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# Electromagnetism

It involves the study of the interaction between electricity and magnetism. It is related to electromagnetic fields. The electromagnetic field is generated by electrically charged object.

The waves of particular frequency like radio and infrared waves etc are electromagnetic fields. The phenomenon of producing the electricity by altering the magnetic field is called electromagnetic induction.

## What is Electromagnetism?

In the electromagnetism, an electric current is produced by magnet. So magnetic field is generated by passing a current through a conductor and conductor behaves like a magnet. This process is known as electromagnetism and the magnet in which magnetic field is produced by current flow is called an electromagnet. This magnetic field disappear with lose of current.

This generated magnetic field is also called magneto motive force. this force is proportional to the product of passing current through magnet and number of turns in wire or coil. Like in the case of straight wire, the magnetic field pattern is in the form of concentric circles around the wire as shown in the figure below.

The induced magnetic field is inversed as the direction of current inverse. The direction can be measured by right-hand grip rule or screw rule.

### 1. Right-hand grip rule

If we grip the wire in right hand than the thumb shows the current direction and fingers show the direction of magnetic field around the wire.

### 2. Maxwell Screw Rule

According to this rule, if a right-handed screw is turned then the movement direction shows the direction current while the rotation direction is the direction of the magnetic field.

### Strength of magnetic field

1. The strength of magnetic field is depends upon the magnitude of current.
2. It increases with increasing the magnitude of current.
3. It also decrease as we move further out of the field.
4. Similar in the case of coiled wire, the magnetic field force produced by a coiled wire depends on the current through the wire and the number of turns of wire in the coil.
5. The generated magnetic field force in collie wire is grater than the magnetic field of straight wire.
6. The process of electromagnetism is used in various fields.
7. This is used in the production of transformers, motors, home appliances etc.

## Photo Electric Effect

The photoelectric effect involves the emission of electrons from the surface a metal when an incident light absorb by metal surface. The energy of incident light is absorbed by electrons of metal surface and electrons emit from the surface.
Thus photoelectric effect is the process of emitting the electrons from the matter when matter expose to electromagnetic radiation like photons of light.

When a matter surface is exposed to electromagnetic radiation with sufficient energy then the energy is used for emitting the electrons from the matter surface. The minimum frequency at which the emission of electrons takes place is known as threshold frequency.

The photoelectric effect occurs with the radiation having frequency up to threshold frequency. The value of threshold frequency is different for different materials like for alkali metals (visible light) and for some metals (near-ultraviolet light), and for nonmetals its extreme-ultraviolet radiation. Generally the photoelectric effect takes place with photons of energies from a few electron volts to over 1 MeV.

Compton scattering may take place with the high photon energies up to 511 keV. Einstein suggested the equation for explaining the photo electric effect.
• According to him, the radiations of all frequency (light) have quanta which are called photons.
• The energy of each quantum of light is equal to the multiplication of frequency of light and a constant.
• This constant is known as Plank's constant.
• Only those protons are capable to eject the electrons which have minimum frequency value or threshold frequency and generate the photoelectric effect.

### Einstein equation

The Einstein equation describe the phenomena of photoelectric effect but the validity of this equation is only up to two regions. One is visible and second is for ultraviolet light.

So,

The energy of photon = energy required to remove electron + kinetic energy of the emitted electron

Energy of photon = h $\nu$

Where,
h = Planck's constant
$\nu$ = frequency of incident light or photon.
So
h$\nu$ = W + K

Where,
• W = the work function the work function is defined as the minimum energy needed to remove an electron from the surface of a given metal. It is equal to  $\nu^{o}$.
• E = this is the kinetic energy (maximum) of emitted electrons. This is equal to (1/2) mv2
• where, $\nu^{o}$ is the threshold frequency.
• m = rest mass of the emitted electron
• v = the speed or velocity of the emitted electron. If the energy is less than work function then no electron will be ejected.
According to Einstein's theory of relativity, the relation between energy and momentum (p) of a particle is given by equation below.

E = [( pc )2 + (mc2)2](1/2)

Where, m is mass of the particle and c is the velocity of light (in vacuum).

Thus in the photoelectric effect, The rate of the emitted photo electrons is directly proportional to the intensity of the incident light at particular frequency of incident radiation. The whole photoelectric process takes place with in small time approximately less than 10-9 second or the time between the incidence and emission of a photo electron is very small.

The photo electric effect does not occur at frequency less then threshold frequency. At the frequency above the threshold frequency, the kinetic energy of the ejected electrons depends only on the frequency of the expose radiation not on its intensity.

## Electromagnetic Phenomena

The electromagnetic phenomena are studied under quantum electrodynamics. The electromagnetic phenomena involve the physical process which can be felt by un-aided human senses.

This can be light, electromagnetic radiation, mechanics, magnetism, electricity etc. But the gravitation is not included in that.