Introduction
Nature of Electricity
We know that matter is electrical in nature i.e. it contains particles of electricity viz. protons and electrons. The positive charge on a proton is equal to the negative charge on an electron. Whether a given body exhibits electricity (i.e. charge) or not depends upon the relative number of these particles of electricity.
- If the number of protons is equal to the number of electrons in a body, the resultant charge is zero and the body will be electrically neutral. Thus, the paper of this book is electrically neutral (i.e.paper exhibits no charge) because it has the same number of protons and electrons.
- If from a neutral body, some electrons are removed, there occurs a deficit of electrons in the body. Consequently, the body attains a positive charge.
- If a neutral body is supplied with electrons, there occurs an excess of electrons. Consequently, the body attains a negative charge.
Unit of Charge
The charge on an electron is so small that it is not convenient to select it as the unit of charge.In practice,coulomb is used as the unit of charge i.e.SI unit of charge is coulomb abbreviated as C.One coulomb of charge is equal to the charge on `6.25times10^{16}` electrons, i.e.
1 coulomb = Charge on `6.25times10^{16}` electrons
Thus when we say that a body has a positive charge of one coulomb (i.e. +1 C), it means that the body has a deficit of `6.25times10^{16}` electrons from normal due share. The charge on one electron is given by;
Mechanism of Current Conduction in Metals
Every metal has a large number of free electrons which wander randomly within the body of the conductor somewhat like the molecules in a gas.The average speed of free electrons is sufficiently high (≅`10^5` `ms^{-1}`) at room temperature. During random motion, the free electrons collide with positive ions (positive atoms of metal) again and again and after each collision, their direction of motion changes. When we consider all the free electrons, their random motions average to zero. In other words, there is no net flow of charge (electrons) in any particular direction. Consequently, no current is established in the
conductor.
When potential difference is applied across the ends of a conductor (say copper wire) as shown in above Figure electric field is applied at every point of the copper wire.The electric field exerts force on the free electrons which start accelerating towards the positive terminal (i.e., opposite to the direction of the field). As the free electrons move, they collide again and again with positive ions of the metal. Each collision destroys the extra velocity gained by the free electrons.The average time that an electron spends between two collisions is called the relaxation time (t). Its value is of the order of `10^{-14}` second.
Although the free electrons are continuously accelerated by the electric field, collisions prevent their velocity from becoming large.The result is that electric field provides a small constant velocity towards positive terminal which is superimposed on the random motion of the electrons.This constant velocity is called the drift velocity.
The average velocity with which free electrons get drifted in a metallic conductor under the influence of electric field is called drift velocity.The drift velocity of free electrons is of the order of `10^{-5}` `ms^{-1}`.
Thus when a metallic conductor is subjected to electric field (or potential difference),free electrons move towards the positive terminal of the source with drift velocity.Small though it is, the drift velocity is entirely responsible for electric current in the metal.
Relation Between Current and Drift Velocity
electrons per unit volume
In one second, all those free electrons within a distance `V_d` to the right of cross-section at P (i.e., in a volume `AV_d`) will flow through the cross-section at P as shown in Figure.This volume contains n `AV_d` electrons and,hence, a charge (`nAV_d`)e.Therefore, a charge of ne`AV_d` per second passes the cross-section at P.
Since A, n and e are constant, I∝`V_d`. Hence,current flowing through a conductor is directly proportional to the drift velocity of free electrons.
- The drift velocity of free electrons is very small. Since the number of free electrons in a metallic conductor is very large,even small drift velocity of free electrons gives rise to sufficient current.
- The current density J is defined as current per unit area and is given by ;
Electric Current
- Current is flow of electrons and electrons are the constituents of matter. Therefore, electric current is matter (i.e. free electrons) in motion.
- The actual direction of current (i.e. flow of electrons) is from negative terminal to the positive terminal through that part of the circuit external to the cell.However, prior to Electron theory, it was assumed that current flowed from positive terminal to the negative terminal of the cell via the circuit. This convention is so firmly established that it is still in use. This assumed direction of current is now called conventional current.
Unit of Current:
The strength of electric current I is the rate of flow of electrons i.e. charge flowing per second.
The charge Q is measured in coulombs and time t in seconds. Therefore, the unit of electric current will be coulombs/sec or ampere.If Q = 1 coulomb, t = 1 sec,then I = 1/1 = 1 ampere.
One ampere of current is said to flow through a wire if at any cross-section one coulomb of charge flows in one second.Thus, if 5 amperes current is flowing through a wire, it means that 5 coulombs per second flow past any cross-section of the wire.
Electric Current is a Scalar Quantity
- Electric current, I= `frac QT`
As both charge and time are scalars, electric current is a scalar quantity.
Types of Electric Current
The Building Blocks: Electrons and Charges
Static Electricity
Static power is one of the most familiar sorts of electric phenomena. It takes place whilst objects emerge as charged because of the switch of electrons. When gadgets with specific electrical charges come into contact or are rubbed collectively, electrons may be transferred, main to an accumulation of rate.
The discharge of static electricity can produce wonderful consequences, along with lightning, where the accumulation of price within the atmosphere is released as a bolt of electrical strength. On a smaller scale, static energy can purpose phenomena like hair status on quit while combing or the appeal of small portions of paper to a charged balloon.