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Introduction
Everybody is familiar with the functions that electricity can perform. It can be used for lighting, heating, traction and countless other purposes.The question always arises, “What is electricity”? Several theories about electricity were developed through experiments and by observation of its behaviour.Electricity is a fundamental force of nature that has been harnessed and explored by humans for centuries. It powers our homes, fuels our technology, and underpins the functioning of our bodies. Understanding the nature of electricity is crucial for a wide range of applications, from powering our homes to revolutionizing modern technology. In this blog, we'll delve into the captivating world of electricity, its nature, and its many manifestations.
The only theory that has survived over the years to explain the nature of electricity is the Modern Electron theory of matter. This theory has been the result of research work conducted by scientists like Sir William Crooks, J.J. Thomson, Robert A. Millikan, Sir Earnest Rutherford and Neils Bohr. Here, we shall deal with some basic concepts concerning electricity.
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;
Charge on electron =`-frac1{6.25times10^{16}}`
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.
(≅`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
Consider a portion of a copper wire through which current I is flowing as shown in Figure. Clearly, copper wire is under the influence of electric field.
Let A = area of X-section of the wire
n = electron density, i.e., number of free
electrons per unit volume
electrons per unit volume
e = charge on each electron
`V_d` = drift velocity of free electrons
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.
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.
∴I=ne`AV_d`
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.
- 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 ;
∴Current density, J = `frac IA` = ne`frac{AV_d}A`
The SI unit of current density is `frac{amperes}{m^2}`.
Electric Current
The directed flow of free electrons (or charge) is called electric current. The flow of electric current can be beautifully explained by referring to below Figure.The copper strip has a large number of free electrons. When electric pressure or voltage is applied, then free electrons, being negatively charged, will start moving towards the positive terminal around the circuit as shown in Figure.This directed flow of electrons is called electric current.
The reader may note the following points :
- 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.
∴Current, I= `frac QT`
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 electric current may be classified into three main classes:
1.Steady current: When the magnitude of current does not change with time, it is called a steady current.Figure 1 shows the graph between steady current and time. Note that value of current remains the same as the time changes. The current provided by a battery is almost a steady current (d.c.).
Figure 1 
Figure 2
Figure 3
2.Varying current: When the magnitude of current changes with time, it is called a varying current. Figure 2 shows the graph between varying current and time. Note that value of current varies with time.
3.Alternating current: An alternating current is one whose magnitude changes continuously with time and direction changes periodically. Due to technical and economical reasons, we produce alternating currents that have sine waveform (or cosine waveform) as shown in Figure 3.It is called alternating current because current flows in alternate directions in the circuit, i.e., from 0 to T/2 second (T is the time period of the wave) in one direction and from T/2 to T second in the opposite direction. The current provided by an a.c. generator is alternating current that has sine (or cosine) waveform.
The Building Blocks: Electrons and Charges
Electricity is intimately connected to the behavior of subatomic particles, specifically electrons. At the heart of electrical phenomena lies the concept of electric charge. Electrons, which orbit the nucleus of an atom, carry a negative charge, while protons, found in the nucleus, carry a positive charge. Like charges repel each other, while opposite charges attract.
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.
Conclusion
Electricity is a dynamic and essential force that permeates our international, from the atoms and molecules that make up rely to the era that shapes our daily lives. By know-how the character of strength and its behavior, we've unlocked its outstanding capability, leading to transformative improvements and conveniences. Whether you are flipping a light switch,charging your smartphone, or marveling at the splendor of a thunderstorm, the captivating nature of strength is all around us, waiting to be explored and harnessed for the betterment of humanity.