollution
  • A small introduction about modes of heat transfer

Introduction

Heat is the random kinetic energy of the atoms, molecules, or ions in a substance or body. It is restricted to energy being transferred. Energy stored in a body is not heat. Absorption of energy by a system as heat takes the form of increased kinetic energy of its molecules, thus resulting in an increase in temperature of the system. Heat can be converted into work.


Heat can be measured by the amount of ice melted, or by change in temperature of the system. Such methods are known as “Calorimetry”.


Heat is a form of energy and temperature is a measure of the amount of that energy present in a body. It transfers from one body to another as a result of a difference in temperature. If two objects have different temperatures, they are not in thermal equilibrium with each other. If they are at the same temperature, they can be named as objects in thermal equilibrium. Higher temperature means faster-moving molecules in a substance.

Followings are differences between heat and temperature.

  • Heat is the degree of hotness and coldness of a body. But the temperature is the measurement of the thermal energy or average heat of the molecules in a substance.
  • Heat flows from hot body to cold body. But temperature rises when heated and falls when cooled.
  • Energy of heat is the total kinetic and potential energy contained by molecules in an object. But energy in temperature is the average kinetic energy of molecules in a substance.

Heat is denoted by “Q” and measured by “Calorimeter”. The SI unit is “Joules”.

Temperature is dented by “T” and measured by “Thermometer”. The SI unit is “Kelvin”.

When most substances are heated, they expand. The amount they expand depends on the material and temperature. That means the molecules speed up and tend to move farther apart. This means things that are cooled slowly down their particles, which get closer…causing it to contract or shrink. At higher temperatures, molecules have more kinetic energy and on average move farther apart from each other. This means the substance expands.

A substance may absorb heat without an increase in temperature, by changing from one physical state to another. Relationships exist between the amounts of heat added to or removed from a body and the magnitude of the effects on the state of the body.

  • Solids have vibrating particles, not moving freely
  • Add heat and the solids melt to liquids
  • Liquids have less restricted moving particles
  • Add heat and the liquid evaporates

The opposite of the above is true when you remove heat, from gas to liquid to solid.

Latent heat:

The amount of heat energy required to change the state of 1 gram of substance.

Heat of fusion: Latent for changes between the solid and liquid phases.
Heat of vaporization: Latent heat for changes between the liquid and gas phases.

What is heat transfer?

When there is a suitable path between two systems with different temperatures, heat transfer occurs necessarily, immediately and spontaneously from the hotter to the colder system. If two bodies at different temperatures are brought together, energy is transferred.

The definition of heat transfer does not require that the process be in any sense smooth.

Example: A belt of lightning may transfer heat to a body.

Heat transfer is also takes place within the system due to temperature difference at various points inside the system. At the thermodynamic equilibrium heat transfer is zero.

There are processes in which there is no transfer of heat between the system and the surrounding. Such processes are known as “adiabatic processes”. The wall or boundary which does not allows the flow of the heat between the system and the surrounding is called as “adiabatic wall” and the wall that allows the flow of heat between the system and the surrounding is called “diathermic wall”.

Conductors of heat:

Conductors are the materials which transfer heat. There are two types of conductors of heat.

  • Good conductors of heat
    Examples: Most metals
  • Poor conductors of heat
    Examples: Wood, plastics, glass, gases

Insulations:

Insulations are used to reduce the amount of heat loss by conduction.

  • Poor conductors are good insulators
  • Materials that tarp air are good insulators

Heat transfer and laws of thermodynamics

In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. According to the thermodynamics, heat transfer is the movement of heat across the boundary of the system due to the temperature difference between the system and the surroundings.

In thermodynamics, energy transferred as heat contributes to change in the system’s cardinal energy variable of state.

If you want to transfer heat from low temperature to high temperature body, external work has to be done.

Heat transfer and zeroth law of thermodynamics

Zeroth law of thermodynamics: If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other.

Example:
You can follow the following steps to observe the zeroth law of thermodynamics.

  • Take two objects of which have two different temperatures and a thermometer. Name the two objects as A and B and the thermometer as C.
  • The thermometer is placed in contact with A until they achieve thermal equilibrium.
  • The reading on C is recorded.
  • The thermometer is then placed in contact with object B until they achieve thermal equilibrium.
  • The reading on C is recorded again.
  • If two readings are the same, A and B are also in thermal equilibrium.

The zeroth law of thermodynamics introduces the concept of the thermodynamic equilibrium in which two objects have the same temperature. If we bring two objects which are not at same temperatures in to physical contact, then they achieve thermal equilibrium. During this process, heat is transferred between the 0bjects.

Q α dТ
Q = C dT

Q = the amount of heat transfer
dT = temperature difference between objects
C = heat capacity of the object

Heat transfer and first law of thermodynamics

First law of thermodynamics: Energy neither be created nor be destroyed within a process. Only thing that can be done is conversion of one energy form to another energy form.

That is the law of conservation of energy. That means the amount of energy in the universe is remains constant.

Whenever heat flows in to or out of a system, the gain or loss of the thermal energy is equal to the amount of heat transferred. That means the total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it.

dU = W + Q

dU = change in the thermal energy of the system
W = work done on the system (W = Fd or W = dK)
Q = heat added to the system
(Q is + if absorbed, Q is – if released)
All measured in Joules

Applications of first law of thermodynamics with heat are as follows.

  • Heat engines
  • Refrigerators and air conditioning units
  • Heat Pumps
  • Internal Combustion Engines
  1. Heat engines
    Heat engines convert thermal energy to mechanical energy. They reduce an existing temperature difference to another system. That means they require high temperature source and low temperature heat to sink. That is they take the advantage of heat transfer process.

    Heat engines are operated in a cycle, which means that the working substance (water + steam), is returned to its original state at the end of the cycle. Then the change in internal energy is zero.

    Heat engines have three essential features. They are as follows.
  • Heat is supplied to the engine at a relative high temperature; hot reservoir.
  • Part of the input heat is used to perform work by the working substance of the engine, which is the material within the engine that actually does the work.
  • The remainder of the input heat is reject at a temperature lower than the input temperature to a place called cold reservoir.

Examples: Steam engine, Automobile engine

  1. Refrigerators and Air conditioning units
    It is possible to remove heat from a cold environment and deposit it in to a warmer environment. This requires an outside source of energy. A refrigerator pumps heat from the cooler interior to the warmer room.
    An air conditioner uses a coolant to move thermal energy to the outside air.
  2. Heat pumps
    Although heat flows spontaneously from a hotter body to a cooler body, it is possible to construct a heat pump.
    A heat pump expends work to transfer energy from a cooler body to a hotter body. It can removes heat from the outside air and pumps it in to the warmer house.
    Heat pumps are refrigeration units that can work in either direction.4.

4.Internal Combustion Engines
Internal combustion engines are heat engines in which fuel is burnt in a combustion chamber inside the engine.

Many machines including cars, boats, lawn mowers, and airplanes use internal combustion engines.

Most modern cars have fuel-injected internal combustion engines that have a four-stroke combustion cycle.

Thermal energy is converted in to mechanical energy as gasoline is burnt under pressure inside cylinders.

Four-Stroke Cycle:

Heat transfer and second law of thermodynamics

Second law of thermodynamics: Natural processes tend to increase the total entropy (disorder) of the universe.

Kelvin’s statement:
No engine, working in a continuous cycle, can take heat from a reservoir at a single temperature and convert that heat completely to work.

Claudius’s statement:
Heat will never flow from a colder body to a hotter body unless some other process is also involved.

According to the second law of thermodynamics, the entropy of the universe or of an isolated system can only increase or remain constant. It will never decrease. Any process will tend to increase the amount of entropy in the universe. Entropy is a measure of the disorder of a system.

If heat is added, entropy is increased. If heat is removed entropy is decreased. Work with no heat transfer, entropy is unchanged.

As per the second law of thermodynamics, heat transfer from the body of high temperature to the body of low temperature. There won’t be spontaneous transfer of heat from body at low temperature to the body at high temperature. An external work has to be done to transfer heat from the body at low temperature to body at high temperature.

For the mathematical calculations,

  • The heat gained by the system is or body is considered to be positive
  • The heat lost by the system is considered to be negative

That means, heat flowing in to the system is positive and heat flowing out of the system is negative.

Major application of second law of thermodynamics is observed in the field of HVAC where the flow of heat is against its natural tendency.
In addition to that its applications are in,

  • Refrigerators
  • Cold storages
  • Water chilling plants
  • Ice melting plants

Heat transfer and third law of thermodynamics

Third law of thermodynamics: The entropy of a perfect crystal is zero when the temperature of the crystal is equal to the absolute zero (0 K).

As the temperature approaches absolute zero, the entropy of a system approaches its minimum. That means, no system can reach absolute zero.

Because a temperature of absolute zero is physically unattainable, the third law of thermodynamics may be restated to apply to the real world as: the entropy of a perfect crystal approaches zero as its temperature approaches absolute zero. We can extrapolate from experimental data that the entropy of a perfect crystal reaches zero at absolute zero, but we can never demonstrate this empirically.

Why does ice always melt in water?

  • When ice is placed in a cup of water, it always melts
  • The water gets colder
  • The first law of thermodynamics does not prohibit the ice from getting colder and the water getting warmer.
  • The first law only requires that energy is conserve
  • Heat lost by ice = heat gained by water
  • The second law of thermodynamics specifies the direction in which spontaneous process proceed. (from hot to cold)

Heat Absorption

Specific Heat: The specific heat of a material is the amount of thermal energy needed to raise the temperature of 1kg of the material by 1℃.

More thermal energy is needed to change the temperature of a material with a high specific heat than one with a low specific heat

  • Example: Sand on a beach has a lower specific heat than water. Its temperature changes more than the water temperature as both water and sand transfer thermal energy to their surroundings.

Thermal Pollution

Thermal pollution is the increase in temperature of a body of water caused by adding warmer water.

  • Increasing the water temperature can cause fish and other aquatic organisms to use more oxygen.

Cooling towers are used by some power plants and factories to cool the warm water and they produce.

Modes of Heat Transfer

There are three modes of heat transfer. They are as follows.

  • Conduction
  • Convection
  • Radiation

Let’s try to understand the meanings of those simply.

Conduction: Conduction is the transfer of heat by the vibration of particles and also through the free electrons. This is most effective in solids.

Convection: Convection is the transfer of heat through density differences. This is most effective in liquids and gases.

Radiation: In radiation, heat is transferred by wave motion. In this no material is required, can occur in space.

  • Through the next articles, you will be able to take a wide knowledge about the three modes of heat transfer, experiments, laws and theories based on them. Invite all of you to stay with us.

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