JEE MAIN CHAPTER OVERVIEW
Chapter Priority | 1st (HIGHEST) |
Number of Problems in PYQ | HIGH |
Difficulty | EASY |
The problems from HEAT topic are very straightforward once you know all the formulae given in these notes.
KEY CONCEPTS FOR JEE MAIN
HEAT
It is a form of energy which flows from high temperature to low temperature without the help of external agent.
Its SI unit is Joules.
TEMPERATURE
It is the degree of hotness or coldness of a body.
It is a scalar quantity. Its SI Unit is Kelvin.
TEMPERATURE MEASURING SCALES
CELSIUS | FAHRENHEIT | KELVIN | |
Boiling water | 100 | 212 | 373.15 |
Human Body | 37 | 98.6 | 310.15 |
Room Temperature | 21 | 69.8 | 294.15 |
Freezing Point of water | 0 | 32 | 273.15 |
Absolute Zero | -273.15 | -459.67 | 0 |
Conversion between Celsius and Fahrenheit Scales:
[Note: Reading on Celsius scale and Fahrenheit Scale is same at Temperature = -40 degree Celsius]
Conversion between Celsius and Kelvin Scales:
Tk = Tc + 273.15
THERMAL EXPANSION
Upon heating, bodies expand. As a body is heated, the molecules gain more energy because in the form of vibrational kinetic energy. This results in more thermal stress due to more collisions between neighbouring molecules. This eventually results in the expansion of body.
Thermal expansion is of three types:
Linear Expansion
Area Expansion
Volume Expansion
Linear Expansion
Area Expansion
Volume Expansion
Relation between length, area and volume coefficients
TRY SOME JEE MAIN LEVEL QUESTIONS
Q. Length of pendulum increases by 1% on heating. Find % in time period of pendulum.
[Hint: What is the time period of a pendulum?]
Q. Two rods (L1 , L2 ) having coefficient of linear expansion 𝛼1 and 𝛼2 are joined together to form a composite system. Find equivalent coefficient of linear expansion for the system.
Q. A scale reads a length 60 cm of a body at 27°C. Find the new reading of scale if scale is heated to 127°C. [⍺ = 10^(–3)]
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THERMAL STRESS & STRAIN
When a body is unable to expand with rise in temperature. Thermal stress and strain are created.
CALORIMETRY
A system is said to be isolated if no exchange or transfer of heat occurs between the system and its surroundings.
When different parts of an isolated system are at different temperature, a quantity of heat transfers from the part at higher temperature to the part at lower temperature.
The heat lost by the part at higher temperature is equal to the heat gained by the part at lower temperature. This is the Prinicple of Calorimetry.
Calorimetry means measurement of heat.
A device in which heat measurement can be done is called a calorimeter.
SOME IMPORTANT TERMS
SPECIFIC HEAT: It is the heat required to raise the temperature of unit mass of substance by 1°C.
Specific Heat of ice= 0.5cal/g°C
Specific Heat of water = 1 cal/g°C
Specific heat of steam = 0.47 cal/g°C
MOLAR SPECIFIC HEAT: It is the heat required to raise temperature of 1 mole of substance by 1°C.
LATENT HEAT(L):Latent heat is defined as the heat of energy that is absorbed or released during a phase change of a substance keeping its temperature constant.
Latent Heat of fusion (LF ):It is the amount of heat required to convert per unit mass of substance from solid to liquid phase or vice versa. E.g. Lf of ice = 80 cal/g
Latent Heat of vaporization (LV ): It is the amount of heat required to convert per unit mass of substance from liquid to gas phase or vice versa. E.g. Lv of water= 500 cal/g
WATER EQUIVALENT (mw)
A substance’s water equivalent is equivalent to the amount of water that is required to consume the same quantity of heat as that substance does for an equal rise in temperature.
The water equivalent of a body is indeed equivalent to the product of its mass and its specific heat.
CHANGE OF STATE
Consider initially ice at a temperature T = -10 degree celsius. It is heated such that it becomes steam at 120 degree celsius. Following diagram shows the Heat supplied vs temperature of the body.
Q. Find final temp of mixture: 10g of H2O at 20°C + 30g of H2O at 50°C.
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HEAT TRANSFER
There are three modes of heat transfer:
Conduction
Convection
Radiation
CONDUCTION
Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference.
Heat conduction may be described quantitatively as the time rate of heat flow in a material for a given temperature difference.
Consider a metallic bar of length L and uniform cross-section A with its two ends maintained at different temperatures.
In steady state, the rate of flow of heat (or heat current),
H is proportional to the temperature difference (TC – TD) and
H is proportional to the area of cross-section A and
H is inversely proportional to the length L.
K is called the thermal conductivity of the material.
The greater the value of K for a material, the more rapidly will it conduct heat.
The SI unit of K is J s^–1 m^–1 K^–1 or W m^–1 K^–1.
ANALOGY BETWEEN THERMAL & ELECTRIC CURRENT
Thermal current across a conductor can be treated in a similar fashion to electric current across a resistor.
This has applications in many questions. We can apply the concepts of Kirchoff's Laws (Electric Current) in thermal current to solve questions.
TRY A JEE ADVACNED LEVEL QUESTION
Q. A lake starts freezing due to ambient air temperature of -0 degree Celsius. Find time in which ‘H’ depth of lake freezing if density of water is 𝜌, latent heat is L and thermal conductivity of ice is K.
Q. Given to concentric shells of radii R1 & R2 whose surface temperatures are maintained at T1 & T2. The gap between them is filled with a material of thermal conductivity K. Find Req and the rate of conduction of heat between them.
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CONVECTION
It is the mode of transfer of heat in which either the source moves or the receiver.
It is of two types:
Free or natural convection -
Example: Land & Sea Breeze is due to natural convection. Land gets heated faster in day so air near land is warm and rises. Due to this cool air from sea comes to land.
Forced Convection -
Example: Fan passing cool air.
RADIATION
It is the mode of heat transfer without movement of source & receiver & without heating the intervening medium.
Key Points:
Every object above 0K emits thermal radiation
The energy radiated per second by object depends on
More the surface area & temp of body more is energy radiated & vice versa.
A body also absorbs some of Radiations from surroundings in additions to radiate energy.
It can travel through vacuum.
Rate of radiation (R) & Rate of absorption (A) | Body's Temperature |
R>A | decreases |
R=A | stays constant |
R<A | increases |
Blackbody
A black body is the one that absorbs all radiations falling on it. Hence it is a perfect absorber. For example, sun is almost a black body.
A good absorber is also a good emitter. Therefore, a black body which is the best absorber also emits radiation at same rate it absorbs.
Emissive power (e) : The amount of heat radiation emitted by unit area of the surface per second at a particular temperature. SI UNIT : J/(m^2)(s)
Stefan's Law
For a black body the emissive power is proportional to 4th power of it temperature.
For a non-black body
Spectral Emissive Power
A black body radiates a mixture of large no of wavelengths.
The energy corresponding to a unique wavelength is called as spectral emissive power.
It is observed that some particular wavelengths have significant contributiontowards energy radiated.
Wien's Law
The product of maximum wavelength and temperature in the emission spectrum is constant.
𝜆maxT = b
b = 2.88 × 10^(–3) mk
As temperature falls, 𝜆max goes up.
Absorptive Power(a)
It is the ratio of energy absorbed by a body to the energy incident on the body.
a = Energy absorbed/Energy incident
For a black body, a = 1
For non-black body, a < 1
Reflective Power (r)
Ratio of energy reflected by a body to the energy incident on the body
r = Energy reflected/Energy incident
Transmittive Power (r)
Ratio of energy transmitted by a body to the energy incident on the body
t = Energy transmitted/Energy incident
a + r + t = 1
Kirchoff's Law
The ratio of emissive power to absorptive power for all the bodies at a given temp is equal to the emissive power of black body at that temp.
Net Power Radiated
If surrounding temperature is T0, the net power radiated per unit area by a black body is :
Newton's Law of Cooling
Rate of fall of temperautre is directly proportional to excess of temperature of the body over that of surrounding (T - T0).
Newton's law of cooling can be derived from the Stefan's Law given the body's mass and specific heat capacity(s). The constant 'k' depends on the size and shape of the body.
Hope you can REVISE HEAT FOR JEE MAIN PHYSICS from this guide to answer all varieties of JEE MAIN PYQs.
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