Atom is the smallest building block of an element – it can’t be further broken by any chemical process. Everything you see around yourself is made of trillions-of-trillions atoms. They are so small that, it is not possible to see them with a powerful microscope. Only specialized microscopes (electron microscope) are able to zoom the atoms.

Composition of an Atom

hydrogen-atomAn atom is composed of electrons, protons, and neutrons. Neutrons and protons are held together in the center of an atom called nucleus. But, electrons orbit around atoms at fixed distances called orbits or shells. A neutral atom (without charge) contains an equal number of electrons and protons.

The total mass of the atom is concentrated in its nucleus, which is measured in a.m.u (atomic mass unit). One a.m.u is about  kilogram. Mass of the atom is equal to the number of nucleons (sum of the number of protons and neutrons). One nucleon – either a proton or a neutron – has a mass equal to 1 a.m.u.

Elements are distinguished from each other by the number of protons. If somehow number of protons is changed in an element – as it happens during radioactivity – then the original element will be converted into another element. However, changing the number of neutrons doesn’t change the element itself. Instead, it will give a different isotope of the same element. An isotope of an element has the same number of protons but a different number of neutrons.


There 118 different kinds of atoms based on the number of protons, that builds 118 different elements. These elements are classified into “Periodic Table” on the basis of similarities between them.



Electrons are negatively charged particles, they orbit around the nucleus in certain energy levels. The orbiting speed of electrons is very fast – almost impossible to predict 100% accurately where they located. The number of electrons in a neutral atom is equal to protons. Electrons are 1800 times smaller than neutrons and protons.


Protons are positively charged particles. They are held together with neutrons in the nucleus of an atom. They have a mass of 1 a.m.u. Elements are distinguished from each other by the number of protons. Hydrogen is a unique element that contains only one proton in its nucleus, and one electron orbiting around it. If an electron is somehow removed from the hydrogen atom, then only one proton will remain there; you can say a proton is a positively charged hydrogen.



Neutrons are charge-less particles. They are also located in the nucleus of an atom – together with protons. The number of neutrons predicts the isotope of an element. An isotope of the element is the same element but with a different mass. Changing the number of neutrons will change the isotope of an element. For example, Hydrogen has three isotopes (Protium, Deuterium, and Tritium) – each one contains one proton but a different number of neutrons.


History of Atom

  • Democritus developed the idea of an atom in 460 B.C. He thought that, if you break a piece of matter in half, and again break that in half, and continue this process. At some point, you will get the smallest piece of matter which will not divide further. He called that piece of matter “Atom”.
  • In 1800’s, John Dalton – an English chemist – performed some experiments on many chemicals. He showed that atoms are made up of elementary particles. He didn’t know about their structure. But, he found some evidence which showed that there are more fundamental particles than an atom.
  • In 1897, J.J Thomson presented a model of the atom. He knew that electrons have a negative charge, so there must be an equal positive charge to make the atom neutral. He thought that electrons are stuck on the protons like raisins in the pudding.


  • In 1911, Earnest Rutherford performed an experiment by bombarding alpha particles on gold foil. He came up with the result that, most of the volume of the atom is empty. The nucleus containing positive charge lies in the center of the atom. The electrons are orbiting around the nucleus in the empty space like planets around the sun. But, there was a problem with his model. According to the theory of electricity and magnetism – opposite charges attract each other so protons will attract electrons. As a result, electrons – gradually losing its energy – must fall in the nucleus spirally, and give a continuous spectrum of light. But in reality, it was not happening.
  • In 1912, a physicist Neil Bohr came up with an idea. He rectified the error from atomic model of Rutherford. He said that electrons don’t spiral into the nucleus. Instead, he gave two rules about the motion of electrons which are:
    • Rule 1 – Electrons orbit around the nucleus at certain allowed distances called “orbits”.
    • Rule 2 – Atoms radiate energy only when electrons jump from higher energy orbit to lower energy orbit. They absorb radiation when electrons jump from lower energy orbit to higher energy orbit.

Many people came with different ideas and the atomic model was developing until it reached the present day model of the atom.



Heat transfer is the transfer of thermal energy from one object to another. It occurs between two objects when they are at different temperatures – the heat will transfer from hot object to cold object. In our environment heat transfer occurs all the time. The sun is transferring some of its heat energy towards earth, and we get days hotter than nights. We cook our food on stoves – it is possible due to heat transfer.

Methods of Heat Transfer

There are 3 methods of heat transfer, which are conduction, convection, and radiation. Let’s discuss each of them individually.


conduction-heat-transferWhen two objects with different temperatures come in contact with each other, the heat is transferred from the hot object towards the cold object. This happens due to the collision of the molecules at the contact area of two objects.

In solids, molecules of hot objects vibrate more frequently than cold objects. When hot and cold objects come in contact with each other, their molecules collide. As a result, the collisions of molecules transfer the energy from the hot object to cold object. This transfer of thermal energy continues until hot and cold objects reach the same temperature. For example, when you boil water on a gas stove, the heat is conducted to the pot from fire.


convection-heat-transferConvection is the heat transfer method that only occurs in fluids (liquid and gases).  It is caused by the flow and diffusion of fluids. This flow happens in the closed path – from hot regions of fluids towards cold regions and back again.

For example, when water is boiled on the gas stove, the heat is conducted by metal pot. The temperature of metal pot increase, which increases the temperature of the water that is in contact with it. Hot water rises to the surface and is replaced by cold water. This process continues and causes circular motion. As a result, the heat is distributed towards all the regions of water.


radiation-heat-transferIn this process, heat is transferred by electromagnetic waves – also known as light. This process doesn’t depend on any medium or material to transfer the heat. Instead, the radiations travel through space or air to transfer the heat.

The heat is transferred to objects that absorb the radiation. As a result, their temperature increases. For example, the Sun transfer the heat energy as radiations that heats-up our Earth. Furthermore, the reflecting objects like mirrors can’t completely absorb the radiations. So, their temperature doesn’t increase.


  • Insulators don’t conduct the heat very well. So, they are used to cover the objects whose heat we want to preserve. For example, when ice-creams are taken on a ride, they are stored in a styrofoam box to prevent the heat by conduction.
  • Materials that are good conductors of electricity are also good conductors of heat. For example, copper is the best conductor of electricity, so it is also the best conductors of heat. Copper heat sinks are used on computer processors to remove the excess heat quickly by conduction.
  • Sometimes it is necessary to transfer the heat from cold object to hot object – opposite to the natural heat transfer. This kind of heat transfer is called forced heat transfer. Today, we use forced heat transfer in air-conditioners and refrigerators to cool-down things.

earth-gravityHave you ever tried to jump?

Probably, yes! And you know that when we jump from the ground, we just fall back after few seconds.children-jumping

Why? And How?

These are the questions that, almost every human had thought in his childhood. So, the simple answer to all these questions is “gravity”.

So, let’s learn something about gravity and the interesting history about it.

What is Gravity?

Gravity is the force that attracts us and other objects towards each other; much like a magnet, but not as a magnet. Then, why things in your room are not attracted to each other? It is because gravity depends on the mass; Massive objects have more gravity.

Gravitational force is so weak that the object must be very massive to have sufficiently strong gravity to attract other objects. Consider our earth, it so massive that its gravity pulls us towards the ground. When we jump, earth’s gravity pulls us and we fall back on the ground.

Importance of Gravity

What would happen, if gravity suddenly disappears? Take some time and think about it.

galaxyYou will be amazed to find out that, the current appearance of our earth, planets, and stars depends on the gravity. Without gravity, there will be no earth’s pull to attract you back to the ground when jumping. Even the planets and stars would not exist without gravity. Because planets and stars have been formed from the accumulation of cosmic dust and gas that was only possible due to gravity.

Here are some of the effects of gravity on our everyday life

  • We can stand, jump, walk, and run on the ground due to gravity. If the earth doesn’t pull you back while jumping, then you would never come back to the ground, and you may reach space in a single jump! Wow!
  • On the earth, gravity keeps the air around us as a blanket, and it prevents air from escaping into space.
  • Tides in the oceans are caused by the gravitational pull of moon and sun.

Universal Law of Gravitation

law-of-gravitationDon’t be afraid!

It is just a simple law that was proposed by a famous scientist Sir Isaac Newton. This law gives us the relation between the gravitational force, the mass of objects, and the distance between objects.

Here are two simple points that explain the law:

  • The gravitational force between massive objects is more as compared to light objects.
  • The gravitational force between two objects decreases exponentially as they go away from themselves.

Here is the mathematical expression of the universal law of gravitation:

F = G*M1*M2/r^2

G = Gravitational constant, whose value is equal to
M1 = mass of the first object
M2 = mass of the second object
r = distance between two objects

By knowing gravity, you can calculate when a ball will touch the ground after being released into the air. You can measure and graph the path of a projectile. You can calculate how much propellant is required to push a rocket into space, and many more.

Do Heavy and Light Objects Fall at the Same Time?

galileo-galilei-gravity-experimentThe simple answer is, yes. But let’s understand, why it happens.

Gravity at the surface of the earth is 9.8 m/s, which means an object falling towards earth gains 9.8 m/s speed in every second. If a ball is dropped from a building it will have 9.8 m/s speed at the end of the 1st second, 19.6 m/s at end of the 2nd second, and 29.4 at the end of the 3rd second and so on. Every object is accelerated at the same rate towards earth, whether it is a paper clip or a stone. If two objects are dropped from a height, they both will hit the ground at the same time.

But, wait a minute!

You may be thinking, why a paper falls slower than a paper clip when both are dropped from the same height? It is because of the air resistance. The paper clip has lower surface area than the paper, which results in higher air resistance for the paper. But, if you drop paper and paper clip into a vacuum chamber both will hit the bottom at the same time. It is because vacuum chamber does not have air, so there will be no air friction to slow any object.

Difference between Mass and Weight

mass and weightIn everyday life, you may have used the term mass and weight interchangeably. But in physics, mass and weight are two different quantities.

The mass of an object is the quantity of matter in it; usually measured in kilograms. But, weight is the force of an object that it is putting on a platform due to gravity.

For example, if you hold a brick in your hand you will feel a downward force. If you somehow take that brick to the moon, you will notice much less downward force. Because the moon has less gravitation than the earth. In both cases, the amount of the matter in the brick will be same on the earth and the moon.

Difference between Gravity and Gravitation

There is a difference between gravity and gravitation. Gravitation is the force of attraction between objects that have mass. But, gravity is the gravitational force between earth and other objects. It means that gravity is the special case of gravitation.

Gravitational force exists since the formation of our universe. It is one of the four fundamental forces of nature. It is the force that attracts atoms together to form stars and planets. It keeps the moon in orbit around the earth and the earth around the sun.

Fun Facts

  • Gravity keeps all the planets in the orbit of the sun, and it keeps our moon in orbit of the earth.
  • If any object goes upward with a velocity of 11 km/s, it will escape the gravitational pull of the earth. This velocity is called the escape velocity, and it is different for different planets and moon.
  • The gravity of our sun is about 274 m/s^2. Anything weighing 1 kg on the earth will weigh about 28 kg (If it was possible!).

A Brief History

Let’s learn something about the history of gravity, and find out about the people who contributed to explaining the gravity.

Who first pointed to the presence of gravity? And when? It is still unknown. But you may consider that early human beings had probably thought about it. Because, it is the force whose effects we can clearly see, like falling back to the ground after jumping.

  • The first person who gave the hypothesis about gravity was, Aristotle. He hypothesized that “heavier objects are accelerated more towards ground than lighter objects”. He was actually wrong because he didn’t know about air resistance which slows down the objects. But, he at least provided the ground for the great scientists to think and research about gravity.

  • In late 16th century, a famous scientist Galileo Galilei performed an experiment. In this experiment, he dropped two balls from a height and proved that light objects and heavy objects fall at the same rate.

    Galileo Galilei
  • In 1687, Sir Isaac Newton developed the universal law of gravitation. Which explained that lighter objects have less gravitational force of attraction than massive objects. Also, the gravitational force decreases exponentially as the distance between two objects increase. This is also known as the inverse square law.

    Isaac Newton
  • The modern understanding of gravity was described by Albert Einstein. He described the gravity as a curvature in space and time. This curvature is caused by the objects with mass; the massive objects cause more curvature than the lighter objects.

    Albert Einstein

temperatureIn our everyday life, we tell the hotness and coldness of different substances and weather in terms of temperature. We also use temperature to find the severity of the fever. For example, our normal body temperature is 98 ⁰F. This temperature increases when we caught in the fever. We also set our oven and electric stoves at a certain temperature to cook food.

What is Temperature?

We use temperature to describe the hotness. But in physics, temperature is defined as the average kinetic energy of molecules in a substance. If the temperature of a substance is more, then its molecules will have higher kinetic energy.

How temperature is measured?

Temperature is measured with a device called thermometer. Thermometer works on the principle that, when the temperature of a substance rises they expands.

Some materials, like mercury and alcohol expands more. So, they are used in the thermometers. However, electronic and infrared thermometers are also developed. They use special sensors to measures the temperature instead of mercury or alcohol.

Temperature scales

There are various units to measure the temperature of a substance. These units are Celsius, Fahrenheit, kelvin, and Rankine.

  1. Celsius: Celsius is widely used unit in the world. It measures the temperature in degrees, abbreviated as ⁰C. This temperature scale measures the freezing point of water at 0 ⁰C and boiling point at ⁰C.
  2. Fahrenheit: Fahrenheit is famous in US and it is also used by doctors. This temperature scale measures the freezing point of water at 32 ⁰F and boiling point at 212 ⁰F.
  3. Kelvin: This scale is also considered as absolute scale and is mostly used by scientists. This scale is not measured in degrees and is abbreviated as, K. The zero point is set at the absolute zero; the temperature at which average kinetic energy of a substance is zero. The freezing point of water is measured at 273 K, and boiling point at 373 K.
  4. Rankine: Rankine is not used very much today. The zero of this unit is also at absolute zero. This scale is measures the temperature in degrees and is abbreviated as, ⁰


  • The scale Kelvin was founded by Lord Kelvin. He Proposed the unit on the basis of absolute temperature of a substance.
  • The first thermometer was invented in 1600s and was called thermoscope.
  • Electronic thermometers use the electrical property of substances to measure the temperature.


When winter arrives, we all prepare our heating system which includes an electric heater, coal heaters, or gas heater. These heaters provide us heat in winter so we can warm up our bodies. Also, we use heat for cooking our food. Sometimes excess heat disturbs our living, so air-conditioners are used to remove that excess heat.

What is heat?

Heat is actually a kind of energy transfer from one object to another, when there is a temperature difference between them. Heat always flow from hot object to cold object. Don’t confuse yourself with heat and temperature, they both are separate quantities. Heat is measured joule; the unit in which energy is measured. Whereas, temperature tells, how hot a substance is?

Heat is measured in joules; the same unit of energy measurement. There are other units for measuring heat, which are BTU (British Thermal Unit) and calorie.

Heat transfer

Heat can be transferred in several ways. There are three methods in which heat can be transferred. These methods are:

  1. Conduction: Conduction takes place when two objects are in contact with each other.
  2. Convection: In this methods, heat is transferred by flow and diffusion of liquids from high-temperature regions towards low-temperature
  3. Radiation: In this method, heat is transferred by radiation e.g. visible light and infrared rays.

Read more about Heat transfer in our article: Heat Transfer

Effect of heat on states of the matter

As the heat of an object increase, its temperature also increases. Each type of substance has a certain value to remain in its particular state (solid, liquid, or gas). When temperature increases or decreases from these values the state of the substance is changed. For example, water turns into ice when the temperature drops below the 0 ⁰C. When the temperature reaches 100 ⁰C, the water starts to boil and is converted into steam (gaseous state of water).

Note: Pressure of the atmosphere also affects the state of the substance.


  • A substance containing more heat does not always mean that its temperature is also high.
  • A cup of tea at 50 ⁰C has a lot more heat than a drop of water at 99 ⁰
  • Each material has a certain capacity to hold the heat at a particular temperature. It is called the heat capacity.
  • A cup of water at 99 ⁰C will not warm up the bucket of water. But, another half filled bucket at 60 ⁰C can warm up the water in the first It is because half filled bucket has more heat than the cup of water at 99 ⁰C.