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Answered on 30 Aug CBSE/Class 12/Science/Mathematics

Jaswant

400 per Hour
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Lesson Posted on 20 Aug CBSE/Class 12/Science/Physics

N-Type P-Type Semiconductor

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors. Pentavalent impurities (5 valence electrons) produce n-type semiconductors by contributing... read more

The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors.

Pentavalent impurities

(5 valence electrons) produce n-type semiconductors by contributing extra electrons.

Trivalent impurities

(3 valence electrons) produce p-type semiconductors by producing a "hole" or electron deficiency.

N-Type Semiconductor :

The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).

P-Type Semiconductor :

The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.

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Lesson Posted on 20 Aug CBSE/Class 12/Science/Physics

Total Internal Reflection & Critical angle

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

As shown in Figure 1, when a Light Beam hits the Boundary between two different Media, Medium 1 and Medium 2, with a certain Angle of Incidence, i, part of it is Refracted at an Angle of Refraction, r, and other part of it is Reflected back to its original Medium at an Angle of Reflection, i’,... read more

As shown in Figure 1, when a Light Beam hits the Boundary between two different Media, Medium 1 and Medium 2, with a certain Angle of Incidence, i, part of it is Refracted at an Angle of Refraction, r, and other part of it is Reflected back to its original Medium at an Angle of Reflection, i’, equal to the Angle of Incidence,i.

Figure 1: Refraction and Reflection of a Light Beam at the Boundary of two different Media

The Refractive Index, n, of a Medium is the ratio of the Speed of Light in Vacuum, c, to the Speed of Light through that Medium, v. That is, n = c/v. The Higher the Index, the Slower the Speed of Light through the Medium.

When a Light Beam travels from a Medium with Higher Refractive Index, n1, to a Medium with a Lower Refractive Index, n2, there is no Refraction if the Angle of Incidence is greater than a certain angle called the Critical Angle, θ, Figure 2. All the Light Beam will be Reflected back to the original Medium, this is called Total Internal Reflection.

Figure 2: Refraction and Reflection of a Light Beam at the Boundary of two different Media at different Angles of Incidence

Given n1 and n2 as the Refractive Indices of Medium 1 and Medium 2, the Critical Angle θ of the Light Beam going from Medium 1 towards Medium 2 can be determine by the following formula:

The Laser Beam stays inside of the Optical Fibers due to the Total Internal Reflection.

The Laser Beam degrades when propagates within the Medium, mostly due to the Impurities inside it. In real Optical Fibers, very pure Glass is used.

If the Glass has very little Impurities and a proper Frequency is used, the Laser Beam can propagate over distance of several kilometers with very little Attenuation.

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Lesson Posted on 20 Aug CBSE/Class 12/Science/Physics

Magnetic field of a Solenoid

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

A solenoid is a long wire wound in a close-packed helix carrying a current I and the length of the solenoid is much greater than its diameter The figure below shows a section of a stretched out solenoid in xy and yz plane The solenoid magnetic field is the vector sum of the field produced by the... read more
  • A solenoid is a long wire wound in a close-packed helix carrying a current I and the length of the solenoid is much greater than its diameter
  • The figure below shows a section of a stretched out solenoid in xy and yz plane


    Magnetic field of a solenoid 

  • The solenoid magnetic field is the vector sum of the field produced by the individual turns that make up the solenoid
  • Magnetic field B is nearly uniform and parallel to the axis of the solenoid at interior points near its center and external field near the center is very small
  • Consider a dashed closed path abcd as shown in figure .Let l be the length of side ab of the loop which is parallel to the is of the solenoid
  • Let us also consider that sides bc and da of the loop are very-very long so that side cd is very much far away from the solenoid and magnetic field at this side is negligibly small and for simplicity we consider its equal to 0
  • At side a magnetic field B is approximately parallel and constant. So for this side
    ∫B.dl=Bl
  • Magnetic field B is perpendicular to sides bc and da ,hence these portions of the loop does not make any contributions to the line integral as B.dl=0 for the side bc and da

     

  • Side cd lies at external points solenoid where B.dl=0 as B=0 or negligibly small outside the solenoid

     

  • Hence sum around the entire closed path reduces to Bl
  • If N are number of turns per unit length in a solenoid then number of turns in length l is nl.The total current through the rectangle abcd is NIl and from ampere 's law

     

    Bl=μ0NlI
    or B=μ0NI                    (22)
  • we have obtained this relation for infinitely long solenoids considering the field at external points of the solenoid equal to zero.
  • However for real solenoids external field is relatively weak rather then equal to zero
  • Thus for actual solenoids relation 22 holds for internal points near the center of the solenoid
  • Field at internal points of the solenoid does not depend on length and diameter of the solenoid and is uniform over the cross-section of a solenoid
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Lesson Posted on 20 Aug CBSE/Class 12/Science/Physics

Magnetic Field of a Toriod

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

We will now apply Ampere circuital law to calculate magnetic field of a toriod A toriodal solenoid is a hollow circular ring with a large number of turns of a wire carrying current wound around the ring Suppose we have to find the magnetic field B at a point P inside the toriod as shown below in... read more
  • We will now apply Ampere circuital law to calculate magnetic field of a toriod
  • A toriodal solenoid is a hollow circular ring with a large number of turns of a wire carrying current wound around the ring
  • Suppose we have to find the magnetic field B at a point P inside the toriod as shown below in figure


     Magnetic Field of a toriod 

  • In this case amperion loop would be a circle through point P and concentric inside the toriod
  • By symmetry field will have equal magnitude at all points of this circle and this field is tangential to every point in the circle
    Thus

     
  • If there are total N number of turns ,net current crossing the area bounded by the circle is NI where I is the current in the toriod
  • using Ampere law

     
    Thus we see that field B varies with r i.e. field B is not uniform over the cross-section of the core because the path l=2πr is longer at the outer side of the section then at the inner side
  • Imagine a concentric circle through point P' outside the toriod
  • The net current passing through this circular disc is zero ,since the current NI passes in and same current passes out. Thus using Ampere's circuital law, the field B=0 outside the torriod
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Lesson Posted on 20 Aug CBSE/Class 12/Science/Physics

Classification of colloids

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

Colloids are classified in 2 different ways Based on the physical state of the dispersed phase and dispersed medium: S.No Dispersed Phase Dispersed Medium Name of Colloid Examples 1. Solid Solid Solid sol Gem stones 2. Solid Liquid Sol Muddy... read more

 

Colloids are classified in 2 different ways

Based on the physical state of the dispersed phase and dispersed medium:

S.No

Dispersed Phase

Dispersed Medium

Name of Colloid

Examples

1.

Solid

Solid

Solid sol

Gem stones

2.

Solid

Liquid

Sol

Muddy water, Paint, cell fluids

3.

Solid

Gas

Aerosol

Smoke,dust

4.

Liquid

Solid

Gel

Cheese, butter, jelly

5.

Liquid

Liquid

Emulsion

Milk, Hair cream

6.

Liquid

Gas

Aerosol

Fog, mist, cloud

7.

Gas

Solid

Solid Foam

Pumice Stone

8.

Gas

Liquid

Foam

Froth, soap lather

 

Based on nature of Interaction Between Dispersed Phase and Dispersed Medium

Lyophilic Colloids(liquid loving)-

  • Some substances which can from colloids directly on mixing them with a suitable liquid(dispersion medium). These colloids are called lyophilic colloids. Examples of these substances are gum, geltine, starch, rubber.
  • They are also called Reversible sols as in these sols(colloids) when the dispersion phase is separated from the dispersion medium (by say evaporation) , the sol can be formed again by just mixing the dispersion phase and medium again.
  • They are also very stable and cannot be coagulated

 

Lyophobic colloids(liquid hating)

  • Some substances cannot form colloid just by directly mixing them with a liquid. Their colloidal sols are prepared by special methods and are called lyophobic colloids. Examples of these substances are metals, metal sulphides.
  • They are also called Irreversible colloids as on precipitation, they don’t give back the colloid on simply mixing the dispersed phase and the dispersed medium.
  • They are unstable and coagulate easily by heating shaking or adding electrolytes. Stabilising agents are used to preserve them

 

Based on the Types of Particles of the Dispersed Phase-

  • Multimolecular Colloids: Many particles(atoms or small molecules) of the dispersed phase aggregate together to form species having the size of a colloidal particle( 1-1000nm). These colloids are called multimolecular colloids. Example-gold sol, Sulphur sol

 

  • Macromolecular colloids: Substances with large molecules (macromolecules) in suitable solvents form solutions but these macromolecules might be in the colloidal range. These solutions are called macromolecular colloids and resemble true solutions in many ways. Example- Starch, Cellulose, Proteins are natural macromolecules.       Nylon, polythene, polystyrene are man-made macromolecules

 

  • Associated Colloids: Some substances at high concentrations act as colloids due to the formation of aggregates. But at low concentrations they behave like normal strong electrolytes. These aggregates formed are called micelles. Such colloids are called associated colloids.

Kraft Temperature- The formation of micelles takes place only above a particular temperature called Kraft’s temperature

Critical Micelle Concentration(CMC) – The concentration above which micelle formation takes place

Example- Soaps, synthetic detergents

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Lesson Posted on 20 Aug CBSE/Class 10/Science/Unit IV: Effects of Current/Electricity CBSE/Class 12/Science/Physics

Difference Between Motor and Generator

Shubham Tomar

I have good knowledge of Physics and Mathematics and I have 4 years of teaching experience. About my...

Difference Between Motor and Generator Sl. No. Differentiating Property Motor Generator 1 Definition An electric motor is a machine that converts electrical energy to mechanical energy. An electric generator is a machine that converts mechanical energy... read more

Difference Between Motor and Generator

Sl. No.

Differentiating Property

Motor

Generator

1

Definition

An electric motor is a machine that converts electrical energy to mechanical energy.

An electric generator is a machine that converts mechanical energy to electrical energy.`

2

Rule

Electric motor follows Fleming’s left-hand rule.

Electric generator follows Fleming’s right-hand rule.

3

Principle

Motors works on the principle that a current carrying conductor experiences a force when placed in a magnetic field.

Generators work on the principle of electromagnetic induction.

4

Driving force for shaft

The shaft of an electric motor is driven by a magnetic force which is developed between the armature and field.

The shaft of an electric generator is connected to the rotor which is driven by a mechanical force.

5

Current Usage

In a motor, current is supplied to the armature winding.

In a generator, current is produced in the armature winding.

6

Example

Ceiling fans, cars, etc. are all examples of motor.

In power stations, generator is used to generate electricity.

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Answered on 22 Aug CBSE/Class 12/Science/Physics

Ashwini Kumar

Tutor

This is because frequency is the inherent property of any matter. It is fixed and specific for a particular substance. It doesn't vary as its the 'identity' of a particle or a wave.
Answers 6 Comments
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Answered on 20 Aug CBSE/Class 10/Mathematics/UNIT III: Coordinate Geometry/Coordinate geometry/Class 10 Difference between two points and section formula CBSE/Class 12/Science/Mathematics

Ashwani Pandey

Teacher

There is only one solution.
Answers 3 Comments
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Answered on 05 Mar CBSE/Class 12/Science/Physics

Do electromagnetic waves carry energy and momentum?

Ramya

Primary Teacher

An electromagnetic wave, although it carries no mass, does carry energy. It also has momentum, and can exert pressure (known as radiation pressure). The reason tails of comets point away from the Sun is the radiation pressure exerted on the tail by the light (and other forms of radiation) from sun
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