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Where does electricity come from? How can electricity preserve state symbols? Do fashion designers use lightning rods? Why does a beech endure a lightning strike better than an oak? All of these and many other questions are answered in this book. It will lead you in a consistent and clear way from the oldest findings to some of the phenomena which the scientists still do not understand.

Wonders of Electricity

v a l s i m o T ski n a ć n e S


Wonders of Electricity illustrations ić Milan Pavlov pina & Jovan Ukro


I am very grateful to Prof. Svetislav Smiljanić, PhD, Dipl. Ing., Electrical Engineering for his professional assistance in the writing of this book. Tomislav Senćanski


Electric charge


er

b m a t u o b a y r A short sto

M

any thousands of years ago, the ground of northern Europe was covered by vast conifer tree forests. The trees of the prehistoric forests were damaged by hungry insects and fungi. These pines healed their wounds using yellow resin, which then slid down their bark and fell to the ground. Hundreds and then thousands of years passed. These ancient forests stopped growing, and the resin withered and hardened. Sea tides took away the pieces of yellow resin and the waves brought them back to the shore. For centuries, people gathered the hardened resin that the waves threw out on the shore. The pieces of resin varied in size. Some pieces weighed over 3 kilograms. The hardened resin was named amber. Phoenicians were famous seafarers and tradesmen in the ancient times. They voyaged to distant northern parts of Europe to gather the “sea gold�. They spread fantastic stories about amber in order to raise its value. In this way, the price of amber became equal to the price of gold! 5


What gave amber its value was the yellow colour. There were transparent, waterlike pieces of amber, pieces which were white as chalk, or green, reddish and dark-red like blood. Skilful merchants used hot oil to change the colour of the pieces of amber in accordance with the wishes of buyers.

Some pieces of amber keep remains of plant and animal life from the time of their creation. These remains have been preserved fascinatingly well, which enabled today’s scientists to take a good look to the distant past. In Spain in 2003, the remains of a 110 million-year-old spider web was discovered! This proves that these hunters of insects existed at the time of dinosaurs.

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Ancient Greeks believed that pieces of amber were petrified rays of sunshine.

Amber was used to make different ornaments. People used it to adorn themselves. They wore necklaces made of shiny amber, believing that it protected them from disease. Amber is still admired for its aesthetic values. Jewellery made of this hardened resin is still appreciated, but amber today has technological and scientific importance as well.


From the historical point of view, amber played its most important role in the discovery of electricity. The word electricity comes from the word electron, which is the name the ancient Greeks used to signify amber. Even then, people knew that if you rubbed amber with cloth, you would give it the ability to attract other, lighter objects.

The word amber is of Persian origin. It came into the English language through Arabic. In the Persian language, it signifies something that attracts straw, particles, insects... William Gilbert, English physician and naturalist, performed experiments with amber in the 16th century. He rubbed a rod made of amber with cloth and brought it close to a small needle that could rotate. Based on the strength of the needle’s rotation, he roughly assessed the amount of electricity in the piece of amber. Apart from amber, Gilbert also performed experiments with other materials. Depending on whether the needle rotated or not, he would determine the presence of electric charge.

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Benjamin Franklin

American scientist and statesman Benjamin Franklin, who lived in the 18th century, noticed that certain electrically charged bodies attract each other, whereas others repel each other. This led him to introduce the theory of two types of electric charge: the one which arises in glass after being rubbed by cloth he named positive charge, and the one which arises in amber he called negative charge. It was determined through experiments that objects with the same type of electric charge (positive or negative) repel each other, and that objects with different types of electric charge (positive and negative) attract each other.

Objects with the same type of electric charge repel each other.

Objects with different types of electric charge attract each other.

Today we know of many materials that can become electrically charged easily, such as polyvinyl (and other types of plastic), ebonite (a type of hard rubber), glass and many others. When friction occurs, changes arise in objects, which then give them the opportunity to attract other, lighter objects. We cannot perceive these changes by our senses. In order to explain them, we need to have basic knowledge on the structure of objects. 8


Atom –

ty i c i r t c e l e f o the source

E

verything is made of atoms: the book that you are reading, the desk you are sitting at, the house you live in, the tree in front of your house, you and the air you are breathing‌ Atoms existed long before humans appeared on Earth. They existed before Earth itself was born.

The idea of atoms is very old. It stems from the ancient Greek philosopher Leucippus. His idea was further developed by Democritus, one of the most important thinkers of the ancient period. According to their belief, all things consist of parts so small that they cannot be divided further. These smallest parts of which everything is composed in nature are called atoms. The word atom stems from the ancient Greek word atomos, which means indivisible, something that cannot be divided further.

Leucippus

Democritus The idea of atoms did not develop during the following centuries. Scientists began to revisit this idea only in the 18th century. English scientist John Dalton proved the existence of atoms, and defined their size at the beginning of the 19th century, using experiments. John Dalton 9


In the beginning of the 20th century, the news of English physicist J. J. Thomson’s experiments caused quite a stir. Thomson proved that atoms are not indivisible, but rather that they consisted of even smaller parts! People were even more surprised when they found out that these small particles, which were the integral parts of the atom, were electrically charged! J. J. Thomson About ten years after Thomson’s discovery, a scientist from New Zealand, Ernest Rutherford, after performing an experiment understood how these small, electrically charged particles make an atom. Ernest Rutherford In the centre of the atom there is the positively charged nucleus. Electrons, the negatively charged particles, move around it at a great distance. The nucleus consists of two types of particles: positively charged protons and neutral (not electrically charged) neutrons. The amount of charge of electrons and protons is of the same value, but of a different type. This amount of electric charge also represents the smallest possible amount of electric charge that can be found in nature. Since atoms always consist of an equal number Rutherford’s model of the atom of protons and electrons, this means that the atom is a particle which is not electrically charged (it is neutral). 10


130 m 13,000,000 m

In order to better understand the relations that exist within the atom, let us imagine that atoms were the size of our planet. The electrons would move around the surface of the Earth, and the nucleus would be placed in the centre of the Earth and it would be the size of a sphere with a diameter of 130 metres! The entire space between the core and the electron would be empty. This image is similar to the appearance of the Solar System.

Rutherford’s interpretation was complemented by the work of his student, Danish physicist Niels Bohr, and by many other scientists. Today, we know a lot about the atom, but there is much that science still cannot grasp. It has still been impossible to faithfully represent the appearance of an atom. The human eye could never see an atom, and the reason is simple: nerves of the eye are not sensitive to the light that comes from such a small object. People have confirmed the existence of the atom only by that which can be seen through the use of research instruments. Nobody knows what all parts of the atom look like. We only use models that enable us to study their structure.

Niels Bohr

If all people in the world would shrink to the size of an atom and hold hands, this chain would be just a few meters long.

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Due to a great number of experiments and calculations, scientists are constantly reinterpreting the miraculous processes within the atom and are creating a more and more precise picture of it. The facilities scientists use consist of tunnels which are up to ten kilometres long.

Were Leucippus and Democritus wrong when they envisaged the atom as an indivisible particle – a particle without any components? No. They were right! Dalton was wrong for concluding that the particles he discovered were in fact the smallest existing particles. Dalton’s “atom” is today often referred to as the chemical atom and it consists of electrons, protons and neutrons. Moreover, even protons and neutrons consist of smaller particles – quarks!

Can we split grains of sand to infinity? No!

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ts c e j b o g n i g r trically cha

Elec

E

very atom has an equal number of electrons and protons. This is why they show no signs of being electrically charged, and we say that they are electrically neutral. Through external activities (mechanical, heat, light, electric, etc.), atoms can receive or release one or more electrons, thus becoming charged. Atoms with a surplus or shortage of electrons are called ions, and the process of obtaining “electrically charged atoms� is called ionization. If an atom receives an electron, it becomes a negatively charged ion. If it an atom releases an electron, it becomes a positive ion.

Since all bodies consist of atoms, they are not electrically charged, just like atoms. If, through a procedure we manage to ionize atoms that make up an object, that object will become electrically charged. We will describe some of the ways how objects can be charged.

A metal ball will keep its electric charge longer if it is kept in oil than in air. This happens because in the air there are much more electrically charged particles that can remove the electric charge.

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Charging objects by friction Friction is the most common way of charging objects. As we know, people have electrically charged amber since the ancient times by rubbing it with cloth. What makes bodies become charged by friction? When two different materials come into contact, the electrons spontaneously move from one object to the other. A book exchanges electrons with the desk it is on, the human body exchanges electrons with the clothes we wear... The number of electrons that will be exchanged, and the answer to the question which object will receive and which will release electrons, depend on the structure of the objects in question.

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In most cases, the exchange is so small that no effects can be noticed without sensitive instruments. Sometimes a much greater number of electrons pass from one object to the other than the other way around, which gives these objects a strong electric charge. In order for the exchange of electrons to have even more impact, the contact between objects needs to be improved. That is why we rub objects against each other, producing friction which leads to electric charge. In fact, during friction, both types of electricity appear. The object that releases more electrons becomes positively charged and the object that receives more electrons becomes negatively charged. Experiments have shown which pairs of objects are good at exchanging electrons. For example, glass and silk, polyvinyl and fur, sealing wax and fur, precious stones and fur... Wood, metal and some other materials obtain very weak electric charge by friction, since it is difficult for them to release electrons. Electrons can pass through metals freely, and that is why metals are excellent conductors of negative electricity.

A

B Both atoms contain the same number of electrons and protons, and that is why they are neutral.

A

B

Atom A has given one electron to atom B. Now, atom A is positively charged, and the atom B is negatively charged. 15


By rubbing a polyvinyl rod with a wool cloth, electric energy is obtained.

After rubbing a glass rod with a silk cloth, glass releases electrons. They pass to the silk cloth, so the glass becomes positively charged, and the silk negatively charged.

In the beginning, the silk cloth and the glass rod are neutral. Each of them contain the same number of protons (+) and electrons (-). During the contact between the cloth and the rod, electrons move from the cloth to the rod, and the other way round. Much more electrons move from the glass rod to the silk cloth.

After the friction, the silk cloth has a surplus of electrons, so it is negatively charged. The glass rod has a deficit of electrons, which means that it is positively charged. 16


When a polyvinyl rod is rubbed by fur (or wool), electrons move from the fur to the polyvinyl rod, so polyvinyl becomes negatively charged and the fur becomes positively charged.

Fur and the polyvinyl rod are neutral in the beginning. Each of them contain the same number of protons (+) and electrons (-).

While there is contact between fur and the polyvinyl rod, electrons move from the fur to the rod, and the other way around. Much more electrons move from fur to the polyvinyl rod.

After friction, the polyvinyl rod contains a surplus of electrons, so it is negatively charged. The fur is left with a shortage of electrons, so it is positively charged.

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