December 28, 2015

ELECTROMAGNETISN

1. Charges and force between charges

Electricity is related to charges. What is charge? Charge is a strange thing given in nature. It is not seen or felt ordinarily. For example if you touch a table you feel no effect due to charge.

(Picture-1)

Around 200 years back, charges were discovered to be in matter by the simplest process of rubbing two substances. Much later they came to understand the fundamental essence of charges. This happened when they discovered that all matter is made up of atoms and molecules (molecules are only atoms joined together).

In an inch of any material, there are crores and crores of atoms / molecules ! ( see the next chapter for the details) Now electricity is a property of electrons and protons. Electrons and protons are in atoms. Electrons are negatively charged, protons are positively charged and there is a third type of particle called neutrons. Neutrons have no charge. The negative and positive are only names to say that there are two kinds of charges. The protons and neutrons reside in a central nucleus. The electrons orbit around the nucleus. Now, the property of these two kind of charges (positive and negative) is that positive and positive repel (go away from each other), negative and negative repel and negative and positive attract! This is given in nature !

When two substances are rubbed with each other, electrons from one substance (Say A) escape into another (Say B). B has now more electrons than protons so it has negative charge. B has lost electrons, so it has more protons than electrons.

So it has positive charge. Thus A and B attract each other since there are unlike charges. If A is taken to similar negatively charge body C, A and C repel. If B is taken close to a similar positively charged body D then B and D also repel. Repulsion happens because like charges repel. The force between charges (the repulsion or attraction) is called electrical force. The closer the charges are, greater the force. More the charges, more the force. This is expressed in the equation f = k q1 q2 / r2 , where q1 and q2 are the charges of the two bodies and r is the distance between them. This is called Coulomb’s Law.

(Picture-2)

2. Currents, Ohm’s Law and Batteries

Currents are flow of charges in a conducting wire. How can such a flow of charges be created. Currents can be created using batteries. It was Volta created the first battery.

Working of a battery :

A battery has three main parts: a positive electrode (terminal), a negative electrode, and a liquid or solid separating them called the electrolyte. The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder. When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions and electrons at the negative electrode. The positive ions flow through the electrolyte to the positive electrode. Meanwhile, the electrons flow around the outside circuit to the positive electrode and make the lamp light up on the way.

(Picture-3)

parts of a battery

(Picture-4)

Principle of a battery

understand the mathematics of current. The current in a wire is directly proportional to what is called a potential difference between two points. Potential difference is defined as the work done to carry a unit positive charge from one place to another.

The more the potential difference, more the amount of current. V a directly proportional to i. So V = i x R Where R is the resistance in the wire to the flow of current. Current is measured in amperes and potential is measured in volts. Resistance is measured in Ohms. This relationship between potential and current is called Ohm’s Law.

(Picture-5)

3. Magnetic effect of current

We have learnt about charged particles. There are positive and negative charges. We know that like charges repel and unlike charges attract. We have also seen that a current can be created through a battery and we have also understood Ohm’s Law.

It was first observed by Oersted that a magnetic needle is affected near a current carrying wire. He accidentally discovered this phenomenon while giving a lecture. It was one of the greatest turning points in the history of science !

Current in a wire brings about a magnetic effect in the space around it. To put it simply current in a wire behaves like a magnet. What is the direction of the magnetic field created by current carrying wire. This is given by the right hand thumb rule. If you imagine holding the current carrying wire in your right hand and with your thumb pointing in the direct of the current, then the curl of your fingers gives the direction of the magnetic field. See figure.

(Picture-6)

4. Electro magnetic induction

We have seen that a current carrying wire produces a magnetic effect. Is the reverse true ? Is there a relationship between magnets and currents ? Michael Faraday wondered about it and tried many experiments and finally discovered that if you move a magnet near a wire, current is induced in it. This is called electro magnetic induction and this was used to make a generator.

So in effect we have two principles. The current carrying wire brings about a magnetic effect and moving magnet induces current ! These are the fundamental laws of electricity and magnetism !

Electromagnetic induction

(Picture-7)

5. The working of motors

Motor work on the principle that a current in a wire creates a magnetic field. What we do is we take a coil bent it into a loop and place this coil between two magnets. The right hand side and left hand side will experience opposite forces of repulsion. Thus they will be turning motion. A turning motion is a called a torque. In practical motors many loops are taken and magnets are replaced by electromagnets.

(Picture-8)

6. The working of generators

Generators work on exactly the opposite principle. Here the loop is given a motion so that electric current is induced in it.

(Picture-9)

7. Electromagnetic waves (Spectrum)

We have two principles till now. Firstly we saw that when a current flows in a wire it becomes like a magnet creating a magnetic field around it. When such a wire in the form of a loop is kept between two magnets, we have a motor.

Secondly we saw that if there is relative motion between a magnet and a wire a current is induced in the wire. This principle is used to make generators.

Now, the current in a wire is steady. What if the current oscillates ? In other words, what if the current is not steady. A wondrous thing happens ! The changing current creates a changing magnetic field. But according to Faraday’s Law this changing magnetic field should create in turn electric field. But the changing electric field again creates a changing magnetic field and so on.

So when a current is not steady, electric and magnetic fields are created and they propagate outwards as waves. And these waves are called electro - magnetic waves. Maxwell found out the velocity of such waves and it was exactly equal to the velocity of light. Thus they discovered that light waves actually are electro magnetic waves. Electro magnetic waves have various frequencies and wave lengths and the whole range is called the electro magnetic spectrum.

(Picture-10)