Introduction
The ability to distinguish between cold and hot is built into our human system. Also, our liking or disliking of them depends on the given condition we are going through. In summer, the heat of the sun is unbearable, and in winter, we like to spend more time under it. Hot tea remains hot in the summer, while in the winter it cools down very quickly. We can sense the heat and coldness of an area or substance by feeling or touching them. But have you ever thought about how all these phenomena work? What creates heat? What makes hot tea cold in winter? How can coldness and heat be quantified and measured? Answers to all these questions and more are explained in this unit of heat.
Define Heat
Heat is basically energy. Heat is produced when the atoms and molecules of a substance move at a faster rate. The faster they move, the more heat is generated, and vice versa. Temperature is the comparative measurement of heat. Let’s take an example from our daily lives: a hot coffee cup that is placed on a table. We must have observed that, with the passage of time, the hot coffee cools down. This means the temperature of the body, like a hot coffee cup, is different from its surroundings, and a transfer of heat has taken place to bring the body temperature into equilibrium with the surrounding environment’s temperature.
Thus, heat is defined as the transfer of energy from one object or system to another because of a difference in their temperature.
Unit of Heat
The SI unit of heat energy transferred is measured in Joule (J). The SI unit of temperature is Kelvin (K). Celsius (C) or Fahrenheit (F) is widely used to measure temperature.
Measurement of Temperature
Temperature is measured with a thermometer. Clinical thermometers are used to measure body temperature; min-max thermometers are used to measure the temperature of the atmosphere; and laboratory thermometers are used in laboratories for high accuracy.
Any scale requires two points for measurement: the high point and the low point. In a thermometer, it is the freezing point and the boiling point. It is 32–212 degrees Fahrenheit and 0–100 degrees Celsius.
The relation between Fahrenheit and Celsius can be expressed as below
℃/100 = (℉-32)/180
℉ = (9/5)℃+32
or ℃ = 5(℉-32)/9
The relation between Kelvin (K) and Fahrenheit or Celsius can be expressed below
K= (℉-32)x(5/9) + 273.15
K= ℃+273.15
For example: The average body temperature of a room is 95.5℉. Calculate the temperature in Celsius.
Solution.
According to Problem,
The temperature in Fahrenheit is 95.5℉.
By putting the value in the equation, we find
TC= 9/5(95.5 – 32)
114.3℃. is the average temperature of the room in Celsius.
Thermal Expansion
Substances have the property of expansion when exposed to heat and contracting when exposed to cold. The change in temperature leads to a change in the dimension of the substance. The increase in dimension due to an increase in temperature is called “thermal expansion.”
Thermal expansion can take three forms: linear expansion, area expansion, and volume expansion.
Linear expansion is the length expansion of a substance and is expressed as
∆l/l = αL∆T
Where ∆l fractional change in length, αL is the coefficient of linear expansion and ∆T is a small change in temperature.
Expansion of the substance in the area is known as area expansion, and expansion in volume is known as volume expansion and is expressed as follows:
∆A/A = 2αA∆T
Where ∆A fractional change in the area, αA is the coefficient of area expansion and ∆T is a small change in temperature.
∆V/V = 3αV∆T
Where ∆V fractional change in volume, αV is the coefficient of volume expansion and ∆T is a small change in temperature.
The coefficient of thermal expansion is the rate of material expansion with the increase in temperature.
Specific Heat Capacity
Specific heat capacity is the property of a substance that calculates the change in temperature of the substance when a given amount of heat is absorbed or rejected by it. The specific heat capacity is denoted by C. The S.I. unit of specific heat is Joule per Kilogram per Kelvin (JKg-1K-1).
C = ∆Q/(m∆T)
Where m mass of substance, ∆Q amount of heat absorbed or changed, ∆T change in temperature.
Change in State
The change in form of a substance (solid, liquid, or gas) from one state to another is called a change in state. It occurs due to the exchange of heat between the substance and its surroundings. The change from a solid to a liquid state is called melting, and vice versa is called fusion. The melting point is the temperature at which a solid or liquid substance is at thermal equilibrium. The transition from a liquid to a gaseous state is known as vaporization. The temperature at which both states coexist is called the boiling point.
Latent heat is the amount of heat transferred during a state of change and is expressed as
L = Q/m
Where L is latent heat, Q is the amount of heat required and m is the mass of the substance.
Heat Transfer
Heat is the transfer of energy from one system to another or from a source to another object because of a temperature difference. Heat can be transferred in three distinct ways: by conduction, convection, and radiation.
Conduction: Here, when two objects of different temperatures come into physical contact with each other, heat transfer takes place. This is called conduction. Food cooked in a utensil on the stove, for example. The utensil and stove are at different temperatures in this case, but when they come into physical contact, the raw food in the utensil begins to cook due to heat transfer from the stove to the utensil.
Convection: Heat is transferred through the movement of fluid in an object. Fluid is a matter made of loosely moving molecules. E.g.:- When water is boiled on an electric stove, the heat is transferred through the fluid present in the water, which creates a circular current of heat in it to boil the water.
Radiation: When heat is transferred from the source to the object through electromagnetic waves, it is called radiation. For example, heat from the sun or fire.
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Heat is the transfer of energy from one object or system to another because of differences in their temperature. Joule (J) It is the change in temperature of the substance when a given amount of heat is absorbed or rejected by it. They are conduction, convection, and radiation. Unit of Heat FAQs
What is heat?
What is the SI unit of heat?
What is the specific heat capacity?
What are the three modes of heat transfer?
