Physics-2

Basics of Electricals

Is electron negative or positive charge particle?

R.A. Franklin identified electric charge by rubbing series of material. Electron is arbitrarily given negative polarity.

when Franklin rubbed two material together, he found that rubbed material attracts small particles. But, when rubbed material kept in contact then it did not attract small particles.

It means, charge on rubbed materials are opposite in nature. Therefore, they cancel each other on keeping on contact(Like positive and negative numbers, +5 and -5)

What is charge on electron?

Only electron can carry the charge?

  1. In animals, the electric charge carriers are primarily sodium, potassium, calcium, and magnesium ions, all positively charged
  2. In the ionosphere, the charge carriers are oxygen, hydrogen, and helium ions along with electrons
  3. In a gas, discharge sign, the electrical current is due to ions and electrons
  4. In lightning, it’s both ionized air molecules and electrons that are moving
  5. The solar wind is in fact a blast of electrical current from the sun comprised of protons and electrons
  6. In semiconductors, like those used in computer chips, the charge carriers are holes and electrons
  7. In the ocean, it’s the movement of salt ions, and not electrons, that sustains an electrical current.
  8. In metals, like in household electrical wire, the charge carriers are indeed just electrons

Conductor, Semiconductor & Insulator

Conductor

The elements which allow the flow of electric current through it by the application of voltage.

Properties of conductor

1.Good conductor. Ex: Copper, Mercury, Silver, Aluminum, Iron, etc.

2. Conduction band and valence band overlap each other.

3. These have positive temperature Coefficient of resistance

4. Charge carriers is Electrons

5. Current Flow due to electrons.

6. Number of Charge Carriers is the order of 1028 electrons per cubic metre.

7. Effect of Temperature on conductivity: Conductivity decreases

8. On Increasing Temperature: The number of  current carries decreases.

9. Effect of doping: Resistance Increases.

10. Behavior at Absolute OK Temperature: Behaves like super conductor.

11. Bonding Types: Ionic bond.

Insulator

The elements which do not allow any flow of electric charge

Properties of Insulator

1.Bad conductor. Ex: Wood, Rubber, Glass, Ebonite, Mica, Sulphur. Dry air

2. Conduction band and valence band are separated by 6 eV.

3. These have Negative temperature Coefficient of resistance

4. Do not have Charge carriers.

5. Current Flows almost negligible.

6. Number of Charge Carriers is negligible

7. Effect of Temperature on conductivity: Conductivity Increases

8. On Increasing Temperature: The number of  current carries Increases.

9. Effect of doping: Resistance remains unchanged.

10. Behavior at Absolute OK Temperature: Behaves as Insulator only .

11. Bonding Types: Ionic bond and Covalent.

Semiconductor

The elements whose conductivity lies between insulators and conductors.

Properties of Semiconductor

1. Ex: Silicon, Germanium

2. Conduction band and valence band are separated by 1 eV.

3. These have Negative temperature Coefficient of resistance

4. Charge carriers are Electrons & Holes

5. Current Flows due to both Electrons & Holes.

6. Number of Charge Carriers is low

7. Effect of Temperature on conductivity: Conductivity Increases

8. On Increasing Temperature: The number of  current carries Increases.

9. Effect of doping: Resistance decreases.

10. Behavior at Absolute OK Temperature: Behaves as Insulator .

11. Bonding Types: Covalent.

Concept of Electric Charge

Electric charge is the property of matter by which it experiences force in electromagnetic field.

Atom consists of electrons, protons and neutrons. where electron has negative charge, proton has a positive charge while  neutron is neutral.

In an atom, the number of electrons is equal to the number of protons therefor atom is electrically neutral.

Properties Of Charge

  • The electrons may be added to or taken away from an atom. This causes the atom to be charged negatively or positively.                                          
  • If an atom has less number electrons than number of proton; it becomes positive ion. While number electron is more than number of proton; it becomes negative ion.
  • A body having ionized atom is said to be electrically charged.
  • An electron has charge of -1.6 x 10-19 Coulomb while proton has +1.6 x 10-19.
  • Total charge on body is integral multiple of electronic charge

                          q = n.e , where n= 1,2,3,4,5,6,7,………..                                 

  • Charge is a scalar quantity. The SI unit of charge is  the Coulomb (C).
  • Charge can neither be created nor destroyed, only it can be transferred from one body to another body.
  • Charge is independent of frame of reference.
  • Static charge produces electric field only.
  • Charge moving with constant velocity produces both electric field and magnetic field.
  • Accelerated charge produces electric field, magnetic field and radiates energy.
  • Similar charge repel each other while dissimilar charge attract.
  • Charge is always associated with mass while mass can exit with and without charge.
  • A body can be charged by friction, electrostatic induction and conduction.

Coulomb’s Law

The force of attraction or repulsion between two electrically charged objects is directly proportional to the magnitude of their charge and inversely proportional to the square of the distance between them.

Where,

‘F’ is the repulsion or attraction force between two charged objects.

‘Q1’ and ‘Q2’ are the electrical charged of the objects.

‘d’ is distance between center of the two charged objects.

‘k’ is a constant that depends on the medium in which charged objects are placed

.

Current

  • Flow of charge in electric circuit is called as current.
  • If dq charge flows in time dt then average electric current flowing through cross section area of conductor
  • Current is scalar quantity and its SI unit is Ampere (A).
  • The conventional direction of current is the direction of flow of positive charge or it is opposite to the flow of negative charge (electrons).
  •  Current flows from high voltage to low voltage or in direction of electric field
  • For a given conductor, current does not change with change in cross-section.

Potential and Potential Difference

Energy is required for the movement of charge from one point to another i.e. Work per unit charge is called voltage.

The electrical potential difference is defined as the amount of work done to carrying a unit charge from one point to another in an electric field. In other words, the potential difference is defined as the difference in the electric potential of the two charged bodies

Electromotive Force (EMF) of a source

The electromotive force(e.m.f.) is the energy provided by a cell or battery per coulomb of charge passing through it, it is measured in volts (V). 

  • It is equal to the potential difference across the terminals of the cell when no current is flowing.
  • The electromotive force is not the force, but it is the energy expended on each charge.

Voltage Drop

  • Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance or resistance.
  • The electromotive force, potential difference and voltage drop are all measured in units of voltage.

Ohm’s Law

According to Omh’s law, at constant temperature, current density through the conductor is directly proportional to the applied electric field

Where, J is current density with unit of A/m2.

E is electric field with unit of Volt/m.

                        SIGMA is conductivity of material with unit of Siemens/meter or (ohm-meter)-1

OR

If the physical condition of conductor is remain same, current flowing through the conductor is directly proportional to the potential difference across its,

Limitation of Ohm’s Law

  1. This law cannot be applied to unilateral networks. (A unilateral network has unilateral elements like transistor, diode, etc., which do not have same voltage and current relation for both directions of current.)
  2. Ohm’s law is also not applicable for nonlinear elements. (Non-linear elements are those which do not have current exactly proportional to the applied voltage that means the resistance value of those elements changes for different values of voltage and current. Examples of non – linear elements are SCR,  Electric arc etc.)

Power

Electric power is the rate, per unit time, at which electrical energy is transferred by a circuit.

The SI unit of  Power is the Watt or Jule/ second.

Energy

where W is the energy transferred during the time interval (t1 , t2 )

  • Unit of energy is watt-second or Joule

 Resistance

electrical resistance of an object is a measure of its opposition to the flow of electric current.

  • Unit of resistance is Ohm
  • Resistance of conductor increases with increase in temperature.

Where R is the resistance at temperature t, R0 is the resistance at temperature t0 centigrade.

Alpha  is temperature coefficient of resistance with unit of per centigrade.

  • Resistance of semiconductor and insulator decrease with increase in temperature therefor temperature coefficient of resistance for these material is negative.

ELECTRIC FIELD

It is the space surrounding an electric charge q in which another charge q¢ experiences a force of attraction, or repulsion.

  • If a charge q is placed at a point where the electric field due to other charge or charges is E, then the charge q will experience a force F given by
Electric Field formula
Field direction due to Positive and Negative Charge
  • No charged particle experiences force due to its own electric field.

Electric field due to an isolated point charge

Electric field at a distance r from a source charge q is given by

Electric Field due to Point Charge
Electric Field due to Point Charge

Electric field due to an electric dipole

Electric field at a point on the axis of a dipole

Electric field at a point on the axis of a dipole
  • p = q(2a) is the dipole moment, 2a is the vector distance between charges –q and +q.
  • Dipole moment p is a vector quantity directed from –q to +q.

Electric field at a point on the perpendicular bisector of a dipole

Electric field at a point on the perpendicular bisector of a dipole

Electric field at a point on the perpendicular bisector of a dipole derivation
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