physics - science4fun

Radioactivity is a very famous term in nuclear physics and chemistry that describes how unstable atoms release certain radiations for the purpose of getting stability. You may have probably heard about uranium, a radioactive element, used for making atomic bombs and electricity from nuclear power plants. It is possible due to the radioactive nature of uranium.

What is Radioactivity?

Radioactivity is a natural phenomenon in which atoms of elements that have unstable nuclei (nuclei = plural of nucleus), disintegrate for getting stability. There are 3 main reasons behind an unstable nucleus in an atom. These reasons are:

higher number of protons
higher number of neutrons
higher energy of the nucleus

When an element decay, it can change from one element to another for achieving stability. Sometimes when an element decay, it doesn’t change from one element to others but simply gets stability by releasing extra energy in the form of gamma rays.

Isotopes

periodic-table — Periodic table of elements

There are around 118 known elements that differ from each other by the number of protons in their atoms. For example, hydrogen has 1 proton and helium has 2 protons. The nucleus also has neutrons which when change in an element, creates a new isotope of that element.

Isotopes of an element have the same chemical properties, but they differ only in weight due to a difference in the number of neutrons. For example, hydrogen has three isotopes; protium, deuterium, tritium. These isotopes differ from each other due to a difference in the number of neutrons, while the number of protons remains the same. Elements from hydrogen to bismuth in the periodic table have at least one stable isotope. But, elements after bismuth have no stable isotope; they are all radioactive.

Types of Radioactivity

Radioactive isotopes of elements can emit 3 three types of radiations known as alpha rays, beta rays, and gamma rays.

Alpha rays – Alpha rays are positive charged particles that are composed of two protons and two neutrons (also called nuclei of helium). Alpha rays have a positive charge due to the presence of protons; the positively charged particles.
Beta rays – Beta rays are the streams of negatively charged particles, e.g. electrons, or positrons.
Gamma rays – Gamma rays are neutral and are different from alpha rays and beta rays because they are not made of matter particles. Instead, they are high energy electromagnetic radiations; a form of light that is very energetic and can’t be seen with naked eyes.

Half-Life and Measurement of Radioactivity

A sample of a radioactive isotope of an element decays continuously at a fixed rate until the whole material has change into a stable isotope of an element. The term half-life is often used to define time in which half of the sample will turn into stable isotope. For example, the half-life of carbon-14 is around 5,730 years. It defines, if you place 1 kg sample of carbon-14, half of the carbon-14 will turn into a stable isotope in around 5,730 years.

Radioactivity and Human Health

Radiations from the decay of radioactive isotopes are not considered good for health due to several dangers involved in it. For example, when these radiations act on the human body, it damages the DNA and structures of cells. This results in the production of cancerous cells in our body and ultimately make a person very sick.

Facts

Radioactivity was first discovered by Henry Becquerel while working on an ore of uranium element called pitchblende.
Not all radiations are bad! We use X-rays to take photographs of the human body’s inside for detecting any damage or diagnosing some diseases.
Gamma rays are very penetrating radiations, they can easily penetrate through 1 meter thick wall of concrete – much like visible light penetrates through a transparent glass.

Do you remember the movies where alien forces use laser guns in battles? Well, lasers are a very famous piece of technology. These were once considered as high-tech gadgets, but today modern science made lasers very common technology used in everyday life.

What is LASER?

A laser is just a device that emits light, but it is different from light bulbs or LEDs. So, what makes them different? The main difference is that lasers emit coherent light. So, what is coherent light? Coherent light has all its waves in phase with each other – It means their crests (upward bulges) and troughs (downward bulges) line up together. Also, the lasers emit monochromatic light or a single wavelength of light – they don’t have a bunch of colors, but only one.

So, what is special about coherence? The coherence of light allows a laser to focus on spots very tightly. Other sources of light such as light bulbs don’t give coherent light. As a result, their light is spread in all directions throughout the empty regions e.g. rooms. Furthermore, Laser’s coherence allows it to focus very precisely, which is very useful in many applications.

How Does Laser work?

The working of LASER lies in its name. The LASER is not just a name but it is an acronym, which stands for “Light Amplification by Stimulated Emission of Radiation”. Let’s understand its working step by step.

The electricity from a battery cell excites the atoms of semiconductors material inside the laser. As result, the electrons of these atoms move to higher energy level.
The electrons of excited atoms when falling back their normal level emit photons of light.
The emitted photons of light are allowed to reflect back and forth on two parallel mirrors. This results into stronger and stronger beam of light as suggested by word “amplification”.
The photons that are not able to reflect back and forth due to the wrong angle, they go back and strike the atoms again. As a result, two coherent photons are emitted – called “stimulated emission”.
The one mirror from the two reflects light partially – means it allows some of the light to pass through it. When the beam become sufficiently strong, it goes through the partial reflecting mirror and hits the spot where the laser points.

Uses of Laser

Lasers are very useful technology and it has revolutionized many fields. Some uses of lasers are discussed below.

Communication

optical-fiber-cabler-laser — Optical fiber cable with laser beam

Today we are able to access very fast internet connections. This is due to the use of optical fiber cables. Optical fibers carry the information signals in form of laser-light from one place to another. Before the use of lasers, information was sent and received as electrical signals through electrical cables.

Industry

Today, we are able to measure precisely in nano-meters (billionth of a meter), and cut the materials very precisely with the help of a laser. The other uses of lasers in industries are welding, cutting, and carving.

Electronic Goods

We use DVD players regularly to watch movies, programs, and songs. The DVD players get the data from a disc with the help of a laser. A laser points at the very fine details of a disc where the information is stored. After laser light hit the disc, some light is reflected and falls on a sensor. Then, the sensor interprets the data into an electrical signal for sending towards a TV.

The other common electronic goods that use lasers are laser pointers used by the teacher, laser pointers used in celebrations, and toys.

Medicine

Lasers are playing a vital role in today’s medicine. The main uses of laser in medicine are laser surgery of eyes, healing the skin, treating the kidney stone, cosmetics, and many more.

Weapons

on-board-navy-laser-gun — Navy on-board laser gun

Today, humans have developed very sophisticated weapons of destruction. The missiles are able to reach the target location precisely with the help of GPS and laser guidance.

Laser guns are also introduced that can make a person temporary blind from distance of few kilometers. Furthermore, the laser that can damage and destroy objects directly on the skies, oceans, or ground are under testing.

Research

Lasers are used by scientists around the world to study the atoms, stars, and other cosmic objects. The most important part that lasers play is in the field of spectroscopy. In spectroscopy, the spectra of the atoms and molecules are analyzed to find out about unknown atoms.

The other uses of lasers in the field of science are in heat treatment, weather, photochemistry, nuclear fusion, and microscopy etc.

Facts

Lasers can be used to push an object! But, you need at least 30 billion common laser pointers to a move a coin.
In 2016, Stephen Hawking announced “Breakthrough Starshot proposal”. This proposal was aimed to push a small spacecraft to space with the one-fifth of the speed of light.
Recently, scientists have discovered gravitational waves with the use of two large lasers.

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.

Atoms 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

An 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.

Atomic Number and Elements

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.

Electron

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.

Proton

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.

Neutron

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

When 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

Convection 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

In 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.

Facts

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.

Have you ever tried to jump?

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

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.

You 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

Don’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?

The 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 1^st second, 19.6 m/s at end of the 2^nd second, and 29.4 at the end of the 3^rd 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

In 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.
Aristotle
In late 16^th 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