Electricity(EDUCATIONPOINT.CO.CC)

8/14/2019 Electricity(EDUCATIONPOINT.CO.CC)

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LECTURE NOTES – PHYSICS STEPS … A TCY Program

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STEPS ____________________________________________________________ G e t f r e e c h a p t e r w i s e t e s t s f o r C l a s s X t h o n

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BASICS OF ELECTRICITY AND

IT’S HEATING EFFECTS

Nature of Charge

1. There are two kinds of charge, positive and negative 2. Like charges repel, unlike charges attract3. Positive charge comes from having more protons than electrons; negative charge

comes from having more electrons than protons4. Charge is quantized, meaning that charge comes in integer multiples of the

elementary charge e (Q = n × e) e = 1.6 × 10 –19 C

5. Charge is conserved

Law of conservation of charge Total electric charge in the Universe is a constant quantity, and if additional charge

appears in some region, it is only at the expense of the charge deficit in some otherregions.

Coulomb’s Law“The magnitude of the electric force t hat a particle exerts on another particle is directlyproportional to the product of their charges and inversely proportional to the square of thedistance between them. The direction of the force is along the line joining the particles. "

F 2

21

R

Q.Qwhere Q 1 and Q 2 are the magnitude of the charges and R the separation

between them.

Conductors and InsulatorsA conductor is a material that permits the motion of electric charge through its volume.Examples of conductors are copper, aluminium and iron. An electric charge placed on theend of a conductor will spread out over the entire conductor until an equilibriumdistribution is established. Conductors have low resistivity.Electric charge placed on an insulator stays in place: an insulator (like glass, rubber andMylar) does not permit the motion of electric charge in under normal physical conditions.

They have very high resistivity.

The Potential at a PointElectric potential is a measure of the potential energy per unit charge.

The potential of a point is the work done in carrying unit positive charge from infinity tothat point.

• The potential of a point in an electric field is a characteristic constant of that point.

• It does not depend on how much charge is carried.




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• It does not depend upon the distance that is actually covered but on thedisplacement of the point from infinity

• It is a scalar quantity.

• The unit is volt.

Potential Difference Between Two Points (V) The potential difference between two points is the work done in moving a unit positivecharge from one point to the other.If the potential at point A is V A and the potential at point B is V B then the potentialdifference, V, between the A and B is given byV = V B – V A If a charge, q, is moved through a potential difference, V, we can write

V =q W

The units of V are JC -1 or Volt.

1 Volt:

A potential difference of 1 volt between two points means that in carrying a charge of +1coulomb from one point to the other 1 joule of work would be done.

Electric Current:It is a sustained flow of charge requiringa closed circuita power sourceIt is measured as the rate of flow of charge:

I = Q/tUnit: Ampere.1 Ampere = 1 Coulomb per second.

• A current in a metal is due to the movement of electrons. In a conducting solution,the current is due to the movement of ions.

• Current is measured using an ammeter.

• An ammeter measures the rate of flow of charge.Note: When current is due to the flow of electrons there is an associated mass transfer.However mass transfer if any in electron conduction is insignificant.

Conventional current and electron flow

Conventional Current assumes that current flows out of the positive terminal, through thecircuit and into the negative terminal of the source. This was the convention chosenduring the discovery of electricity.Electron Flow is what actually happens and electrons flow out of the negative terminal,through the circuit and into the positive terminal of the source




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Resistance The resistance of any object is measured as the potential difference required per unitcurrent in that object. Hence the resistance R is quantified as:

R =IV

where V is the applied potential difference and I the current in the object. It can be said

that resistance is the property of an object to resist flow of current through it.

George Ohm investigated the resistance of metals.He first tried to find how the resistance of a piece of metal depends on its dimensions. Hefound that resistance depends on(i) the length of the piece of metal, l (ii) the cross-sectional area of the piece of metal, A(iii) the type of metal.His results showed that

R l

and R l /A

R = ρ l /A ( ρ is the resistivity of material)

Resistance and TemperatureResistance occurs because conduct ing electrons repeatedly collide with the comparativelymassive vibrating atoms losing their kinetic energy. The vibrating atoms having gainedthis kinetic energy now vibrate more. The resulting increase in the average vibrationalkinetic energy is rise in temperature.

Ohm’s lawPhysical conditions remaining constant, the current in a conductor is directly proportionalto the potential difference across the ends of the conductor.

V I

V = R × I where R is a constant

R =IV

where R is a constant for a particular conductor under given physical conditions. This constant is called the resistance of the wire.[Ohm’s law does not state V=IR; It states that in the expression V=IR, the R is a constant ]

Unit : ohm ( Ω) = Volt/Ampere.

1 ohm1 ohm is the resistance of that wire in which 1 volt of p.d. is required in order to maintaina current of 1 Ampere.

V

I




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Ohmic and non-ohmic resistorsSince only metals have constant resistance as stipulated by Ohm’s law, they are calledohmic conductors. All other materials which conduct current such as electrolytes andsemiconductors etc. do have resistance. But they are known as non-ohmic conductorssince their resistance is not constant. Hence for non-ohmic conductors the V-I graph is acurve.

Resistivity

Resistivity ( ρ) of a material is the resistance offered by a cube of dimension 1m made of the material when the potential difference is applied perpendicular to the opposite faces of the cube. It is an intrinsic property of that medium and does not change with the shapeand size of the sample. It is however dependent on the temperature of the material.Unit of resistivity is ohm.metre

Series circuitsA series circuit is a circuit in which resistors are arranged in a chain, so the current hasonly one path to take. The current is the same through each resistor. The total resistance

of the circuit is found by simply adding up the resistance values of the individualresistors:Equivalent resistance of resistors in series: R = R 1 + R 2 + R 3 + ...

Let V 1 , V 2 and V 3 be the potential across resistors R 1 , R 2 and R 3 . Since current is same

throughout: by the definitions of Resistance.R1 =

IV1 , R 2 =

IV2 , R 3 =

IV3

Adding together

R1 + R 2 + R 3 =I

VVV 321 ++=

IV (In series V = V 1 + V 2 + V 3)

R1 + R 2 + R 3 =IV = R

R is the equivalent resistance of the three resistors.

It should be noted that in a series connection of resistances,

• the equivalent resistance is higher than any of the components,• the current is the same in all components

• the potential differences across the different components are directly proportional totheir resistances.

Parallel circuitsA parallel circuit is a circuit in which the resistors are arranged with their headsconnected together, and their tails connected together. The current in a parallel circuit

V1 V2 V3




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breaks up, with some flowing along each parallel branch and re-combining when thebranches meet again. The voltage across each resistor in parallel is the same.

The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:equivalent resistance of resistors in parallel: 1 / R = 1 / R 1 + 1 / R 2 + 1 / R 3 +...

We know that I = I 1 + I 2 + I 3 We know that all resistance work at same potential difference in the parallel circuit.

So I 1 =1R

V , I 2 =2R

V and I 3 =3R

V

I = I 1 + I 2 + I 3 = V ++321 R

1R1

R1

VI =

321 R1

R1

R1 ++

If all three resistors are replaced by a single equivalent resistor R then

VI

R1 =

Hence321 Rl

Rl

R1

R1 ++=

It should be noted that in a parallel connection of resistances,• The equivalent resistance R is in this case smaller than even the smallest of the

individual resistances.

• In a parallel connection, the current subdivides, the sum of all the currents beingequal to the main current in the circuit.

• The current in each arm is inversely proportional to the resistance of that arm.

Power of a device

We know that potential difference is given by work done per unit charge carried. Therefore,

W = V x QIf time taken is t, thenW/t = V x Q/tor, P = V x Ior, P = I 2Rand, P = V 2 /RAll these expressions are applicable to all materials, both ohmic and non-ohmic.




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CHEMICAL EFFECTS OF ELECTRIC CURRENT

Voltaic and electrolytic cellsElectrochemical cells which generate an electric current are called voltaic cells or

galvanic cells, and common batteries consist of one or more such cells.

In other electrochemical cells an externally supplied electric current is used to drive achemical reaction which would not occur spontaneously. Such cells are calledelectrolytic cells .

ElectrolytesAn electrolyte is a substance which dissociates into free ions when dissolved (or molten),

to produce an electrically conductive medium. Because they generally consist of ions insolution, electrolytes are also known as ionic solution.

What is electroplating?Electroplating is the deposition of a metallic coating onto an object by putting a negativecharge onto the object and immersing it into a solution which contains a salt of the metalto be deposited. The metallic ions of the salt carry a positive charge and are attracted tothe part. When they reach it, the negatively charged part provides the electrons to reduce"the positively charged ions to metallic form.




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The laws of electrolysis.Law-I : The amount of chemical change produced by a current is proportional to thequantity of electricity passed, ie equal currents produce equal amounts of decomposition.

m Q or m ItLaw-II: The quantities of substances liberated or deposited on the electrode by thepassage of a given quantity of electricity (current) are proportional to the chemicalequivalent weights of those substances.

m z

z =valency

massatomic

Together the law yields the formulaM = Z I tM is the mass deposited when a current of I flows for time t to deposit a metal of electrochemical equivalent Z

Faraday’s constantIt represents the electric charge carried on one mole of electrons. It is found bymultiplying Avogadro's constant by the charge carried on a single electron, and is equal to9.6483 × 10 4 coulombs per mole.One faraday is this constant used as a unit. The constant is used to calculate the electriccharge needed to discharge a particular quantity of ions during electrolysis

Dry Cell The common dry cell relies on chemical changes occurring between the electrodes – thecentral carbon rod and the outer zinc casing – and the ammonium chloride electrolyte to

produce electricity. The mixture of carbon and manganese is used to increase the life of the cell.

Zn(s) -> Zn 2+ (aq) + 2e - (Anode)2NH4 +(aq) + 2MnO2(s) + 2e Mn2O3(s)+ H2O(l) + 2NH3(aq) (Cathode)

Cathode reaction simplified The reduction of the ammonium ion produces two gaseous products

2NH 4 +(aq) + 2e - 2NH 3(g) + H 2(g)Which must be absorbed to prevent the buildup of gas pressure. That is accomplished

with two further reactions in the paste electrolyte. Zinc chloride reacts with ammonia toform solid zinc ammonium chloride and manganese dioxide reacts with hydrogen to formsolid dimanganese trioxide plus water.ZnCl 2(aq) + 2NH 3(g) Zn(NH 3)2Cl 2 (s)2MnO 2(s) + H 2(g) Mn 2O 3(s) + H 2O(l)

Insulatingtop seal

ammoniumchloride jelly

carbon and(+ ve)

insulatingouter cover

cardboarddisc

brass cap(+ ve contact)

zinc can (– ve)

mixture of powdered

carbon andmanganese

(IV) oxide – ve contact

trade here




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Primary cellsIn primary cells the electrochemical reaction is not reversible. During discharging thechemical compounds are permanently changed and electrical energy is released until theoriginal compounds are completely exhausted. Thus the cells can be used only once.

Secondary cells (accumulators)In secondary cells this electrochemical reaction is reversible and the original chemicalcompounds can be reconstituted by the application of an electrical potential between theelectrodes injecting energy into the cell. Such cells can be discharged and recharged manytimes.




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MAGNETIC EFFECTS OF ELECTRIC CURRENT

What is magnetism?Magnetism is the force of attraction or repulsion between substances made of certain

materials, such as iron, nickel, cobalt, and steel. The force of magnetism, simply put, isdue to the motion of electric charges.

What is a magnetic field?A magnetic field is the region in space where a magnetic force can be detected. Themagnetic field strength and direction can be measured in terms of strength and direction.

What are magnetic poles?All magnets have points, or poles, where their magnetic strength is concentrated. Thosepoints are called poles. We label them north and south because suspended magnetsorient along north-south planes. On different magnets, like poles repel each other,opposite poles attract

Magnetic field lines

Magnetic field lines are a way to vis ualize the magnetic field. When drawn, the distancebetween them is an indication of the strength of the field. The closer they are, thestronger the field. Also, the direction of the tangent to the field line is the direction of themagnetic field at that point. A free north pole would move along the magnetic field line.

Permanent magnetsA permanent magnet is one that will hold its magnetic properties over a long period of time.

MagnetiteMagnetite is a magnetic material found in nature. It is a permanent magnet, but it isrelatively weak.

Magnetic Alloys

Most permanent magnets we use are manufactured and are a combination or alloy of iron,

nickel and cobalt. Rare-earth permanent magnets are a special type of magnet that canhave extreme strength.

Temporary magnetsA temporary magnet is one that will lose its magnetism. For example, soft iron can bemade into a temporary magnet, but it will lose its magnetic power in a short while.




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ElectromagnetBy wrapping a wire around an iron or steel core and running an electrical current throughthe wire, we can make an electromagnet. If the core is soft iron, the magnetism willdiminish as soon as the current is turned off. This feature makes electromagnets good forpicking up and dropping objects. Typically DC electricity is used in electromagnets

Properties of magnetsWe know that a magnet has two poles. A north pole and a south pole. All the properties of magnets arise because of their poles.If you bring two magnets close to each other, you will observe that like poles repel eachother and unlike poles attract each other. Also the poles exist only at the free ends of themagnet.

If you break up a magnet, you will see, new north and south poles form immediately. Amagnetic pole cannot be isolated.

If you spread iron filings around a magnet, they will align themselves along the lines in acurved fashion, starting from one pole and ending on the next pole. These lines are calledlines of force of the magnet.

Uses of MagnetsCommon uses of magnets are based on simple magnetic attraction or repulsion, as inmagnetic refrigerator latches. Large electromagnets are used to move loads of steel scrapand to process magnetic ores. Smaller electromagnets are used in relays, switches, loudspeakers, microphones, clocks, ammeters, voltmeters and speedometers etc. Magneticmaterials are used to store information in tapes, Floppies etc. MRI is a medical procedure

that uses powerful magnets and radio waves to construct pictures of the body

What are permanent magnets made of? Today’s permanent magnets are made of alloys. Alloy materials includeAluminum-Nickel-Cobalt (Alnico)Carbon, chromium, cobalt or tungsten steelsNipermag (iron, nickel, Aluminium and titanium)Neodymium-Iron-Boron (Neodymium magnets or "super magnets", a member of the rareearth category)Samarium-Cobalt (a member of the rare earth category)Strontium-Iron (Ferrite or Ceramic)

How are magnets made?Magnets are made from materials that contain nickel, iron, or cobalt. When thesematerials are exposed to a magnetic field, the structure of the material is actually changedon a microscopic level. The molecules are rearranged into lines called polarized). Whenenough of the metals are polarized, it becomes a magnet.




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Oersted’s DiscoveryA Danish professor, Hans Christian Oersted , prepared a science demonstration whichincluded the heating of a wire by an electric current from a battery. Oersted noted thatevery time he connected the current, the compass needle moved, too , somethingcompletely unexpected. Thus for the first time it was established that electric current hadmagnetic properties.

The direction of the magnetic field linesThe right-hand rule : if you held the wire with your thumb pointing in the direction of thecurrent, the magnetic field would make a circular path around the wire, in the directionthat your fingers curl.

The magnetic field strength due to a current carrying current

Magnetic field strength, B I/r where I is the current flowing through the conductor and r the distance from the

conductor

Field due to a Current in a Straight Conductor

Field due to a circular loop

Field due to a Current in a Long Coil (Solenoid)




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Field Due to a bar magnet

Field due to a horse shoe magnet

The Motor Effect- Force on a current carrying conductorWhen the current passes at right angles through a magnetic field, it experiences a force.

The direction of the force is determined by Flemmings “Left Hand Motor Rule”Case 1: Conductor parallel to magnetic field

No force experiencedCase 2: Conductor at right angles to magnetic fieldMaximum force experiencedCase 3: Conductor at an angle to the magnetic fieldIntermediate force experienced

Fleming ′ s Left Hand RuleAlso known as the Motor Rule this is a way of determining the direction of a force on a

current carrying conductor in a magnetic field.

The thumb, the first and the second fingers on the left handare held so that they are at right angles to each other.

If the first finger points in the direction of the magnetic fieldand the second finger the direction of the current in the wire,then the thumb will point in the direction of the force on theconductor.




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Van Allen Radiation Belts The particles originating in periodic solar flares, get carried by the solar wind, and aretrapped by the earth's magnetic field. The charged particles of which the belts arecomposed circulate along the earth's magnetic lines of force extending from the area abovethe equator to the N Pole, to the S Pole, and circles back to the equator. This can presenta dangerous hazard to satellites orbiting the earth.

The Simple D.C. Electric MotorA simple d.c. electric motor consists of a coil of wire placed in a magnetic field. Whencurrent flows through the coil, a torque is produced. The brushes and commutatorconduct the current from the supply to the coil.Each of the carbon brushes makes contact with one half of the commutator. Thecommutator rotates with the coil.

This arrangement ensures that the torque produces a constant sense of rotation. In thediagram below, the force on side a – b of the coil will be directed downwards (Fleming’s left

hand rule) so the rotation is anti-clockwise (viewed from the front).When the coil has rotated 180°, side d – c is on the left but, as the commutator has alsorotated, the torque is still in the same sense.

BrushesA device which conducts currentbetween rotating and stationary partsof a generator or motor

Slip ringA metal ring mounted on a rotatingpart of a machine to provide acontinuous electrical connectionthrough brushes on stationarycontacts.

N

S




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Commutator The commutator is a single slip ring split into left and right halves that are insulated fromeach other and are attached to opposite ends of the coil. It allows current to leave thegenerator through the brushes in only one direction.

Electromagnetic induction

Faraday discovered that a voltage would be generated across a length of wire if that wire was exposed to a perpendicular magnetic field of changing intensity. An easy way tocreate a magnetic field of changing intensity is to produce a relative motion between apermanent magnet and a coil of wire. This phenomenon where a changing magnetic fieldinduces a current in a coil is called electromagnetic induction.

The direction of induced current is given by fleming’s right hand rule

Right hand RuleAlso known as the Generator Rule this is a way of determining the direction of the induced emf of a conductor

moving in a magnetic field.

The thumb, the first and the second fingers on the righthand are held so that they are at right angles to each other.

If the first finger points in the d irection of the magnetic fieldand the thumb in the direction of the motion of theconductor then the second finger will point in the direction of the induced emf in theconductor.

Simple A.C. GeneratorConsider a coil of wire rotating in a uniform magneticfield, of flux density, B, as shown below.

When the coil is in the position shown in the diagram ,side 2 is moving down and side 1 is moving up. Wecan use Fleming’s right hand rule to decide that end q

will (at that instant) be the positive terminal of the generator.When the coil has rotated through half aturn, end p will be the positive terminal.

Therefore, a coil of wire rotating in amagnetic field has an alternating emf induced in it.

To connect the coil to a light bulb (or anyother component) brushes made of carbonmake contact with slip rings made of brass, as shown in the diagram.




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DC Generators:In a dc generator, the ac output from a rotating armature is converted to pulsating dc byusing a commutator in place of slip rings. The rotating coil and the magnetic field are thesame for both AC and DC generators. The difference is in the method used for removingthe voltage induced in the armature. In an AC generator the armature coil is attached toslip rings which make contact with brushes.In a DC generator the armature coil isattached to a commutator which also makes contact with brushes.

A commutator is basically a slip ring which is split into two or more parts. These parts arecalled commutator Segments . The segments are insulated from each other and from theshaft. Each end of the armature coil is attached to one of the segments. The brushesmake contact to opposite sides of the commutator segments.

Domestic CircuitsColor coding of wiresRed- Live (positive)Black- Nuetral (Negative)Green Wire- EarthPotential difference in domestic circuits is 220 V A.C.Amperage of wiring- 5A for bulbs and other low power devices

15 A for high power devices like Geyser etc.

Earthing of appliances The earth wire gives a safe route for the current if the live wire touches the outer casing.




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If the live wire inside an electric cooker comes loose and touches the metal casing, you willget an electric shock. The earth terminal is connected to the metal casing, so the currentgoes through the earth wire instead of through you.

The earth wire route has a very low resistance, so a big current flows which blows thefuse, disconnecting the cooker.

Parallel connection of devicesDevices are connected in parallel because(i) It ensures that each device can be controlled without affecting others(ii) All devices would operate under same potential difference

OverloadingAn overload is the flow of electricity into conductors or devices when normal load exceedscapacity. It may occur when(i) Higher power appliance is connected to a circuit having lower capacity.

(ii) More number of appliances are used at a single point than recommended(e.g. multiplugs)

(iii) A faulty device consuming more power than its ratingAn over load results in heating of conductors and hence melting of insulation resulting inshort circuit and fire.

Short circuitA short circuit occurs when the current finds a way to bypass the appliance on a path thathas little or no resistance - for example, where frayed insulation bares a wire and allows itto touch the frame of the appliance, so the current can flow straight to ground. In this

situation, a very large current can occur, producing a lot of heat and a fire hazard.

The fuse The fuse does two jobs. It protects the wiring if something goes wrong, and it can alsoprotect us. The fuse contains a piece of wire that melts easily. If the current through thefuse is too great, the wire melts and breaks the circuit. The thicker the fuse the more itscurrent carrying capacity. It is usually made of pure tin or copper - tin alloy.Fuses in plugs are made in standard ratings. The most common are 5A and 15 A. Thefuse should be rated at a slightly larger current than needed for the device.• A 5A fuse is used in the wires feeding fans bulbs etc.

• A 15 A fuse is used to protect geysers, refrigerators (1000W or more) etc.

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