Madhava Publications
Calorimetry
Calorimetry is made up of two words calorie which means heat and metry which means measurement.
When two bodies of different temperature are allowed to share heat, they attain a common temperature. If it is assumed that no heat is received from or given to anybody outside the system and if there is no chemical reaction involved in the process of sharing then,
Heat gain by the colder body= Heat loss by the hotter body
The above statement is called the principle of calorimetry and this is based on the law of conservation of energy.
A calorimeter consists of a metallic vessel and a stirrer both are made of the same material (copper or aluminium) and the vessel is kept in a wooden jacket so that there is no heat loss .A mercury thermometer can be inserted through a small opening in the outer jacket.
Change of state
Depending on temperature and pressure, all matter can exist in a solid, liquid and gaseous state. These states of matter are also called phases of matter.
The change of state from solid to liquid is called melting and from liquid to solid is called fusion. It is observed that the temperature remains constant until the entire amount of solid substance melts. That is both the solid and the liquid state of the substance co-exist in thermal equilibrium during the change in state from solid to liquid.
The temperature at which the solid and the liquid state of the substance is in thermal equilibrium with each other is called its melting point. The change of state from the liquid to vapour is called vaporisation.
It is observed that the temperature remains constant until the entire amount of liquid is converted into vapour.
The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point. The change from solid to vapour state without passing through the liquid state is called sublimation and substance is said to sublimate.
As altitude increases, the density of the air becomes thinner, and thus exerts less pressure. At high altitudes, external pressure on water is therefore decreased and will hence take less energy to break the water. If less energy is required it means less heat and less temperature which means that water will boil at a lower temperature.
Latent heat
Latent heat is defined as the heat or energy that is released during a phase change of a substance. It could be either from a gas to a liquid or liquid to solid and vice versa. Latent heat is related to a heat property called enthalpy.
However, an important point that we should consider regarding latent heat is that the temperature of the substance remains constant. As far as the mechanism is concerned, latent heat is the work that is needed to overcome the attractive forces that hold molecules and atoms together in a substance.
Let’s take an example. Suppose a solid substance is changing to a liquid, it needs to absorb energy to push the molecules into a wider, more fluid volume. Similarly, when a substance changes from a gas phase to a liquid, their density levels also need to go from lower to a higher level wherein the substance then needs to release or lose energy so that the molecules come closer together. In essence, this energy that is required by a substance to either freeze, melt or boil is said to be latent heat.
Two types of Latent heat
- Latent heat of fusion
- Latent heat of vaporisation
Latent heat of fusion: It is the amount of the heat which is required to change the phase of the solid into liquid for unit mass at constant temperature.
For example: Latent heat of fusion of water is
Latent heat of vaporisation: It is the amount of heat which is required to change the phase of the liquid into vapour for unit mass at constant temperature.
For example: Latent heat of water is
Heat transfer
Any matter which is made up of atoms and molecules has the ability to transfer heat. The atoms are in different types of motion at any time. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy. The more the motion of molecules, the more will be the heat energy. However, talking about heat transfer, it is nothing but the process of transfer of heat from a high-temperature body to a low temperature one.
There are three mechanisms of heat transfer whose name is given as conduction, convection and radiation.
- Conduction occurs within a body or between two bodies in contact
- Convection depends on the motion of mass from one region to another,
- Radiation is heat transfer by electromagnetic radiations such as sunshine, with no need for matter to be present in the space between bodies.
Conduction
Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference .Suppose one end of a metallic rod is put in a flame the other end of the rod will soon be so hot that I cannot hold it with your bare hands.
Here, heat transfer takes place by conduction from the hot end of the rod through its different parts to the other end. Gases are poor thermal conductors while liquids have conductivities intermediate between solids and gases.
The rate of heat energy flowing through the rod becomes constant at steady state. It is given by, Rate of flow of heat 
Where K= thermal conductivity of material
A= cross-section area; d= distance between the two end
T2 and T1 are the temperatures of hotter and colder bodies. Following are the examples of conduction:
- Ironing of clothes is an example of conduction where the heat is conducted from the iron to the clothes.
- Heat is transferred from hands to ice cubes resulting in the melting of an ice cube when held in hands.
- Heat conduction through the sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.
Convection
Convection is a mode of heat transfer by actual motion of matter. It is possible only in fluids. Convection can be natural or forced.
In natural convection, gravity plays an important part. When a fluid is heated from below, the hot part expands and therefore becomes less dense. Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated and rises up and is replaced by a relatively colder part of the fluid. The process goes on.
In forced convection material is forced to move by a pump or some other physical means. Examples of forced convection are the circulatory system, cooling system, and heat connector of an automobile.
Radiation
Radiation is the transfer of heat by electromagnetic waves such as visible light, infrared and ultraviolet rays. Everyone has felt the warmth of the sun’s radiations and intense heat from a charcoal grill or the glowing coal in the fireplace. Most of the heat from these bodies reaches you not by conduction or convection in the intervening air but by radiation. This heat transfer would occur even if there were nothing but a vacuum between you and the source of heat.
Black body radiation
All bodies emit radiant energy, whether they are solid, liquids or gases. The electromagnetic radiation emitted by a body by virtue of its temperature like the radiation by a red hot iron or light from a filament lamp is called thermal radiation.
When this thermal radiation falls on other bodies, it is partly reflected and partly absorbed. The amount of heat that a body can absorb by radiation depends on the color of the body.
Everybody both radiates and absorbs energy from their surroundings. The amount of energy absorbed is proportional to the color of the body.
A black body is an idealization in physics that pictures a body that absorbs all electromagnetic radiation incident on it irrespective of its frequency or angle.
Black-body radiation is the thermal electromagnetic radiation emitted by a black body within or surrounding a body in thermodynamic equilibrium with its environment (an idealized opaque, non-reflective body).
It has a specific spectrum of wavelengths that are inversely related to intensity and are only affected by the body's temperature, which is assumed to be uniform and constant for the sake of calculations and theory.
Emissive power: The amount of heat energy radiated per unit area of the surface of a body, per unit time and per unit wavelength range is constant which is called emissive power
Absorptive power: The ‘absorptive power ‘of a surface at a given temperature is the ratio of the heat energy absorbed by a surface to the total energy incident it at a certain time. It is represented by
Wein’s displacement law
The blackbody radiation curve for different temperature peaks at a wavelength is inversely proportional to the temperature.
Stefan- Boltzmann law
The Stefan- Boltzmann law explains the relationship between total energy emitted and the absolute temperature
Stefan's Law states that the radiated power density of a black body is directly related to its absolute temperature T raised to the fourth power.
Greenhouse Effect
The greenhouse effect is the way in which heat is trapped close to Earth's surface by “greenhouse gases.” These heat-trapping gases can be thought of as a blanket wrapped around Earth, keeping the planet toastier than it would be without them.
The main gases responsible for the greenhouse effect include carbon dioxide, methane, nitrous oxide, and water vapor (which all occur naturally), and fluorinated gases (which are synthetic)
The largest source of greenhouse gas emissions from human activities in the United States is burning fossil fuels for electricity, heat, and transportation.
Newton’s law of cooling
Newton’s law of cooling describes the rate at which an exposed body changes temperature through radiation which is approximately proportional to the difference between the object’s temperature and its surroundings, provided the difference is small.
Definition: According to Newton’s law of cooling, the rate of loss of heat from a body is directly proportional to the difference in the temperature of the body and its surroundings.
Greater the difference in temperature between the system and surroundings, the more rapidly the heat is transferred and faster the body changes its temperature.
Newton’s law of cooling is given by,
Where,
- Tt = temperature of the body at time t and
- Ts = temperature of the surrounding,
- k = Positive constant that depends on the area and nature of the surface of the body under consideration
A fun thing to try: Virtual lab
Below is the link of the simulation of the Black Body radiation
This simulation gives us lots of information. In this simulation, we have a graph between spectral Power density and the wavelength. This graph can be drawn for various objects at different temperatures Like Star Sirius at temperature 10000 K to earth at 300 K.
- For the selected temperature of the black body, we can see at which wavelength(
- We can also in which part of electromagnetic spectrum do it falls like ( Infrared, Visible, UV etc)
- We can verify the wien's displacement law (
