electricity

Electricity: Static and Current electricity

In this article we are going to explore electricity and we will focus on the very basics of the topic and cover static and current electricity.

Electricity involves charges, either static or in current form.

Electric charges: Like charges & unlike charges

  • In 1752 Benjamin Franklin, an American scientist, showed that lightning & the spark from clothes are essentially the same phenomena.
  • When a plastic refill is rubbed with polythene, it acquires a small electric charge. 
  • Similarly, when a plastic comb is rubbed with dry hair, it acquires a small charge. 
  • These objects are called charged objects. 
  • The electrical charges produced by rubbing are called static charges.
  • When charges move, they constitute an electric current.
  • Electrical charge can be transferred from a charged object to another through a metal conductor.
  • The process of transferring of charge from a charged object to the earth is called earthing.
  • Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electrical current.
  • In the process of charging the refill & the plastic comb, polythene & hair also get charged (loss of charge in one object is compensated by the gain of charge in the other).
  • An object that loses charge (electrons), is said to be positively charged because it has an excess of protons
  • An object that gains charge (electrons), is said to be negatively charged
  • Like charges repel while the unlike charges attract.

Experiment

  • Inflate two balloons. Hang them in such a way that they do not touch each other. Rub both the balloons with a woollen cloth & release them. What do you observe? (They repel each other)
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Like charges repel each other
  • Rub a refill with polythene & place it gently in a glass tumbler. Bring an inflated charged balloon near the refill & observe. The charged balloon will attract a charged refill.
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Unlike charges attract each other
Electroscope
A Simple Electroscope
  • The aluminium foil strips in the jar receive the same charge from the charged refill. 
  • The strips carrying similar charges repel each other & they become wide open. 
  • Such a device can be used to test whether an object is carrying a charge or not. 
  • This device is known as an electroscope.

Lightning

  • During the development of a thunderstorm, the air currents move upward while the water droplets move downward. These vigorous movements cause the separation of charges
  • By a process, not yet completely understood, the positive charges collect near the upper edges of the clouds & the negative charges accumulate near the lower edges
  • There is an accumulation of positive charges near the ground also
  • When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. 
  • Negative & positive charges meet, producing streaks of bright light & sound. 
  • We see streaks as lightning. The process is called an electric discharge.
The image on the Accumulation of charges leading to lightning will be uploaded soon.

Do’s & Don’ts during a Thunderstorm

Outside the house

  • Open vehicles, like motorbikes, tractors, construction machinery, open cars are not safe. 
  • Open fields, tall trees, shelters in parks, elevated places do not protect us from lightning strokes.
  • Carrying umbrella is not a good idea at all during thunderstorms.
  • If in a forest, take shelter under shorter trees.
  • If no shelter is available & you are in an open field, stay far away from all trees. 
  • Stay away from poles or other metal objects. 
  • Do not lie on the ground. Instead, squat low on the ground. This position will make you the smallest target.
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Inside the house

  • Lightning can strike telephone cords, electrical wires & metal pipes. During a thunderstorm contact with these should be avoided. 
  • It is safer to use mobile phones & cordless phones. However, it is not wise to call up a person who is receiving your phone through a wired phone.
  • Bathing should be avoided during thunderstorms to avoid contact with running water.
  • Electrical appliances like computers, TVs, etc., should be unplugged.

Lightning Conductors

  • Lightning Conductor is a device used to protect buildings from the effect of lightning
  • A metallic rod, taller than the building, is installed in the walls of the building during its construction. 
  • One end of the rod is kept out in the air & the other is buried deep in the ground. 
  • The rod provides easy route for the transfer of electric charge to the ground.
  • The metal columns used during construction, electrical wires & water pipes in the buildings also protect us to an extent. But do not touch them during a thunderstorm.
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Electric current

  • Electric current is expressed by the amount of charge flowing through a particular area in unit time
  • In other words, it is the rate of flow of electric charges
  • A continuous & closed path of an electric current is called an electric circuit.
  • Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons.
  • If a net charge Q, flows across any cross-section of a conductor in time t, then the current I, through the cross-section is I = Q/t.
  • The SI unit of electric charge is coulomb (C), which is equivalent to the charge contained in nearly 6 x 1018 electrons. (electron possesses a negative charge of 1.6 x 10-19 C
  • The electric current is expressed by a unit called ampere (A), named after the French scientist, Andre-Marie Ampere (1775-1836). 
  • One ampere is constituted by the flow of one coulomb of charge per second
  • Small quantities of current are expressed in milliampere (1 mA = 10-3 A) or in microampere (10-6 A). 
  • An instrument called ammeter measures electric current in a circuit. 
  • It is always connected in series in a circuit through which the current is to be measured. 
The credit for the invention of the electric bulb is usually given to Thomas Alva Edison, though others before him had worked on it. 
Edison made some 1300 inventions including the electric bulb, gramophone, the motion picture camera & the carbon transmitter, which facilitated the invention of the telephone.

‘Flow’ of charges inside a wire

  • A low-energy electron would have great difficulty passing through a solid conductor as the atoms are packed together with very little spacing between them. 
  • But it turns out that the electrons are able to ‘travel’ through a perfect solid crystal smoothly & easily. 
  • The ‘motion’ of electrons in a conductor, however, is very different from that of charges in empty space. 
  • When steady current flows through a conductor, the electrons in it move with a certain averagedrift speed’. 
  • For a typical copper wire carrying a small current, drift speed it is found to be of the order of 1 mm per second (electrons move at snail’s pace in a conductor)
  1. How is it then that an electric bulb lights up as soon as we turn the switch on? 
  • Think of a tube filled with marbles: if you add a marble at one end, a marble will come out the other end at almost the same instant, even though each marble did not move that far on its own. 
  • Likewise, a copper wire contains lots of electrons. The electrons are packed in so tightly that even a small movement will travel down the wire from electron to electron at an impressive speed, letting you turn on the lights without having to wait for electrons to travel the whole way there.

Electric potential & potential difference

  • Charges do not flow in a copper wire by themselves, just as water in a perfectly horizontal tube does not flow. 
  • For flow of charges in a conducting metallic wire, the electrons move only if there is a difference of electric pressure – called the potential difference – along the conductor. 
  • This difference of potential may be produced by a battery, consisting of one or more electric cells
  • The chemical action within a cell generates the potential difference across the terminals of the cell. 
  • When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor & produces an electric current. 
  • In order to maintain the current in a given electric circuit, the cell has to expend its chemical energy.
  • We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other.
  • Potential difference (V) between two points = Work done (W)/Charge (Q) = V = W/Q
  • The SI unit of electric potential difference is volt (V), named after Alessandro Volta (1745-1827), an Italian physicist. 
  • One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.
  • The potential difference is measured by means of an instrument called the voltmeter
  • The voltmeter is always connected in parallel (ammeter is connected in series) across the points between which the potential difference is to be measured.

Questions

  • Name a device that helps to maintain a potential difference across a conductor. (Ans: Battery)
  • What is meant by saying that the potential difference between two points is 1 V? (Ans: One Joule of work done by One Coulomb of charge)

Components of an electric circuit & their symbols

  • It is convenient to represent electric components by symbols. 
  • Using these, an electric circuit can be represented by a circuit diagram. 
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Circuit diagram
A simple circuit diagram

Ohms law

  • In 1827, a German physicist Georg Simon Ohm (1787-1854) found out the relationship between the current I, flowing in a metallic wire & the potential difference across its terminals. 
  • He stated that the electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm’s law. 
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  • R is a constant for the given metallic wire at a given temperature & is called its resistance.
  • It is the property of a conductor to resist the flow of charges through it
  • Its SI unit is ohm, represented by the Greek letter Ω
  • According to Ohm’s law, R = V/I.
  • If the potential difference across the two ends of a conductor is 1 V & the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. 
  • Ohm’s law holds true for semiconductors, but for a wide variety of materials (such as metals) the resistance is fixed & does not depend on the amount of current or the amount of voltage.

Resistance

  • The electrons, not completely free to move within a conductor. They are restrained by the attraction of the atoms among which they move. Thus, motion of electrons through a conductor is retarded by its resistance. 
  • A component of a given size that offers a low resistance is a good conductor. 
  • A conductor having some appreciable resistance is called a resistor
  • A component of identical size that offers a higher resistance is a poor conductor. 
  • An insulator of the same size offers even higher resistance.
  • Resistance of the conductor depends on
  1. its length (greater the length, greater is the resistance, & greater is the transmission loss), 
  2. its area of cross-section (greater the cross-section, lesser is the resistance), & 
  3. the nature of its material. 
  • Precise measurements have shown that resistance of a uniform metallic conductor is directly proportional to its length (l) & inversely proportional to the area of cross-section (A). 
  • That is,
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where ρ (rho) is a constant of proportionality & is called the electrical resistivity of the material.

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  • The SI unit of resistivity is Ω m. It is a characteristic property of the material. 
  • The metals & alloys have very low resistivity. They are good conductors of electricity. 
  • The resistivity of an alloy is generally higher than that of its constituent metals
  • Alloys do not oxidise (burn) readily at high temperatures
  • For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc. 
  • Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper & aluminium are generally used for electrical transmission lines.
  • Insulators like rubber & glass have resistivity. 
  • Both the resistance & resistivity of a material vary with temperature.

Resistor

  • A resistor is an electrical component that implements electrical resistance as a circuit element. 
  • In electronic circuits, resistors are used to reduce current flow, adjust signal levels, etc.
  • Current through a resistor is inversely proportional to its resistance
  • If the resistance is doubled the current gets halved. 
  • A component used to regulate current without changing the voltage source is called variable resistance
  • In an electric circuit, a device called rheostat is often used to change the resistance in the circuit. 

Resistors in Series

resistance in series
  • The potential difference V is equal to the sum of potential differences V1, V2, & V3. 
  • That is the total potential difference across a combination of resistors in series is equal to the sum of potential difference across the individual resistors. 
  • That is,
V = V1 + V2 + V3 = IR = IR1 + IR2 + IR3 = I (R1 + R2 + R3)

That is, Rs = R1 + R2 + R3

  • We can conclude that when several resistors are joined in series, the resistance of the combination Rs equals the sum of their individual resistances, R1, R2, R3, & is thus greater than any individual resistance.

Disadvantages of series circuit

  • In a series circuit the current is constant throughout the electric circuit
  • Thus, it is obviously impracticable to connect an electric bulb & an electric heater in series, because they need currents of widely different values to operate properly. 
  • Another major disadvantage of a series circuit is that when one component fails the circuit is broken & none of the components works.
  1. Resistors in Parallel
resistance in parallel
  • It is observed that the total current I, is equal to the sum of the separate currents through each branch of the combination.
I = I1 +12 +13 = V/R = V/R1 + V/R2 + V/R3 or 1/Rp = 1/R1 + 1/R2 + 1/R3
  • Thus, we may conclude that the reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.
  • That is, the total resistance in a parallel circuit is decreased.
  • Parallel circuits are helpful when each gadget has different resistance & requires different current to operate properly.

Electric power

  • Electric power is the rate of consumption of energy = Voltage x Electric Current.
P = VI = (IR) I = I2R = V2 / R
  • The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. Thus, 1 W = 1 volt x 1 ampere = 1 V A.
  • The unit ‘watt’ is very small. Therefore, we use a much larger unit called kilowatt (1000 watts)
  • Since energy is the product of power & time, the unit of electric energy is, therefore, watt hour (Wh)
  • One watt hour is the energy consumed when 1 watt of power is used for 1 hour
  • The commercial unit of electric energy is kilowatt hour (kWh), commonly known as unit.
  • 1 kW h = 1000 watts x 3600 seconds = 3.6 x 106 watt second = 3.6 x 106 joule (J)

Questions

  • Why is the tungsten used almost exclusively for filament of electric lamps?
  • Why are the conductors of electric heating devices, such as bread-toasters & electric irons, made of an alloy rather than a pure metal?
  • Why is the series arrangement not used for domestic circuits?
  • Why are copper & aluminium wires usually employed for electricity transmission?

Chemical Effects of Electric Current

  • The passage of an electric current through a conducting liquid causes chemical reactions. 
  • The resulting effects are called chemical effects of currents.
  • British chemist, William Nicholson (1753-1815), had shown that if electrodes were immersed in water, & a current was passed, bubbles of oxygen & hydrogen were produced. 
  • Oxygen bubbles formed on the electrode connected to the positive terminal of the battery & hydrogen bubbles formed on the other electrode.
  • The passage of an electric current through a conducting solution causes chemical reactions. 
  • As a result, bubbles of a gas may be formed on the electrodes. 
  • Deposits of metal may be seen on electrodes. 
  • Changes of colour of solutions may occur. 
  • The reaction would depend on what solution & electrodes are used.

Electric Cell

  • All electric cells have two terminals: a positive terminal (anode) & a negative terminal (cathode).
  • An electric cell produces electricity from the chemicals stored inside it (chemical energy to electrical energy). 
  • The electric circuit provides a complete path for electricity to pass between the two terminals. 
  • In an electric circuit, the direction of current is taken to be from the positive to the negative terminal.
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Direction of current in a closed circuit

  • A switch is a simple device that either breaks the circuit or completes it. 
  • Materials which allow electric current to pass through them are conductors of electricity.
  • Insulators do not allow electric current to pass through them. 

Electric Conductivity in liquids

  • Most liquids that conduct electricity are solutions of acids, bases & salts.
  • Distilled water (pure water free of salts) doesn’t conduct electricity as there are no ions in it.
  • Tap water may contain several salts dissolved in it & hence it is a good conductor of electricity. 
  • Some liquids such as oil or alcohol do not form ions & do not conduct electricity.
  • Vinegar is mostly water with a small amount of acetic acid in it. 
  • The acetic acid separates into ions. Hence vinegar (weak acid) conducts electricity.
  • Soda compounds (containing sodium) contain ions when dissolved in water & ions help conduct electricity. 
  • Milk is a good conductor of electricity because it contains water & lactic acids & other salts. 
  • Lubricants are normally only slightly conductive & therefore can work as insulators in transformers or switches. 
  • Sugar solution does not conduct electricity, because there are no ions in the solution

Electroplating

  • When electric current is passed through the copper sulphate solution, copper sulphate dissociates into copper & sulphate. 
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  • The free copper gets drawn to the electrode connected to the negative terminal of the battery & gets deposited on it. But what about the loss of copper from the solution?
  • From the other electrode, a copper plate, an equal amount of copper gets dissolved in the solution. 
  • Thus, the loss of copper from the solution is restored & the process keeps going. 
  • This means that copper gets transferred from one electrode to the other.
  • The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating. It is one of the most common applications of chemical effects of electric current.

Examples of electroplating

  • Chromium plating is done on many objects such as bath taps, wheel rims & many others. 
  • Chromium has a shiny appearance. It does not corrode. It resists scratches
  • However, chromium is expensive, & it may not be economical to make the whole object out of chromium. 
  • So, the object is made from a cheaper metal & only a coating of chromium over it is deposited. 
  • Jewellery makers electroplate silver & gold on less expensive metals. 
  • Tin cans, used for storing food, are made by electroplating tin onto iron. Tin is less reactive than iron
  • Thus, food does not come into contact with iron & is protected from getting spoilt.
  • Iron is used in bridges & automobiles to provide strength. However, iron tends to corrode & rust. 
  • So, a coating of zinc is deposited on iron to protect it from corrosion & formation of rust.
  • In the electroplating factories the disposal of the used conducting solution is a major concern. 
  • It is a polluting waste & there are specific disposal guidelines to protect the environment.

Questions

  1. To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F) (Hint: arrangement of batteries in a torch light)
Image result for battery of two cells
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Heating effect of the electric current

  • A part of the source energy in maintaining the current may be consumed into useful work. 
  • Rest of the source energy may be expended in heat to raise the temperature of gadget. 
  • For example, an electric fan becomes warm if used continuously for longer time etc. 
  • On the other hand, if the electric circuit is purely resistive, the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current. 
  • This effect is utilised in devices such as electric heater, electric iron etc.

Joule’s law of heating

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  • Consider a current I flowing through a resistor of resistance R. 
  • Let the potential difference across it be V. Let t be the time during which a charge Q flows across. 
  • The work done in moving the charge Q through a potential difference V is VQ. Therefore, the source must supply energy equal to VQ in time t. Hence the power input to the circuit by the source is
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  • Or the energy supplied to the circuit by the source in time t is P × t, that is, VIt. 
  • What happens to this energy expended by the source? 
  • This energy gets dissipated in the resistor as heat. 
  • Thus, for a steady current I, the amount of heat H produced in time t is
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  • This is known as Joule’s law of heating
  • The law implies that heat produced in a resistor is :
  1. directly proportional to the square of current for a given resistance, 
  2. directly proportional to resistance for a given current, & 
  3. directly proportional to the time for which the current flows through the resistor. 
  4. Practical Applications of Heating Effect of Electric Current
  • The electric laundry iron, electric toaster, electric oven, electric kettle & electric heater are some of the familiar devices based on Joule’s heating.
  • The electric heating is also used to produce light, as in an electric bulb. Here, the filament must retain as much of the heat generated as is possible, so that it gets very hot & emits light. 
  • It must not melt at such high temperature. A strong metal with high melting point such as tungsten (melting point 3380°C) is used for making bulb filaments
  • The filament should be thermally isolated as much as possible, using insulating support, etc. 
  • The bulbs are usually filled with chemically inactive nitrogen & argon gases to prolong the life of filament. 
  • Another common application of Joule’s heating is the fuse used in electric circuits
  • It protects circuits & appliances by stopping the flow of any unduly high electric current. 

Electric fuse

  • A wire gets hot when electric current passes through it. This is the heating effect of the electric current.
  • The amount of heat produced in a wire depends on its material, length & thickness. 
  • Wires made from some special materials melt quickly & break when large electric currents are passed through them. These wires are used for making electric fuses.
  • One reason for excessive currents in electrical circuits is the direct touching of wires. 
  • This may happen if the insulation on the wires has come off (causing short circuit).
  • Another reason for excessive current can be the connection of many devices to a single socket (overload). 
  • These days Miniature circuit breakers (MCBs) are increasingly being used in place of fuses. 
  • MCBs are switches which automatically turn off when current in a circuit exceeds the safe limit.
  • The fuse is placed in series with the device. 
  • The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc. 
  • For an electric iron which consumes 1 kW electric power when operated at 220 V, a current of (1000/220) A, that is, 4.54 A will flow in the circuit. In this case, a 5 A fuse must be used.

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