Free PDF download of Class 12 Physics revision notes & short key-notes for Chapter 3 - Current Electricity to score high marks in exams, prepared by expert. Current. Electricity. 95 number of electrons travelling in any direction will be equal to the .. See Eq. () of Chapter 13 from Class XI book. . Page State Ohm's law. Ans. Ohm's law states that the current (I) flowing through a Ans. The reciprocal of electrical resistivity of material of a conductor is called.
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(b) a velocity with which electrical energy is transferred along the wire (about ms-1). (c) a drift velocity of the electrons as a whole when a current flows through the wire. (this depends on the applied voltage but is usually a few Page Get PDF download of Class 12 Physics Chapter 3 - Current Electricity Class 12 Notes, Video lectures and more made by expert teachers of Vidyakul to score. Current Electricity class 12 Notes Physics chapter 3 in PDF format for free download. Latest chapter wise notes for CBSE board exams.
Like ferromagnetism and atomic spectral lines , superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect , the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
Semiconductor Main article: Semiconductor In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron. This is the case in a p-type semiconductor.
A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator. In the classic crystalline semiconductors, electrons can have energies only within certain bands i. Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material.
The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy closer to the nucleus are occupied, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few semiconductor or virtually none insulator of them are available in the conduction band, the band immediately above the valence band.
The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on the band gap between the bands. The size of this energy band gap serves as an arbitrary dividing line roughly 4 eV between semiconductors and insulators. With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond.
For a net current to flow, more states for one direction than for the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap.
Often this is stated as: full bands do not contribute to the electrical conductivity. However, as a semiconductor's temperature rises above absolute zero , there is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band.
The current-carrying electrons in the conduction band are known as free electrons, though they are often simply called electrons if that is clear in context. Current density and Ohm's law Main article: Current density Current density is the rate at which charge passes through a chosen unit area. Conventionally, if the moving charges are positive, then the current density has the same sign as the velocity of the charges.
For negative charges, the sign of the current density is opposite to the velocity of the charges. Current Density: Current density j gives the amount of charge flowing per second per unit area normal to the flow.
If j is normal to a cross — sectional area A and is constant over the area, the magnitude of the current I through the area is Mobility: Mobility is defined to be the magnitude of drift velocity per unit electric field. Now, where q is the electric charge of the current carrier and is its mass. Thus, mobility is a measure of the response of a charge carrier to a given external electric field.
Resistivity: Resistivity is defined to be reciprocal of conductivity. It is measured in ohm-metre Resistivity as a function of temperature: It is given as, Where, is the temperature coefficient of resistivity and is the resistivity of the material at temperature T. If the mass of a charge carrier is large, then for a given field its acceleration will be small and will contribute very little to the electric current.
Electrical Conductivity: When a conducting substance is brought under the influence of an electric field ,free charges e.
Consider a cylindrical material with cross-sectional area A and length L through which a current is passing along the length and normal to the area A, then, since and are in the same direction, Where A is cross-sectional area and L is length of the material through which a current is passing along the length, normal to the area A.
Measuring resistance: R is measured in ohm , where EMF: Emf Electromotive force is the name given to a non-electrostatic agency. Typically, it is a battery, in which a chemical process achieves this task of doing work in driving the positive charge from a low potential to a high potential.
The effect of such a source is measured in terms of work done per unit charge in moving a charge once around the circuit. It fails if V depends on I non- linearly.
The relation between V and I depends on the sign of V for the same absolute value of V. The relation between V and I is non- unique. For e.