REVISION PLUS - CURRENT ELECTRICITY

REVISION PLUS – CURRENT ELECTRICITYLS-18/AS 17th FEB-10 7.30 pm : REVISION PLUS – CURRENT ELECTRICITYLS-18/AS 17th FEB-10 7.30 pm

. : . . ELECTRIC CURRENT CURRENT CARRIERS. The charged particles whose flow in a definite direction constitutes the electric current are called current carriers. e.g. Electrons in conductors Ions in electrolyte Electrons and holes in semiconductor.

ELECTRIC CURRENT : ELECTROMOTIVE FORCE of a cell is defined as the maximum potential difference between the two electrodes of a cell when the cell is in the open circuit. FACTORS ON WHICH E.M.F DEPENDS E.M.F. of a cell depends upon nature of electrodes, nature and concentration of electrolyte used in the cell and its temperature. ELECTRIC CURRENT

. : . . ELECTRIC CURRENT ELECTRIC CURRENT. The flow of charge in a definite direction constitutes the electric current. Electric current =

ELECTRIC CURRENT : S.I. Unit of current is ampere (denoted by A). Direction of current is Direction of flow of positive charge, whereas the direction of flow of electrons gives the direction of electronic current which is opposite to that of conventional current. Current is a scalar quantity. ELECTRIC CURRENT

. : . . ELECTRIC CURRENT DRIFT VELOCITY: It is defined as the average velocity with which free electrons get drifted towards the positive end of the conductor under the influence of an external electric field. Drift velocity of electrons is given by:

. : . . ELECTRIC CURRENT where e is the charge on electron, m is the mass of electron, E is the electric field applied and t is the time of relaxation i.e. the average time that has elapsed since each electron suffered its last collision with the ion or atom of the conductor ,while drifting towards the positive end of the conductor under the effect of external electric field applied.

ELECTRIC CURRENT : The value of DRIFT VELOCITY of electron is about 10-5 m/s & value of relaxation time is about 10-14 second. Mobility. Mobility of electron is defined as the drift velocity (vd) per unit electric field applied i.e. ELECTRIC CURRENT

RELATION BETWEEN CURRENT AND DRIFT VELOCITY/MOBILITY. : where n is the electron density or no. of electrons per unit volume of the conductor and A is the area of cross-section of the conductor. RELATION BETWEEN CURRENT AND DRIFT VELOCITY/MOBILITY.

ELECTRIC CURRENT : The small value of the drift velocity Produces a large amount of current due to presence of large number of free electrons in a conductor The propagation of current is at the speed of light. ELECTRIC CURRENT

ELECTRIC CURRENT AND RESISTANCE : OHM’S LAW: It states that the current(I) flowing through a conductor is directly proportional to the potential difference (V) across the ends of the conductor,provided physical conditions of the conductor such as temperature,mechanical strain etc. are kept constant i.e. where R is known as resistance of the conductor,which depends upon the nature and dimensions of the conductor. The S.I.unit of R is ohm. ELECTRIC CURRENT AND RESISTANCE

ELECTRIC CURRENT : WHAT IS RESISTANCE Resistance of a conductor. It is the obstruction posed by the conductor to the flow of current through it. WHAT IS THE CAUSE OF RESISTANCE Resistance of a conductor is due to collisions of free electrons with the ions or atoms of the conductor while drifting towards the positive end of the conductor. ELECTRIC CURRENT

ELECTRIC CURRENT AND RESISTANCE : The resistance of a conductor can be given by the expression: Where m is the mass of electron,e is charge of electron, n is the number density of electron, is the relaxation time, l is the length of conductor and A is its area of cross section, is the specific resistance of the conductor. ELECTRIC CURRENT AND RESISTANCE

SPECIFIC RESISTANCE : DEFINE SPECIFIC RESISTANCE Specific resistance or electrical resistivity of the material of a conductor is defined as the resistance of unit length and unit area of cross-section of the conductor. The S.I.unit of resistivity is ?m.Resistivity of a conductor depends upon the nature of the conductor but is independent of the length of area of cross section of the conductor. In fact resistivity, SPECIFIC RESISTANCE

CURRENT DENSITY (J) : Current density (J) at a point is defined as the amount of current flowing per unit area of cross-section of the conductor, provided the area is held in a direction normal to the current. The S.I.unit of current density is Am-2 CURRENT DENSITY (J)

ELECTRICAL CONDUCTIVITY : Electrical conductivity of a conductor is the inverse of its resistivity The S.I. Unit of is ELECTRICAL CONDUCTIVITY

EFFECT OF TEMPERATURE ON RESISTANCE : Effect of temperature on resistance. The resistance of a metal conductor at a temperature to C is given by where Ro is the resistance of a conductor at 0oC and ? is the temperature co-efficient of resistance. For metals ? is positive i.e.resistance increase with rise in temperature. EFFECT OF TEMPERATURE ON RESISTANCE

EFFECT OF TEMPERATURE ON RESISTANCE : For semi conductor and insulators ? is negative i.e. resistance decreases with rise in temperature. For alloys like manganin,eureka and constantan,the value of ? is very small as compared to that of conductors. That is why these alloys are used in making standard resistances. EFFECT OF TEMPERATURE ON RESISTANCE

EFFECT OF TEMPERATURE ON RESISTANCE : If are the resistances of the same conductor at temperature Here ? is the temperature coeff. Of resistance averaged over the temperature range t1oC and t2oC EFFECT OF TEMPERATURE ON RESISTANCE

NON-OHMIC CONDUCTOR : Non-ohmic conductor.Those conductors which do not obey Ohm’s law are called non-ohmic conductor e.g. vacuum tube, liquid electrolyte etc. NON-OHMIC CONDUCTOR

SUPER-CONDUCTORS : Super-conductors. Those materials which often least resistance to the flow of current through them are called super-conductors. Examples :mercury at temp 4.2K, lead at 7.25 K and niobium at temperature 9.2 K become super-conductors. The conductivity of superconductor is called superconductivity. SUPER-CONDUCTORS

COLOUR CODE FOR CARBON RESISTORS : Colour code for carbon resistors. The number attached from 0 to 9 to the various colours can be recollected by the sentence B.B.ROY Great Britain Very Good Wife. Black-0, Brown-1,Red-2,Orange-3,Yellow-4,Green-5,Blue-6,Violet-7,Grey-8,White-9. The strip of gold,silver and no colour shows the accuracy of 5%, 10% and 20% of the given carbon resistor. COLOUR CODE FOR CARBON RESISTORS

RESISTANCE IN SERIES : Resistance in series. The total resistance (Rs) is given by Rs=R1+R2+R3+ …….. RESISTANCE IN SERIES Call me at……………9814123832 Email ………………. hksidhuinstitute@gmail.com

RESISTANCE IN PARALLEL : Resistance in parallel. The total resistance is given by RESISTANCE IN PARALLEL

INTERNAL RESISTANCE : Internal resistance of a cell is defined as the resistance offered by the electrolyte and electrodes of a cell when electric current flows through it. INTERNAL RESISTANCE

INTERNAL RESISTANCE : Internal resistance of a cell depends upon: (I) distance between the electrodes. (ii) the nature of electrodes. (iii) nature of electrolyte. (iv) area of the electrodes immersed in the electrolyte. INTERNAL RESISTANCE

TERMINAL POTENTIAL DIFFERENCE : Terminal potential difference of a cell is defined as the potential difference between the two electrodes of a cell in a closed circuit. Is the terminal potential difference greater or less than emf of cell? Terminal potential difference of a cell decreases if the current is drawn from the cell and increases when cell is charged. TERMINAL POTENTIAL DIFFERENCE

TERMINAL POTENTIAL DIFFERENCE : Terminal potential difference of a cell(V) is less than the e.m.f.of a cell(e)by an amount equal to potential drop across the internal resistance of the cell i.e. Where R is the external resistance in the circuit and r is the internal resistance of a cell. TERMINAL POTENTIAL DIFFERENCE

ELECTRIC MEASUREMENT : KIRCHHOFF’S LAWS First Law: The algebraic sum of the current meeting at a junction is zero i.e. . The current reaching a junction is taken positive and the current leaving the junction is taken negative. This law supports the concept that moving charges are not accumulated at a juntion. ELECTRIC MEASUREMENT

KIRCHHOFF’S SECOND LAW : In a closed loop,the algebraic sum of all the potential difference is zero i.e. SIGN CONVENTIONS While traversing a closed loop(in clockwise or anti clock wise direction),if negative pole of the cell is encountered first then its emf is-ve, otherwise +ve. KIRCHHOFF’S SECOND LAW

KIRCHHOFF’S SECOND LAW : SIGN CONVENTION FOR IR The product of resistance and current in an arm of the circuit is taken positive if the direction of current in that arm is in the same sense as one moves in a closed loop and is taken negative if the direction of current in that arm is opposite to the sense as one moves in the closed loop. KIRCHHOFF’S SECOND LAW

KIRCHHOFF’S SECOND LAW : KIRCHHOFF’S SECOND LAW

Wheatstone bridge Principle : It states that if four resistances P,Q,R,S are arranged to form as shown in Fig. On pressing battery key K1 first and then galvanometer key K2,if the galvanometer shows no deflection,then the bridge is balanced. In that case. Wheatstone bridge Principle

Wheatstone bridge Principle : The practical application of Wheatstone principle is in metre bridge and post office box which are used to find the unknown resistance or specific resistance of the given metallic wire. Wheatstone bridge Principle

METER BRIDGE : METER BRIDGE

Principle of potentiometer : It is based on the fact that the fall of potential across any portion of the wire is directly proportional to the length of that portion, provided the wire is of uniform area of cross section and a constant current is flowing through it. Principle of potentiometer

Principle of potentiometer : V ? l (if l and A are constant ) or V =Kl Where K is called potential gradient I.e. fall of potential per unit length of the given wire. Principle of potentiometer

Potentiometer Applications : Expression for comparison of emf of two cells by using potentiometer. where l1,l2 are the balancing lengths of potentiometer wire for the emfs E1 and E2 of two cells. Potentiometer Applications

Potentiometer Applications : Expression for the determination of internal resistance of a cell(r) by potentiometer method Potentiometer Applications

Potentiometer Applications : where, l1= balancing length of potentiometer wire corresponding to e.m.f. of the cell. l2 = balancing length of potentiometer wire corresponding to terminal potential difference of the cell when a resistance R is connected in series with the cell whose e.m.f. is to be determined. Potentiometer Applications

HEATING EFFECT OF CURRENT : 26. Joule’s law of heating. It states that the amount of heat produced in a conductor is directly proportional to the: (i) square of the current flowing through the conductor. (ii) resistance of the conductor and (iii) time for which the current is passed. HEATING EFFECT OF CURRENT

HEATING EFFECT OF CURRENT : ELECTRIC POWER. It is defined as the rate at which work is done in maintaining the current in electric circuit. Electric power, P = VI or I2 R = V2/R watt or joule/second. HEATING EFFECT OF CURRENT Call me at……………9814123832 Email ………………. hksidhuinstitute@gmail.com

HEATING EFFECT OF CURRENT : Electric energy. The electric energy consumed in a circuit is defined as the total work done in maintaining the current in an electric circuit for a given time. Electric energy=Vit = Pt = I2 Rt = V2t/R S.I.unit of electric energy is joule(denoted by J) where 1 joule = 1 watt x 1 second = 1 volt x 1 ampere x 1 second. Commercial unit of electric energy is kilowatt hour(k Wh) where 1kW h = 1000 Wh = 3.6 x 106 J. HEATING EFFECT OF CURRENT

IMPORTANT QUESTIONS : IMPORTANT QUESTIONS .A piece of wire is cut into 4 parts and the pieces are bundled together closely to form a thick wire. Compared with the original wire, find the resistance of thicker wire. Ans. On cutting,each part will have an equal resistance of R/4. As they are bundled they will be in parallel and so the equivalent resistance will be R/16.

IMPORTANT QUESTIONS : IMPORTANT QUESTIONS Two wires have their length and mass in the ratio5:3 and 1:3 respectively.Find the ratio of their resistance. Ans. Since resistance where d is the density and p is the resistivity,we get

IMPORTANT QUESTIONS : IMPORTANT QUESTIONS Which two of the resistors 3,4,12 and 16 ohm will you choose singly or in groups to form all the four values? Ans. 4 and 12 ohm we will choose as (i) when connected in parallel they give 3 ohm (ii) when connected in series give 16 ohm.

IMPORTANT QUESTIONS : IMPORTANT QUESTIONS A resistance is made by joining two wires of the same material. The radii of the two wires are 1 mm and 3 mm, while their lengths are 3 cm and 5 cm respectively. A battery of emf 16V and negligible internal resistance is connected across the resistances. What is the potential drop along the shorter wire?

IMPORTANT QUESTIONS : IMPORTANT QUESTIONS Ans.

IMPORTANT QUESTIONS : Since in series connection voltage is divided among resistors in the ratio of their resistances. IMPORTANT QUESTIONS

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS . A wire of resistance X ohm is drawn out so that its length is increased to twice its original length. If the new resistance becomes 20 ?, then find the value of X.

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS Ans. We know that when a wire is stretched its volume remains constant i.e., v = Al

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS On doubling the length by stretching And its new resistance will be

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS A potential difference of 10 V is applied across a conductor of 1 k?. Find the number of electrons flowing through it in 5 minutes. Call me at……………9814123832 Email ………………. hksidhuinstitute@gmail.com

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS Ans. The current passing through the conductor, Also q = i x t ? Total charge flowing through the conductor in 5 minutes = 10-2 x 5 x 60 = 3 C As q = ne, 3 = n x 1.6 x 10-19 ? n = 1.875 x 1019

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS In a discharge tube, the number of hydrogen ions (i.e., protons) drifting across a cross-section per second is 1.0 x 1018, while he number of electrons drifting in the opposite direction is 2.0 x 1018 per second. If the voltage is 220 V, what is the effective resistance of the tube ?

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS Ans. As the electron and proton carry equal and opposite charge of 1.6 x 10-19 C, the total electric current constituted by their flow will be I=(ne + np) e Or I=(2.0 x 1018) + 1.0 x 1018) x 1.6 x 10-19 A

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS = 3.0 x 1018 x 1.6 x 10-19 A = 0.480 A Also

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS In a discharge tube, the number of hydrogen ions (i.e., protons) drifting across a cross-section per second is 1.0 x 1018, while he number of electrons drifting in the opposite direction is 2.0 x 1018 per second. If the voltage is 220 V, what is the effective resistance of the tube ?

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS Ans. As the electron and proton carry equal and opposite charge of 1.6 x 10-19 C, the total electric current constituted by their flow will be I=(ne + np) e Or I=(2.0 x 1018) + 1.0 x 1018) x 1.6 x 10-19 A

CURRENT ELECTRICITY EXPECTED QUESTIONS : CURRENT ELECTRICITY EXPECTED QUESTIONS = 3.0 x 1018 x 1.6 x 10-19 A = 0.480 A Also

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