Heat

How is Heat Transferred?

Heat can travel from one place to another in several ways. The different modes of heat transfer includes
1.Conductio 2. Convection3.  Radiation.
Meanwhile, if the temperature difference exists between the two systems, heat will find a way to transfer from the higher to the lower system.

What is Conduction?
Conduction is defined as
The process of transmission of energy from one particle of the medium to another with the particles being in direct contact with each other.
An area of higher kinetic energy transfers thermal energy towards the lower kinetic energy area. High-speed particles clash with particles moving at a slow speed, as a result, slow speed particles increase their kinetic energy. This is a typical form of heat transfer and takes place through physical contact. Conduction is also known as thermal conduction or heat conduction.

Conduction Examples
Following are the examples of conduction:
1. Ironing of clothes is an example of conduction where the heat is conducted from the iron to the clothes.
2. Heat is transferred from hands to ice cube resulting in the melting of an ice cube when held in hands.
3. Heat conduction through the sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.

What is Convection?
Convection is defined a
The movement of fluid molecules from higher temperature regions to lower teperature regions.


Natural Convection:
Natural convection occurs without external forces, driven solely by the buoyancy forces arising from temperature variations. A classic example is the rising of warm air near a heat source, such as a radiator or a heated surface.

Forced Convection:
Forced convection involves the use of external forces, such as fans or pumps, to enhance fluid movement and heat transfer. Common examples include the use of fans in ovens or the circulation of water in a radiator by a pump.
Convection Examples:
Convection is present in various natural and artificial processes. Examples include the heating of air in a room, ocean currents driven by temperature variations, the rising of hot air and sinking of cool air in the Earth's atmosphere, and the movement of molten rock in the mantle beneath the Earth's crust
1. Boiling of water, that is molecules that are denser move at the bottom while the molecules which are less dense move upwards resulting in the circular motion of the molecules so that water gets heated.
2.Warm water around the equator moves towards the poles while cooler water at the poles moves towards the equator.
3. Blood circulation in warm-blooded animals takes place with the help of convection, thereby regulating the body temperature.

What is Radiation?
Radiant heat is present in some or other form in our daily lives. Thermal radiations are referred to as radiant heat. Thermal radiation is generated by the emission of electromagnetic waves. These waves carry away the energy from the emitting body. Radiation takes place through a vacuum or transparent medium which can be either solid or liquid. Thermal radiation is the result of the random motion of molecules in matter. The movement of charged electrons and protons is responsible for the emission of electromagnetic radiation. 
Radiation heat transfer is measured by a device known as thermocouple. A thermocouple is used for measuring the temperature. In this device sometimes, error takes place while measuring the temperature through radiation heat transfer.

Radiation Example

Following are the examples of radiation
1. Microwave radiation emitted in the oven is an example of radiation.
2. UV rays coming from the sun is an example of radiation.
   
Ex 1 : Convert -10⁰C into Fahrenheit scale.
Sol:    C=-10⁰C
         \( F = \frac{9} {5}C + 32 = \frac{9} {5}\left( { - 10} \right) + 32 = 14^0 F \)   

Ex 2: Convert the normal temperature of a human body into Celsius scale.
Sol:  \( F = 98.4^0 F \)      
       \( C = \frac{5} {9}\left( {F - 32} \right) = \frac{5} {9}\left( {98.4 - 32} \right) = \frac{5} {9}\left( {66.4} \right) = 36.9^0 C \)
        
Ex 3 : Show that  measure the same temperature on Celsius and Fahrenheit scales.
Sol:   \( \begin{gathered} Let\,C = F = x,Substituting\,\,\,in\,\, \hfill \\ \frac{{C - 0}} {{100}} = \frac{{F - 32}} {{180}} \Rightarrow \frac{{x - 0}} {{100}} = \frac{{x - 32}} {{180}} \Rightarrow \frac{x} {5} = \frac{{x - 32}} {9};9x = 5x - 160 \hfill \\ \Rightarrow 4x = - 160 \Rightarrow x = - 40^o \hfill \\ \end{gathered} \)     

Ex 4: Express 200K on Fahrenheit  scale.
Sol: On any scale of temperature,  \( \frac{{\operatorname{Re} ading - LFP}} {{UFP - LFP}} = a\,cons\tan t \)  
    On the Kelvin and Fahrenheit scales,     
      \( \frac{{K - 273.15}} {{373.15 - 273.15}} = \frac{{F - 32}} {{212 - 32}} \Rightarrow \frac{{K - 273.15}} {{100}} = \frac{{F - 32}} {{180}} \)
 Where K and F are the readings on kelvin and Fahrenheit scales respectively. In this problem, K=200K
     \( \begin{gathered} \frac{{200 - 273.15}} {{100}} = \frac{{F - 32}} {{180}} \Rightarrow 200 - 273.15 = \frac{{100}} {{180}}\left( {F - 32} \right) \hfill \\ - 73.15 = \frac{5} {9}F - \frac{{160}} {9} \Rightarrow F = \frac{9} {5}\left( { - 73.15 + \frac{{160}} {9}} \right) = - 99.68K \hfill \\ \end{gathered} \)   
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Ex 5: What is the temperature for which the readings on Kelvin and Farenheit scales are same?
Sol: On the Kelvin and Farenheit scales
   \( \begin{gathered} \frac{{K - 273.15}} {{100}} = \frac{{F - 32}} {{180}},Here,K = F \hfill \\ \frac{{F - 273.15}} {{100}} = \frac{{F - 32}} {{180}} \Rightarrow F - 273.15 = \frac{5} {9}F - \frac{{160}} {9} \hfill \\ \end{gathered} \)  
    \( \Rightarrow F = \frac{{2298.35}} {4} = 574.59 \)  

Ex 6: A faulty thermometer has its fixed points marked as 5⁰ and 95⁰. The temperature of a  body as measured by the  faulty thermometer is 59⁰ . Find the correct temperature of the body on Celsius scale.
Sol : On any scale of temperature \( \frac{{\operatorname{Re} ading\, - LFP}} {{UFP - LFP}} = a\,cons\tan t \).
 Let the reading on the faulty thermometer be x and the corresponding reading on Celsius scale be C, By the above expression for the faulty thermometer and  Celsius scale,
    \( \begin{gathered} \frac{{x\, - LFP}} {{UFP - LFP}} = \frac{{C - 0}} {{100 - 0}} \Rightarrow \frac{{x - 5}} {{95 - 5}} = \frac{C} {{100}} \hfill \\ \Rightarrow C = \frac{{100}} {{90}}\left( {x - 5} \right) = \frac{{10}} {9}\left( {59 - 5} \right) = 60^o C\left( {\because x = 59^o } \right) \hfill \\ \end{gathered} \)