STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT .

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www.Padasalai.Net - Page No - 1STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inChapter : 3Current ElectricityCurrent ElectricityThe branch of Physics which deals with the study of electric charges in motion is called currentelectricity.Electric currentThe flow of electric charges in a particular direction constitutes electric current.The electric current is defined as the rate of flow of charges across any cross sectional area of aconductor is known as electric current.Where, I electric currentq charge passed through any cross section of conductort timeIf the rate of flow of charge is not uniform then current varies with time and is given by-Note Current is a scalar quantity. The direction of conventional current is taken as the direction of flow of positive charges oropposite to the direction of flow of electrons. Units of electric current: Ampere (in S.I. System)- One ampere represents the passage of onecoulomb of charge per second. An electric current is due to the drift of 1. Electrons in a metallic conductor.2. Positive and negative charges in an electrolyte.3. Electrons and ions in gases in discharge tubes.4. Electrons and holes in a semiconductor.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 2STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inHow Electric Current Flow in the ConductorWhen no potential difference is applied across a conductor, the electrons are in random motion.The average velocity of electrons is zero. Thus the motion of electrons does not constitute anytransport of charge in any direction. The current in the conductor is zero.When potential difference is maintained (i.e. battery is connected) across a conductor, the electronsgain some average velocity in the direction of positive potential (towards positive terminal ofbattery). And thus current is set up in the conductor.Ohm’s LawIt states that physical conditions like temperature pressure etc. remaining constant, the currentflowing through a conductor is always directly proportional to the potential difference across theconductor.i.e.I VorV IorV IR ---------(1)Where R is constant of proportionality, called resistance ofconductor.From (1) constantSo, if we plot a graph between V and I it will be a Straight line as shown in figure.ResistanceIt is the property of a body due to which it opposes the flow of current through the conductor. It isequal to ratio of potential difference applied to the current flowing through the conductor.i.e.Unit of R: unit of resistance is ohm (Ω) in S.I. system.One Ohm: Resistance of a conductor is said to be one ohm if a potential difference of one volt producesone ampere current through it.Cause of Resistance: As we know that there are large numbers of free electrons in a conductor.When a potential difference is applied across the ends of conductor the free electrons moves withhigher energy and collide among themselves and with the atoms of conductor and causes obstacleFor free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 3STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.into flow of current. This obstacle produces the opposition to flow of electrons i.e. producesresistance.Conductance (G)The reciprocal of resistance is known as conductance. i.e.G Unit of conductance is Simen or mho.Resistivity ( )As we know, resistance of a conductor (R) is(i)(ii)Directly proportional to length(l) ofconductori.e.R lInversely proportional to the area of cross-section (A) of the conductorR Combining these two points, we have R Where, is the constant of proportionality, called resistivity of the material.If l 1, A 1 then RSo, is defined as the resistance of a material having unit length and unit area of crosssection. depends on the nature of material, but not on the dimensions of the material. Unit of is ohm-metreConductivity ( )The reciprocal of resistivity ( ) is called conductivity ( ) i.e. Unit of conductivity is Simen per metreCurrent density (J)The electric current per unit area ,taken normal to the current, is known as Current Density.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 4STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inJ Unit of J is Ampere/metre2The current density is a vector quantity directed along the flow of current.When the plane of small area A makes an angle with the direction of current, thenJ Or I JA cos OrI If E is the magnitude of uniform electric field in the conductor whose length is l, then thepotential difference V across its ends is given byV ElAlso(asV I R I OrEl J lOrE J In vector formOrWhere is conductivity.Drift VelocityWhen no Electric field is applied across a conductor, the electrons are in random motion. Theaverage velocity of electrons is zero. Thus the motion of electrons does not constitute any transportof charge in any direction. The current in the conductor is zero.But when Electric field is applied, the free electrons at negative end experience a force F eE in adirection opposite to the electric field i.e. towards positive end.Thus due to collision, there act backward force on electron. So, the electron drift slowly towardspositive end with a constant average velocity called, drift velocity vdSo, drift velocity is defined as the velocity with which free electrons get drifted towards the positiveterminal, when electric field is applied is known as drift velocityExpression for drift velocity:If average time between two successive collisionsa acceleration producedThen, For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 5STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYAlso (F force on electron, m mass of electron)But -eelectric field.So,www.rsnotes.inwhere negative sign shows that direction of force is opposite to that ofa Or where negative sign shows that direction of drift velocity is opposite to thatof electric field.Which is expression for drift velocity.Note Mobility:Magnitude of drift velocity is given byvd vd EOrwhere vd is mobility, and is defined as the drift velocity acquired per unit electric field.Unit of mobility m2V-1s-1Relation between current and drift velocityConsider a conductor of length l and area of cross section A. An electric field E is applied between itsends. Let n be the number of free electrons per unit volume. The free electrons move towards theleft with a constant drift velocity vd.The number of conduction electrons in the conductor nAlThe charge of an electron eThe total charge passing through the conductor q (nAl) eThe time in which the charges pass through the conductor, t Current flowing through the conductor, I For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 6STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYSo,www.rsnotes.inI nAeOr neOrJ neThis is the relation between current and drift velocity.Note:J ne J in vector form Also Comparing above two equations Deduction of Ohm’s Law:The current flowing through a conductor is given by, Alsovd Therefore,I nAe OrI nAVWhereI nAeas constant ( resistance)Therefore I VWhich is ohm’s law.Try example 3.1 and 3.2 from NCERT BOOK Download it from www.rsnotes.in or website of ncertLimitations of Ohm’s Law1. For some good conductor graph between V and I is not exactly linear.2. In diodes relation between V and I depends on sign of V.3. For semiconductor graph between V and I is not linear.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 7STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inNote: the conductors which do not obey ohm’s law completely are called non-ohmic conductor e.g.semiconductors, and diodes.RESISTIVITY OF VARIOUS MATERIALSDepending on resistivity, the materials can be classified as conductor, semiconductors and insulators.The conductors (metals) have low resistivities in the range of 10-8 Ωm to 10-6 Ωm .The insulators have very high resistivities in the range of 108 Ωm to 1014 Ωm.The semiconductors have resistivities between conductors and insulator i.e. in the range of 10-2 Ωm to104 Ωm.Temperature Dependence of ResistanceAs we know for metallic conductorR i.e. R So, when temperature of a conductor is increased, the thermal energy of electron increases. Due towhich the frequency of collision of free electrons with atoms or ions also increase and hence decreases. Hence, resistance increases.IfR0 resistance of conductor at 00C &Rt resistance of conductor at t0CAndt rise in temperatue.ThenRt R0 ( 1 t) ---(1)Where temperature coefficient or resistance.From (1) Rt R0 R0 tOr i.e.Temperature coefficient of resistance Hence is defined as the change in resistance per unit original resistance per degree rise intemperature.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 8STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inUnit of 0C-1 or kelvin-1For metals: is positive. So, from (1) Rt R0Therefore resistance of metals increases with rise of temperature.(this implies that conductivitydecreases with rise in temperature.)For insulators and semiconductors: is negative. So, from (1) Rt R0Therefore resistance of insulators or semiconductors decrease with rise of temperature.(this impliesthat conductivity increases with rise in temperature.)For Alloys: for alloys like Nichrome, Manganin etc is very-very small. So, from (1) Rt R0Therefore resistance of alloys almost remains same with rise in temperature. That is why we use thesealloys to make standard resistance coil.Similarly resistivity of a metallic conductor is given by t 0 ( 1 t)Colour Code of Carbon ResistorsThe value of resistors and their percentage accuracy are indicated on carbon resistor by colour codeprinted on them.The colour code of carbon resistors is as given in the following tableColour blackbrown redorange yellow green blueviolet greywhite goldNo.123456789Mult. 100101102103104105106107108109Colourgreen silverNo ColourTolerance5%20%010%10-1silver10-2Method1: when there is a set of rings of colours on the carbon resistors1st ring colour gives the first significant figure2st ring colour gives the second significant figure3rd ring colour gives the multiplier4th ring (if any) gives the toleranceFor free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 9STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.ine.g. if 1st , 2nd,3rd and 4th colour are red, yellow, orange and golden respectively, then its value will be24 103 5%Method2:Colour of body gives the first significant figureColour of ends gives the second significant figureColour of dots gives the multiplierRing (if any) gives the tolerancee.g. if body colour be blue, end colour be green, dot be red and ring is silver, then its value will be65 102 10%COMBINATION OF RESISTANCESResistances in SeriesResistance are said to be connected in series , if the same current is flowing through each resistor, whensome potential difference is applied across the combination.Let , andare three resistances connected in series asshown in figureIfV , and is the potential drop acrossrespectively. Then, ,and----------(1)Also from Ohm’s Law , V ISo,,whereis the total resistance of series combination.------------(2)So, from (1) and (2), we haveIOr I I I If there are n resistances connected in series then . Hence if a number of resistances are connected in series then their resultant resistance is equal to sumof individual resistances.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 10STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inResistances in ParallelA number of resistances are said to be connected in parallel if potential difference across each of themis the same and is equal to applied potential difference.Let,andare three resistances connected in parallel as shown in figure.Let V potential difference applied.If , and is the current flowing throughrespectively. Then,Then total current, I Also V ,and Or,,Or Or whereis total resistance of parallel combination.Similarly, if a number of resistances are connected in parallel, then the resultant resistance is given by . Hence , the reciprocal of resultant resistance in parallel is equal to sum of reciprocals of the individualresistances.Terminal Potential DifferenceThe potential difference between two electrodes of the cell in closed circuit i.e. when current is beingdrawn from it, is called terminal potential difference.E.M.F.Electromotive force is the maximum potential difference between two electrodes of the cell in poencircuit i.e. when no current is being drawn from it.Internal Resistance of a CellThe internal resistance of a cell is the resistance offered byelectrolyte and electrodes of a cell when electric current flowsthrough it.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 11STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inInternal resistance depends on1.2.3.4.Distance between the electrodesNature of electrolyteNature of material of electrodesArea of electrodes immersed in electrolyte.Expression for Internal ResistanceConsider a circuit with a battery, key and a resistance R as shown in figure.LetE emf of the cellr internal resistance of the cell.R resistance connected &I current clowing through the circuitThen, I OrE IR IrOrE V IrOrV E - IrOrr Orr which is expression for internal resistance of a cell.Difference between emf and potential differenceEmfMaximum value of potential difference betweenelectrodes of a cell in open circuit is called emfPotential DifferenceThe potential difference between two electrodesof the cell in closed circuit i.e. when current isbeing drawn from it, is called terminal potentialdifference.It is independent of resistance of circuit.It is measured by potentiometerIt depends upone the resistance of the circuit.It is measured by voltmeter.GRUPING OF CELL1. Cell in Series:For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 12STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inLet us consider n cells each of emf E and internal resistance r are connected in series, as shown in figure.Then, total emf of circit E E E E nEAnd total internal resistance of circuit r r r nrTotal resistance of circuit nr RI If R nr , then I current in external circuit is n-times the current due to single cell. So, seriescombination is useful. current in external circuit is same as due to single cell. So, seriesIf R nr , then I combination is not useful in this case.2. Cell in ParallelLet us consider m cells each of emf E and internal resistance r are connected in parallel, as shown infigureThenE total emf or circuit&R external resistanceThen , total internal resistance is given by OrTotal resistance of the circuit R Or I R If R r , then I current in external circuit is n-times the current due to single cell. So, parallelcombination is useful.If R r , then I current in external circuit is same as due to single cell. So, parallelcombination is not useful in this case.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 13STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inNote in mixed groupingn-cells are connected in each series and m such combination are connected in parallel, as shown infigureThen, in the similar way,Total emf nETotal resistance R Then , I Combination is useful when internal resistance is equal to external resistance.Note : Ohm ‘s law is unable to give current in complicated circuits.Kirchoff’s Law:Kirchoff’s gave two laws which are very useful to give current in complicated circuits.Kirchoff’s 1st law or Junction Law: It states that, the algebraic sum of the currents at any unction in acircuit is zero. i.e.So, according to this law there is no accumulation of electric current at any junction (the points wherewires meets in an electric circuit).By convention, the currents which flow towards a junction are taken aspositive & the currents which flow away from the junction are taken asnegative.From figure, we have-Kirchoff’s 2nd Law or Loop Law: it states that , in any closed circuit, the algebraic sum of the products ofthe current and resistance of each part of the circuit is equal to the total emf in the circuit. i.e.For free study material: notes/MCQ/Assignments etc of 9th, 10th,10 1, 10 2,NDA,TET, AIEEE/IIT-JEEPhysics, Chemistry and Mathematics visit our website: www.rsnotes.inNOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams.

www.Padasalai.Net - Page No - 14STUDY MATERIAL FOR CLASS 10 2 - Physics- CURRENT ELECTRICITYwww.rsnotes.inBy convention, the product of current and resistance is taken as positive when we move in the directionof flow of current. The emf is taken positive when we move from negative to positive terminal throughthe cell and vice-versa.Applying Kirchoff’s to the circuit as shown in fig.For the loop ACDBA,For the loop EFDBAFor the loop EFBAEILLSTRATION: taking the current in the clockwise direction alongABCDA as positive5I 10I 8I 10I 5I 50 - 20 - 30 40 38I 40 I AWheatstone’s Bride:It is based on Kirchoff’s Law. It is consist of four resistances P,Q,Rand S t

Physics, Chemistry and Mathematics visit our website: www.rsnotes.in NOTES ALSO AVAILABLE FOR – NTSE, BANKING, SSC, HCS and other competitive Exams. J Unit of J is Ampere/metre2 The current density

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