Heat Sources and Classification – Thermodynamics

The rise in temperature leads to an increase in molecule vibrations in a body. The vibrations are capable of transferring from one location to another. Heat is the measurement of the molecule vibration inside the body.

The definition of heat according to physics is the movement of energy from a warmer body to a cooler body. This movement is from a higher temperature substance to a lower temperature substance.

This is done to move energy from faster vibrating molecules to slower ones. This vibrational energy is the heat content that makes the body warm and cold.

The greater is the heat content, the hotter is the body.

If the subject matter is changing from one state to another, heat transfer is possible with a temperature rise. Heat as energy can convert to work and other forms of energy. The automobiles see its conversion to mechanical energy.

This was just one example, this application is relevant in most of the things around us. The temperature on the other hand is measuring the hotness and the coldness of the body. It basically represents heat in a numerical form.

A thermometer is an instrument that measures the temperature. Celsius, Fahrenheit scale, and Kelvins are the few units to measure by this instrument.

Heat Transfer

This process of transferring the vibrating molecules from one substance to another occurs by three mechanisms –

• Conduction
• Convection
• Radiation

1. Conduction

This method of heat transfer follows direct contact of molecules and the transfer is between the atoms. There is movement from high temperature to low temperature.

The kinetic energy of the particles with higher vibration comes in contact with slower particles and as a result transfer the energy. The thermal conduction or heat conduction is another name for this method. Metal bodies are the best option when it comes to this method.

The equation for conduction is –

Q = [K.A.(Thot−Tcold)]d
(Q – transfer of heat per unit time, K – thermal conductivity, A – area of heat transfer, Thot – hot region temperature, Tcold – cold region temperature, and d – body thickness)

Conduction Examples

• Ironing of clothes as the heat conduction takes place from the iron to the clothes
• Melting of ice cube follows the same process
• Sand conducts heat at the beaches mostly in summers.

2. Convection

This method of heat transfer follows the movement of heated fluid substances. There is movement from higher temperature to lower temperature regions. There is the process of displacement in this method. The rise in temp leads to rising in volume as well. The equation of convection is –

Q = hc ∙ A ∙ (Ts – Tf)
( Q – heat transferred per unit time, Hc – coefficient of convective heat transfer, A – area of heat transfer, Ts – surface temperature, and Tf – fluid temperature)

Convection Examples

• Boiling water is an example of this. This is because there is continuous movement of molecules and up and down direction according to their density to generate heat.
• Warm water directs at the poles while the cooler water directs at the equator.
• Convection regulates blood circulation in warm-blooded animals.

3. Radiation

This method of heat transfer uses electromagnetic waves to facilitate the process. The waves carry the energy from the body and are a result of random molecular motion in the object. This is possible in a vacuum and transparent medium.

The charged particles are behind this idea. Thermal radiation follows this equation –

P = e ∙ σ ∙ A· (Tr – Tc)4
(P – net power of radiation, A – area of radiation, Tr – radiator temperature, Tc – surrounding temperature, e – emissivity and σ – Stefan’s constant)

Radiation Example

• Microwave radiation when you get your food.
• Sun emitting UV rays is another example.

Unit of Heat Transfer

• SI system – Joule
• MKS system – cal
• Rate of transfer of heat – KW

Classification and Sources of Heat

When there is high heat content, the object becomes hot and when there is low heat content, the object becomes cold. Examples of hot objects are the sun, fire, hot pans, etc. And examples of cold are ice, snow, metal, etc.

The main sources of heat are –

• Sun
• Chemical
• Electrical
• Nuclear

Thermometry

As mentioned above, a thermometer is an instrument for measuring heat with different units of temperature. The temperature changes when there is a change in state to solubility. The kinetic energy has an important role here.

Kinetic Energy

Any energy that is in motion becomes kinetic energy. There is a presence of energy in the object because there is motion.

The direction and dimension have no impact on kinetic energy. The kinetic energy around the object produces heat energy. A daily life example is hot water cools down in some time as there is heat loss as the temperature in the surrounding is lower.

The molecules show a random motion that makes them bounce off each other and thus end up transferring heat. This is because the heat travels from high temperature to lower temperature.

Principles of Thermodynamics

The principle of thermodynamics states “ the flow of heat generally occurs from a high-temperature region to a low-temperature region”. It concludes that coldness is a made-up concept and is just an absence of heat.

The kinetic energy because of higher temperature contributes to smaller volume expansion. This becomes the thermal expansion – the basic principle behind thermometers.

Scales for Measuring Temperature

Kelvin Scale

The magnitude of the temperature uses the Kelvin scale. The International System of Units calls Kevin Temperature’s SI unit. The name comes from 1st Baron Kelvin, a physicist.

The scientific equation and calculation often use this scale. The mass, pressure, temperature, and volume relationship only uses this scale.

Fahrenheit Scale

It is a common form of temperature measurement. The name comes from Daniel Gabriel Fahrenheit, a German physicist. It has a negative temperature too and can measure -459.67 degrees F, that is the coldest.

Rankine scale is another type but is not very relevant. It mainly acts as a companion for the Fahrenheit scale.

Celsius Scale

This scale uses the freezing and boiling point of the water to measure temperature. It is a derived unit of temperature used in day to day lives. The name comes from Anders Celsius, a Swedish physicist. There is a 100-degree division between both the points and there can be a negative value too.

The Thermal Expansion

Thermal expansion is when the temperature change leads to expansion or contraction of the body. The contraction on cooling and expansion of heating is based on this concept. The expansion is either linear or area wise. The hot water loosens the light metallic lid of a bottle when kept upside down.

This is because the lid expands. A similar concept is relevant when the thermometer is put in warm water.

The balloon also inflates in a cooler room while expands in a warmer environment. All these things are due to thermal expansion. The coefficient of thermal expansion is by fractional change in size due to temperature.

The metals have a higher coefficient of linear expansion. This is by the coefficient of volume expansion. The freezing of water in winter is also due to this phenomenon.

Evaporation

The process of converting liquid substance to a gaseous state is evaporation. The temperature facilitates this process of conversion. Vaporization latent heat is the temperature at which water evaporates.

The energy absorbed during the change of state is the latent heat. But this heat maintains the same temperature and remains constant. The temperature rise during the process increases water absorption and retention. The greater evaporation depends on greater air movement.

Humidity

The ratio of vapor and air pressure of the water vapor is humidity. There is no unit to measure the humidity. There are different types of humidity based on different factors. The types are –

Relative Humidity
This method of humidity is to find out water vapor content in the air. It is 100% during the rainy season. It is the ratio of moisture amount in the air to maximum moisture that can be present at the same temperature. RH = Actual vapour density saturation vapour density × 100%

Absolute Humidity
The absolute humidity method measures the weight of water vapor per unit air volume. Though the results are not very reliable due to constant factors affecting the humidity.

The unit for this is
g.m-3 (grams of water vapor per cubic meter of air). Because of continuous factors affecting the temperature, there is no reliability.

Specific Humidity
This one is the most reliable method to measure humidity. This follows the measuring of the weight of water vapor per unit weight of air. This expression is in grams of water vapor per air kilogram. The “g.kg-1” is the specific humidity unit across all measurements.

Condensation

When the vaporous converts back to the water. There is heat loss which converts the vapor back to the liquid state. It depends on the cooling and the humidity in the air. There are different forms of condensation. They are –

1. Dew

The moisture in the air often forms water droplets on solid surfaces that are cool known as dew. The dew needs a clear sky, less air pressure, and high relative humidity to form. The dew must be above the freezing point.

2. White Frost

This follows the same process as the dew formation. The only difference is the temperature is either below or at the freezing point. And instead of water droplets, there are ice crystals on the solid surface.

3. Fog

When the air mass with a huge quantity of water vapor experiences a sudden temperature fall, fine dust particles in the air undergo condensation. Basically, fog becomes the cloud that is closer to the ground and reduces visibility. They are very common in winters and quite thin in nature.

Though the formation of fog by the warm air is thick and persistent. This situation is common in overwater mixing zones in oceans.

4. Mist

The mist is similar to fog but not the same as it has more moisture. The nucleus of mist has a very thick moisture layer and thus the visibility is very low like not more than 1 km.

They are commonly over the mountain as the warm air meets the cooler surface. Though it is less dense and separates quickly in case they are formed of water droplets.

5. Haze and Smog

There is no condensation in the creation of haze. The formation of haze is by dry particles present in the air like dust and smoke. They often block the sky clarity and are common during farming, pollution, wildfires, and more.

The smog undergoes condensation and primary pollutants contribute to its formation. It is basically smoke plus fog by coal burning, vehicles smoke, and more.

6. Clouds

The condensation of water vapour in the free air creates a mass of small water droplets and crystals called clouds. The cooling is below the dew point and is at a height from the surface. There are four types of cloud –

a. Cirrus Clouds

• These are the clouds at high altitudes above 8,000.
• They are thin in nature with a feathery appearance.
• They are always present in white color.

b. Cumulus Clouds

• These look like a cotton wool ball.
• They are present above the height of 4,000 and up to 7,000 m.
• They are present in a scattered manner in patches with a flat base.

c. Stratus Clouds

• These have layers and cover a large sky portion.
• They form due to heat loss or air mass mixing at different temperatures.

d. Nimbus Clouds

• They are dark in color like black or grey.
• They are usually near the earth’s surface.
• These are very dense with a higher opacity.
• They are shapeless with a thick vapour layer.

Another way to see the cloud-based on above division is –

Effect of Heat on Substances

• The process of sublimation converts solids like camphor and naphthalene to gas when heated.
• The pressure also changes the state of matter. The unit atmosphere measures the pressure by the gas.
• The pressure at sea level is 1 atm which reduces with height
• The kinetic energy is higher in some particles of liquids which evaporate because of breaking away.

States of Matters

The liquid, solid and gaseous state of matters is the basic one. The extension of this division is plasma. It is the fourth state of matter made of ionized gas. The CFL bulbs have them to facilitate electricity transfer. It is only possible at high temperatures and creates light.

Bose-Einstein condensate is the fifth state of matter present at the low gas density and super low temperature. The density uses kilograms per meter cube for measurement. Natural gas has methane and other chemicals important for LPG and tyre industries.

Nuclear Energy

Nuclear fission is the concept behind nuclear reactors. The Atom bomb follows the same idea. The neutrons in the reactor slow down to interact with uranium isotope by using graphite or heavy water.

The calcium rods control the reaction and solar energy is also based on nuclei fission. India also has a nuclear programme divided into three stages –

1. The natural uranium acts as fuel and heavy water as a moderator and results in plutonium.
2. Plutonium and uranium oxide makes uranium -233 without a moderator.
3. Thorium and uranium-233 produce energy.

Anomalous Behavior of Water

The water below 4°C shows anomalous behavior. There is a decrease in density as the volume increases. This is why the water bodies freeze in winter as the water density falls and the upper top becomes solid. The animals and plants underwater stay intact.

Effect of Wool and Cotton on Heat

The darker color clothes absorb more heat and the lighter one reflects heat. This is why there is a color preference in different seasons. Wool is a bad heat conductor and thus woolen fibres don’t let body heat escape.

This is why they are prominent for the winter season. Using two blankets also traps the heat between the layers. Cotton clothes absorb sweat which evaporates and makes the body feel cool.

Effect of Air Parcel on Heat

The house construction uses hollow bricks with air-filled inside. This makes sure that the house doesn’t heat up during summer and remains cool. And during the winter season, heat gets trapped in flats keeping the house warm. This is the effect of air parcels on heat.

Newton Cooling’s Law

Newton’s cooling law describes the object’s rate of changing temperature by radiation is proportional to the difference of the object’s surroundings and temperature.

It states “ the rate of loss of heat from a body is directly proportional to the difference in the temperature of the body and its surroundings”. Newton’s law of cooling in the equation is –
dT/dt = k(Tt – Ts)
( Tt – temperature at time t, Ts – temperature of surrounding, and k = Positive constant )

Newton’s Law of Cooling Formula

The heat transfer accelerates with a greater difference in temperature of the system and the surrounding. The temperature of the body changes accordingly. The formula for this law is –

T(t) = Ts + (To – Ts) e-kt
( t – time, T(t) – temperature of the body at time t, Ts – surrounding temperature, To – initial temperature of the body, and k = constant)

Limitations of Newton’s Law of Cooling

• There should be a small difference between the temperature of the body and the surroundings.
• Radiation should facilitate the loss of heat in the body.
• The surrounding temperature has to be constant while the body cools.

Air Conditioning and Refrigerator

The thermal energy used in AC and refrigerator follows the concept of heat. The refrigerator supplies the thermal energy from a lower temperature to the higher one. But in the case of AC, the thermal energy is from the air to release cooler air.

There is a movement of energy from higher to lower temperatures in both cases. There is a heat pump present in all refrigerating appliances. And the ACs have compressors that give cooled air experience.

Conclusion

Heat remains an important physics topic for multiple competitive exams. UPSC, RRB, SSC, etc are some important competitive exams in India interested in this subject. Some important topics were the nature of heat, types, humidity, cooling law, and much more.

The General Science paper of the IAS exam will have some questions from this topic. And this is important for Prelims as well in the general science paper. This is because it comes under the physics basic level module and is present in NCERT books as well.

Candidates will definitely receive reliable and easy to understand information from the article above based on NCERT notes. Candidates will receive benefit from reading this article.