Electromagnetic Waves | Part 3 | Ultraviolet rays, Visible light, Infrared rays, Microwaves

to view Electromagnetic waves | Part 2 | Electromagnetic Spectrum, Gamma rays, X- rays click the link below 

Electromagnetic waves | Part 2 | Electromagnetic Spectrum, Gamma rays, X- rays

Ultraviolet rays (UV-rays)

Explanation

      1. Ultraviolet rays are electromagnetic waves of short wavelength ranging from nearly 10^-8 m to 3.9 ✕ 10^-7 m.

      2. Their wavelengths are shorter than those of violet light.

      3. They can be produced by the arcs of the mercury and iron. They can also be obtained by passing discharge through hydrogen and xenon.

      4. The sun is the most important natural source of UV- rays. 

      5. UV- rays were discovered by Johann Wilhelm Ritter in 1801.

Properties

      1. UV- rays can travel in vacuum with the speed of light.

      2. They can undergo reflection, refraction, interference and polarization.

      3. UV- rays can cause photoelectric effect.

      4. They produce fluorescence in certain materials.

      5. They cannot pass through glass but they can pass through quartz, fluorite, rock salts etc.

      6. UV- rays possess the property of synthesizing vitamin D when the skin is exposed to sunlight.

Uses

      1. UV- rays are used for checking the minerals samples.

      2. They are used to study molecular structure.

      3. UV- rays destroy bacteria and hence they are used for sterilizing surgical instruments.

      4. Being invisible, UV- rays are used in burglar alarm.

      5. They are used in high resolving power microscopes.

      6. They are used to distinguish between real and false gems.

      7. They are used in analysis of chemical compounds.

 Visible light

Explanation

      1. Visible rays are narrow part of electromagnetic spectrum which is detected by human eyes. 

      2. The wavelength range varies between 3.9 ✕10^-7 m to 7.5✕ 10^-7 m.

      3. The frequency range of visible light is 8 ✕10^14 Hz to 4 ✕10^14 Hz.

      4. Visible light is produced when excited atoms return to their normal states.

      5. Visible light consists of different colours ranging from red to violet.

      6. Different wavelengths give rise to different colors. The wavelength of red colour being largest while that of violet colour being least. The wavelength range of various parts of visible rays are as follows: 

                     

Infrared rays 

Explanation

       1. Infrared rays are electromagnetic waves which are responsible for heat radiation.

       2. All hot bodies are sources of infrared rays. 

       3. The wavelengths ranges nearly from 4 ✕10^14 Hz to 3✕ 10^11 Hz 

       4. The frequency ranges from 4 ✕10^14 Hz to 3 ✕10^11 Hz.

       5. About 60% of solar radiations are infrared in nature.

       6. It was discovered by Sir Frederick William Hershell in 1800

Properties

       1. Infrared rays obey laws of reflection and refraction.

       2. They produce interference and polarization.

      3. They affect the photographic plates.

      4. When infrared rays are allowed to fall on the material surface, its temperature increases.

      5. These rays are strongly absorbed by glass.

      6. Thermocouple, thermopile, bolometer etc are used to detect infrared rays.

 Uses

      1. Infrared rays are used in long-distance Photography (photos taken in complete darkness).

      2. They are used in diagnosis of superficial tumours and varicose veins. 

      3. They are used to cure infantile paralysis (polio) and to treat sprains, dislocations and fracture.

      4. They are used in solar water heaters and solar cookers.

      5. Used in medicine.

      6. They use to keep green house warm.

      7. They are used in remote controls of T.V., V.C.R., etc.

Microwaves

Explanation

      1. Microwaves are electromagnetic waves of short wavelength ranging from nearly 1 ✕10^-4 m to 0.3 m.

      2. Frequency range of microwaves is 5✕ 10^9 Hz to 10^12 Hz.

      3. Microwaves are produced by special vacuum tubes called klystrons, magnetrons and Gunn diodes.

Properties

      1. They obey the laws of reflection and refraction.

      2. They travel in vacuum at the speed of light.

      3. They heat an object on which they are incident.

      4. Their absorption by water, fats, sugar, can produce heat.

      5. They can penetrate haze, light rain and snow.

Uses

      1. Microwaves are used in radar system for the location of the distant objects like ships, areoplanes,etc.

      2. In wireless communications, long distance telephone communications system.

      3. Satellite communications for T.V. broadcasting.

      4. Microwaves oven are used for cooking.

      5. For study of atomic and molecular structure.

      

Radio waves

Explanation

      1. Radio waves are electromagnetic waves having a very long wavelengths ranging from about 10 centimeters to few hundred kilometres.

      2. Radio waves are produced by oscillating electric circuits containing an inductor and a capacitor.

      3. Radio waves have frequencies from 3kHz to 300GHz.

      4. The frequency of the waves produced by thr circuits depends upon the magnitudes of the inductance and the capacitance.

Properties

      1. They obey laws of reflection and refraction.

      2. Radio waves get diffracted from the obstacles coming from their path. The size of the obstacles should be large as radio waves are having quite larger wavelengths.

      3. They can penetrate throught rain, snow, clouds.

Uses 

      1. Radio waves are used for wireless communications purposes.

      2. They are used for radio broadcasting and transmission of T.V. signals.

      3. Cellular phones uses radio waves to transmit voice communications in the ultra high frequency (UHF) band.

      4. Radio astronomy.

Electromagnetic waves | Part 2 | Electromagnetic Spectrum, Gamma rays, X- rays

To view Electromagnetic waves Part 1 click the link below
Electromagnetic waves | Part 1

    Electromagnetic Spectrum

    Is defined as 'The orderly distribution (i.e., sequential arrangement) of electromagnetic waves according to their wavelengths (or frequencies) in the form of distinct groups having different properties is called as the electromagnetic spectrum.

    Note - 1. The main parts of the spectrum are: gamma radiation, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, radio waves.

                2.The spectrum of electromagnetic radiation has no upper or lower limits i.e., sharp boundaries. All regions overlap.

                 3.The know electromagnetic waves have wavelength ranging from 6 ✕ 10^-13m to more than 10^4 m.

          S.I. unit : Hertz (Hz)

Diagram of Electromagnetic Spectrum

Types of Rays

Explanation their properties and their uses

          1) Gamma Rays (ɣ)

Explanation 

      1. Gamma rays are electromagnetic waves having wavelength ranging nearly from 10^-15m to 10^-10m.

      2. The gamma rays are high energy electromagnetic waves having very high frequency ranging nearly from 3 ✕ 10^18 Hz to 5 ✕ 10^20 Hz.

      3. Gamma rays are detected with Geiger Counter, photographic plate, fluorescence.

      4. These gamma rays are emitted from the nuclei of some radioactive elements such as uranium, radium etc.

      5. Gamma rays were discovered by Paul Ulrich Villard. 

 Properties

      1. Gamma rays have high penetrating power. They can penetrate through several centimeters of thick iron and lead blocks.

      2. They have moderate ionizing power.

      3. They can produce fluorescence in some substances like willemite, zinc sulphate, barium platinocyanide, etc.

      4. They can affect a photographic plate.

      5. Gamma rays are not detected by electric and magnetic fields.

      6. They knock out electrons from the surface on which they are incident.

      7. They produce heating effect on the surface exposed to them. 

      8. They cause photoelectric effect, Comption scattering and pair production.

Applications:

      1. Gamma rays are used in radiotherapy for the treatment of cancer and tumor.

      2. In food industry, gamma rays are used to kill micro-organisms.

      3. It helps to preserve the food stuff.

      4. Gamma rays are used to produce nuclear reaction. 

      5. Also used in gamma ray astronomy.

                   2) X- rays

Explanation 

      1. X- rays are electromagnetic waves having very short wavelength ranging nearly from 10^-11 m to 10^-8 m.

      2. The frequency range of X- rays is from 1 ✕ 10^16 Hz to 3 ✕ 10^19 Hz.

      3. They are produced, when fast-moving electrons are suddenly stopped by an obstacle.

      4. X- rays are produced in laboratory by Coolidge X-ray tube.

      5. X- rays are detected by ionization chamber, photographic plate, scintillation counters, semiconductor detector, etc.

      6. X- rays were discovered by Wilhelm Konrad  Rontgen in 1895.

Properties 

      1. X- rays can travel in vaccum with the speed of light.

      2. They are not deviated by electric and magnetic fields.

      3. They affect photographic plates.

      4. They ionize the gas through which they pass. 

      5. They produce fluorescence in many substances like zinc sulphate, barium platinocyanide etc.

      6. They travel in straight line, they cast the shadows of the objects falling in their path.

      7. X- rays can undergo reflection, refraction, interference, diffraction, and polarization.

      8. X- rays can penetrate through materials like paper, thin sheet of metal, wood, flesh, skin, etc. But they cannot penetrate denser objects, such as bones, heavy metals, etc.

      9. Prolong exposure can have injurious effect on human bodies.

     10. They cause photoelectric effect.

Note:- When electromagnetic radiation of very short wavelength is incident on metal surface, electrons are emitted from these surfaces. This phenomenon is called photoelectric effect.

 Uses

      1. In surgery, X-ray photographs are useful to detect bone fracture or the presence of foreign objects likes bullets or hidden metal in a human body. 

      2. They are used for detecting faults,  cracks, flaws and gaps in metals.  

      3. They are used to distinguish real diamonds, gems, from artificial ones.

      4. X- rays are used to detect the structure of crystals.

      5. X- rays are used to cure skin diseases and to destroy tumors in the body of the patient. 

      6. They are used for detection of explosives opium etc.

To see Electromagnetic Waves | Part 3 | Ultraviolet rays, Visible light, Infrared rays, Microwaves click the link below
Electromagnetic Waves | Part 3 | Ultraviolet rays, Visible light, Infrared rays, Microwaves

Electromagnetic Waves | Part 1 | Definition, Characteristics, Formula, etc...

Electromagnetic Waves 

   Is defined as, Time varying electric and magnetic fields, propagating in space.

E.g. Infrared rays, microwaves, radio waves, X-rays, UV rays etc.

They are mostly produced by accelerated electric charge. 

  Hertz's Experiment 

For production of electromagnetic waves

   1. An experimental setup used by Hertz's to produce and detect electromagnetic waves.

         

   2. The transmitter consists of two spheres S1 and S2 located near the end of two straight rod A and B seprated by a spark gap S.

   3. With the two rod connected to an induction coil I, spark jump across the gap S, giving rise to an oscillatory current in A & B. The sphere S1, S2 act as plates of a capacitor and the rod A and B provide inductance. Hence, the transmitter act as an oscillator  circuit.

   4. The recevier or detector, consists of a single loop of wire with tiny spark gap at R. 

   5. This circuit too is an oscillating circuit with the spark gap acting  as a capacitor and the loop providing the inductance.

   6. Tuning the frequency so that of the receiver is done by sliding the sphere S1, S2, along the rod A and B. When the two circuits are tuned, a spark appear across R, whenever a spark passes across S.

   7. Wavelengths of electromagnetic waves were found to be 6m.

   8. These waves under go reflection, refraction, interference etc. Similar to light.

Characteristics Of Electromagnetic Waves

   1.Electric field vector and magnetic field vector vibrate perpendicular to each other and also to the director of propagation of wave 

i.e., Electromagnetic Waves (E.M.) are transverse in nature

   2. E.M. waves propagate in the form of time varying electric and magnetic fields.

   3. The ratio of the amplitudes of electric and magnetic fields is always constant and it is equal to velocity of the electromagnetic waves
                                                              c =  E 

                                                                     B

   4. The energy of E.M. waves is equally distributed among the electric and magnetic field vector. 

   5. E.M. waves are produced by accelerated electric charges. 

   6. E.M. waves do not require any material medium for their propagation. They can travel through vacuum as well as through solids, liquids, and gases.

   7. The relation between velocity (c), frequency(ν) and wavelength (λ) of the E.M. waves in vacuum is given by c = νλ

   8. E.M. waves travel with the speed of light in vacuum and their velocity (c) is given by, 

 c =    1         =                        1                        

       √μoεo             4π × 10^-7 × 8.85 ×10^-12

                                             c = 3 × 10^8 m/s

 Where, and are the permittivity and permeability of free space. 

    9. In a given material medium, the velocity (vm) of E.M. waves is given by, 

       vm =   1  

              √με

Where  μ  = Permeability of the given medium

             ε  = Permittivity of the given medium

   10. E.M. waves obey the principle of superposition of waves.

   11. E.M. waves exert pressure on the surface upon which they are incident.

   12. E.M. waves obey  the laws of reflection and refraction.

   13. The low frequency E.M. waves are unaffected by the external electric and magnetic fields.

 Transverse nature of E.M. waves

    1. The electric and magnetic fields are mutually perpendicular to each other. If the accelerated charge is oscillating, both the electric and magnetic fields vary with time and they travel outwards from the charge in the form of E.M. waves.

     2. If E is along the Y-axis and B is along the Z-axis, the direction of propagation is along E × B i.e., along the X-axis 

             

     3. A single E.M. wave propagating perpendicular to both electric and magnetic fields in magnified form as shown below

             

     4. The electric field and magnetic field vary sinusoidally with x and is given by,

                                            Ey = Eo sin(kx - ωt)

                                            Bz = Bo sin(kx - ωt)

Where, Eo = amplitude of electric intensity E

            Bo = amplitude of magnetic induction B

             k = 2π  = Propagation constant 

                     λ

             λ = wavelength of oscillation

     5. Both electric and magnetic fields attain their maximum and minimum values at the same time and at the same point in space 

      6. From the figures it is observed that propagation of electromagnetic fields is in the direction of E × B. 

As the electric and magnetic fields are mutually perpendicular to each other and to the direction of wave propagation, the electromagnetic waves are transverse in nature.

 Quick Question. #QQ

Q.Is it possible to polarise the E.M. waves?

Ans. Yes, E.M. waves can be polarised. A beam of electromagnetic waves is unpolarised, but if the vibration of the electric field vector are confined to one plane containing the direction of propagation, then the waves are said to be plane polarised or linearly polarised.

To view Electromagnetic Waves  Part 2 click the link below
Electromagnetic Waves | Part 2 |

Know Current Electricity | Part 3 | Definitions, Formulas,etc

To view Know Current Electricity Part 2 click the link below

Know Current Electricity Part 2

Colour code for resistors

       Resistor is an important component of electric circuits. We use to limit the current through a particular path of the circuit. Resistance available in the market are mainly of two types:

1)Wire wound resistors
2)Carbon resistors

    Carbon resistors

       This resistors are highly range resistors. They are small and inexpensive. Since they are small in size, their values are colour coded to mark their values in ohms. 

The value of R is printed in the form of colour rings. On one side there is tolerance ring (either golden or silver) start observing from the other side. First, two colour rings give first two digits of value and the 3rd ring indicates the decimal multiplier. In case tolerance ring is absent the colour of the ring lie closer to one terminal yhen start observing from that side.

Example If the code for the resistor is -

Orange white red golden, then its resistance

    ↓            ↓       ↓       ↓

    3            9  ✖️ 10²    5%, Ohm

Or     3900 ± 5%

         3.9  K ±  5%

   Wire wound resistors 

A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material. They can be produced very accurately, and have excellent properties for low resistance values and high power ratings.

Resistances in series and in parallel 

1) Resistance in series:- When a number of resistors are connected in series then the current flowing through each of them will be same.

If R1, R2, R3, be the three resistors connected in series then their equivalent resistance (Rs) is given by 

                                           Rs = R1 + R2 + R3

        Hence, when a number of resistors are connected in series, the equivalent resistance is equal to the sum of individual resistance. For 'n' resistors

                                        Rs = R1 + R2 + .....................+ Rn

2)Resistance in parallel:- When the number of resistors are connected in parallel, then the potential difference across each of them will be same. 

If 'n' resistors R1, R2, ......., Rn are connected in parallel, the equivalent resistance is given by

                                        1 =   1  +  1  +    1    ........... + 1 

                                       Rp     R1      R2      R3               Rn

Note- Hence when a number of resistances are connected in parallel, the reciprocal of the equivalent resistance is equal to the sum of reciprocal of individual resistance.

Know Current Electricity | Part 2 | Definitions, Formula, S.I. units

To view Know Current Electricity Part 1 click below

Know Current Electricity Part 1

Specific resistance (Resistivity)

At a particular temperature, the electric resistance of a conductor is observed to depend on the following factors -

1.      Length of the conductor
2.      Area of cross section of the conductor
3.      Nature of material of the conductor

    It is found that, the resistance (R) of a conductor is directly proportional to its length (l) and inversely proportional to its area of cross section (A).

Formula

     R ∝ l            and           R ∝ 1 

                                                 A  

                    or     R ∝  l

                                    A

                   or      R = l 

                                   A

Where, ρ is a constant of proportionality and is called a specific resistance 

                       ρ  = RA 

                                l

         Where, R is in 

                      A is in m^2

                      l  is in m

                  ρ =  1Ω  1m^2

                            1m

                  ρ = 1        Ωm

       The resistivity of material is the resistance of the wire of unit length and unit area of cross-section.

Note- Resistance of different materials of same dimensions will be different.

Conductivity

     Reciprocal of the resistivity is called conductivity of the material. 

               σ   = 1              (1/ohm - m)
                        ρ

S.I. unit - Siemens/metre

Resistivity depends upon

Conductors: Those materials whose resistivity is negligibly small are called conductor e.g. silver, copper, aluminum, etc.

Insulator: Those materials whose resistivity is very are called insulator e.g. glass, rubber, etc.

Semiconductor: Those materials whose resistivity lies between that of conductor and insulators are called semiconductor e.g. silicon, germanium, etc.

Temperature dependence of resistance

Resistance of any material depends on its temperature. For metals, its found that, the resistance increases with temperature. The resistance temperature relationship for metallic conductor is quite linear as shown below

                       

   

Temperature coefficient of resistance

Formula

             α = Rt - R0

                     R0t

  Note- The temperature coefficient of resistance is defined as the increase in resistance per unit original resistance at 0°C, per degree rise in temperature.

          The resistivity of metal conductor increases with temperature. Such materials have a positive temperature coefficient (PTC), where Semiconductor have a negative temperature coefficient (NTC).

          The resistance of PTC thermistors increase non-linearly with temperature and resistance of NTC thermistors decreases non-linearly with temperature.

Superconductivity 

We know that resistivity of material depends on temperature. For some metals and alloy, resistivity suddenly become zero at a particular low temperature. The temperature is called critical temperature (Tc). That is such materials lose their resistivity completely and become perfect conductors,
e.g. resistance of mercury falls to 0Ω at 4.2K.

Know Current Electricity | Part 1 | Definitions, Formulas, S.I. units...

Current Electricity

Electricity is an important source of energy in our everyday life. Current Electricity is charges in Motion

     The electric current is the rate of flow of charges, flowing across the area of the conductor.

     S.I. unit 'Ampere'.

     When 1 coulomb of charge flow through a conductor in 1 second, the current flowing is said to be 1 ampere.

Ohm's Law 

        As long as physical state (material, dimensions & temperature) of conductor remains the same, the electric current flowing through a given conductor is directly proportional to the potential difference applied across it.

Formula

                            I ∝V

                            I = V 

                                  R

                     or   V=RI

      Where R is constant of proportionality. It represents the opposition to flow of current and is known as 'resistance'.

Formula

                           R=V

                                 I

      The graph of current versus potential difference across conductor is straight line as shown

      Resistance is given by the ratio of applied p.d. across the conductor to the resulting current through it.

Formula

  R = V =                    1                            

          I        slope of I-V characteristics

S.I. unit Ohm (Ω)

1 ohm = 1 volt

              1 ampere

Note:- Reciprocal of resistance is called conductance. S.I. unit (S or Ohm^-1)

Resistance

      Solids conductors contains about 10²² free electrons in 1cc. The atoms are tightly bound are current is carried by negatively charged electrons. 

      When no external potential difference is applied, the electrons move in random direction and collide with atoms. The average number of electrons crossing any section in one direction is equal to average number of electrons crossing that section in opposite direction in given time. Thus there is no net flow of electrons through any section of the conductor. Hence there is no current.

Drift Velocity

     There is continous flow of electrons across the section of the conductor. Atoms of any material are always in state of vibration because of thermal energy. While moving, electrons continuously collide with vibrating atoms and their motion is opposed and electrons move with constant velocity called drift velocity.

Limitations of Ohm's Law 

     Although Ohm's law is obeyed by various materials, there exists some material and devices for which the linearity of relation between V and I is not obeyed.

     The dotted curve represents ideal Ohm's law and the solid line represents actual observations. According to Ohm's law current increases with p.d. But, when current increases temperature of the conductor increases, which inturn increase resistance of conductor and hence slightly decrease in current. Ohm has not considered this temperature dependence in his relation.

This figure shows the nature of I-V curve for semiconductor

      All the ohmic and non-ohmic materials have different applications in electric and electronic circuits. Conductors are used to apply the p.d. to various components and carry the current. 

Learn About Heat It's Thermodynamics! Part 3 | Definitions, Formulas,Units

To view  Learn About Heat It's Thermodynamics! Part  2 Please click the Link below

Learn About Heat It's Thermodynamics! Part 2

Emissive Power And Absorptive Power

Emissive Power: The Power of a body at a given temperature is defined as the quantity of radient energy emitted by the body per unit time per unit surface area of the body at that temperature

If Q = amount of radiant energy emitted 

    A = surface area of body

    t  =  time for which body radiates energy

Formula

Emissive Power of body at given temperature

                E = Q

                       At

S.I. unit of emissions power is J/m^2s or  W/m^2

Dimensions : [M^1 L^0 T^-3]

Emissive Power of a body depends on:

1. Temperature of the body
2. Nature of body
3. Surface area of body
4. Nature of the surrounding

Note:- Emissive Power of a perfectly black body is always greater than any other body at same temperature. Emissive Power of some surfaces are Lampblack 98%, Aluminum Paint 33%, Platinum 11%, Copper 5%, Silver 3%.

Coefficient of Emission (emissivity)

Coefficient of Emission of a body is the ratio of emissive power of the body at the given temperature to the emissive of a perfectly black body at same temperature

Formula

Coefficient of Emission e = E

                                                Eb

Where, E= Emissive power of ordinary body at given temperature.

             Eb = Emissive power of perfectly black body at same given temperature.

For perfectly black body, e = 1

For perfect reflectors, e = 0

For ordinary bodies, e < 1

Note :- Good absorbers are Good emitter of heat.

Absorptive Power

Absorptive Power of a body at a given temperature is defined as the amount of radiant energy absorbed per unit area per unit time by a suface at that temperature. A body which absorbs all radiation of all wavelengths would be called 'Perfectly Black Body'

Kirchhoff's Law of Radiation 

It states that 'the coefficient of absorption of a body is equal to its coefficient of emission at any given temperature.'

                                                                   a = e

But coefficient of emission e  = E

                                                      Eb

                         a = E    or  E = Eb

                               Eb        a 

Hence Kirchhoff's law can be stated as " At any given temperature, the ratio of the emissive power (E) to the coefficient of absorption (a) is constant for all bodies and the this constant is equal to emissive power (Eb) of a perfectly black body at the same temperature."

Stefan's Law of Radiation

It states that the amount of radiant energy emitted per unit time per unit surface area of perfectly black body is directly proportional to the fourth power of its temperature.

 Formula

                                          Q ∝  T^4  Or  Eb ∝ T^4

Let,   Q  =  Amount of radiant energy emitted by perfectly black body.

         A  = Area of the perfectly black body.