Page 1 :
Mob :-9304684650, , ¥ VIVEK CLASSES, , CLAS:, Electricity, , , , , , (a) Definition :, Electric charge may be defined as the intrinsic property, of certain fundamental particles (electron, proton; etc.), due to which they produce.electric and -magnetic, effects., , (b) Types of Electric Charge :, There are two types of charges. They are :, , (i) Positive charge - A body having deficiency of, electrons., , (ii) Negative charge- A body having excess of, electrons., , (ec) Charging of a body :, , There are a number of methods to charge a body as:, (i) Charging by friction, , (ii) Charging by conduction, , (iii) Charging by induction etc., , (i) charging by friction :, , Whenever two bodies (at least one non conductor) are, rubbed against each other, heat is produced due to, friction present between them. Due to this heat, produced, electrons in both the bodies are excited., The body having more electron affinity attracts some of, the electrons from-other body. Both the bodies develop, equal and opposite charges by this method., , , , POSITIVE CHARGE NEGATIVE CHARGE, , , , , , , , 1. Glass Rod 4. Silk cloth, 2. Ebonite, Amber,, 2. Fur or woolen cloth Rubberroe, , , , , , , , , , 3. Woolen coat 3. Plastic seat, '4 Weolen carpet 4. Rubber shoes, 5. Nylon or Acetate 5. Cloth, , 6 6. Comb, , , , , , , , - Dry hair, , , , Note : The object in above table must be in given pair., (ii) Charging by conduetion :, , If an uncharged conductor is touched with a, positively or negatively charged conductor. then the, uncharged conductors also acquires the charged, possessed by the charged conductor :.This process, is called charging by conduction., , Take an uncharged metal rod A and place it on an, insulating stand as shown in figure (a) bring a positively, charged conductor B with an insulating handle near it, and touch the metal rod A figure(b). You will observe, that the uncharged metal rod becomes positively, charged. Try the same activity with a negatively charged, Conductor. Observe the charge on the uncharged, conductor., , uncharged conductor, , , , , , , , |} insulating, stand, , , , handle, , (a) (b), , (iii) Charging by induction : The process of charging, a body by keeping it near a charged body, but not, touching it, is called charging-by induction., , Take a metal rod A on an insulating stand. Bring a, positively charged conductor’B with an insulating, handle near it. Keeping the charged conductor with, your finger. Now, remove your finger first and then the, charged conductor. The uncharged conductor, becomes negatively charged figure., , uncharged metal rod, A SG, , , , , , , , , , , , , , insulating, handle, , (d) Properties of Electric. Charge :, (@ Like charges repel and unlike charges attract each, , other., , NN, Attraction,, , , , Repulsion, , , , (ii) Charge is a scalar quantity, , (iii) Charge is always quantized : The amount of charge, on’a charged body is always in integral multiple of the, elementary charge the fractional multiple is not, possible., , (iv) Charge is conserved: Whenever two bodies are, charged by rubbing, one gets positively charged and, the other negatively charged. The net charge on the, two bodies, however, remains zero—the same as that, before rubbing. In other words, charge is conserved., It can neither be created nor destroyed. The only thing, that happens on rubbing is that charged particles, (electrons) get transferred from one body to the other., , , , VIVEK CLASSES PATNA, , Page 1

Page 2 :
In-some phenomena, charged particles are created., But even then the conservation of charge holds. For, example, a free neutron converts itself into an electron, and the proton taken together is also. zero. So, there is, no change in the conversion of a neutron to an electron, and a proton., , (v) Charge is always associated with mass., , (vi) Total charge of system remains conserved ., , (e) Unit of Charge :, , The S.1. unit of charge is coulomb ‘abbreviated as C., One coulomb of charge is equal to the charge on, 625 x 10" electrons., 1 coulomb = charge on 625 x 10’ electrons, or 6.25 101° electrons, , Thus, when we say that a body has a positive charge, of one coulomb (i.e. + 1C) it means that the body has a, deficit of 625 x 107° electrons from the normal due to, , share., , STATIC AND CURRENT ELECTRICITY ), , (a) Static electricity :, A branch of physics which deals with the study of the, electric charges at rest and their effects:is known as, electrostatic or static electricity., , (b) Current electricity, , A branch of physics which deals with the study of the, electric charges in motion and their effects is known, , as current electricity., , COULOMB’S LAW., , Charles Augustine de Coulomb studied the interaction, forces of charged particles in detail in 1784. He used a, torsion balance. On the basis of his experiments he, established Coulomb's law. According to this law the, magnitude of the electric force between two point, charges is’ directly proportional to. the, product of the magnitude of the two charges and, inversely proportional to the square of the, distance betweén them and acts along the. straight, -line joining the two charges., In mathematical terms, the force that each of the two, point charges q, and q, at a distance r apart exerts on, , the other can be expressed as—, , r, This force is repulsive for like charges and attractive, for unlike charges., 4, k is a constant of i ity. kK = gue:, Where k is a int of proportionality. Anco, here «, is absolute permittivity of free space., , The force is directed along the line joining the centres, of the two charged particles.,, , Sol., , Mob :-9304684650, , , , A charge Q is placed at each of the opposite, corners of a square. A charge. q is placed at, each of the other two corners. If the net, electrical force on Qis zero, then Q/q equals, , Since-F,,, is zero , , , 2, kQ, KaQ (Ya) 442> =, a’ 2a’, , , , , , , , , , , , kqQ kat, 2a*, q kqQ, Q a, Q q, -2 2, , , , a, q, , , , [ELECTRIC FIELD AND ELECTRIC POTENTIAL), , (a) Electric Field :, , Electric field due to a given charge is defined as the, space around the charge in which electrostatic force, of attraction or repulsion due to charge can be, experienced by any other charge. If a test charge, experiences no force at a point, the electric field at that, point must be zero. ., , Electric field intensity at any point is the strength of, electric field at that point. It is defined as the force, experienced by unit positive charge placed at that point., , If F.is the force acting on a test charge +q, at any point”, then electric field intensity at this point is given by, , ga., , 40, Electric field is a vector. quantity and its S.1. unit is, Newton per coulomb (N/C)., (b) Electric Potential :, , The electric potential at a point in an electric field is, defined as the amount: of work done in moving a unit, +ve charge from infinity to that point, without acceleration, or without a change in K.E., against the electric force, due to the electric fleld., , Mathematically,, , , , Ww, q, , Since work is measured in joule and charge in, coulomb, therefore electric potential is ‘measured in, joule per coulomb (J/C). This unit occurs so often in, , , , VIVEK CLASSES PATNA, , Page 2

Page 3 :
our study of electricity, so it has been named as volt, in, honour of the scientist Alessandra Volta (the inventor, of the voltaic cell)., 4 joule, 1 Volt= Teoulomb, , Potential is a scalar quantity, therefore it is added, algebraically. For a positively charged body potential is, positive and for a negatively charged body potential is, negative., , , , We can say potential is the electrical state of a, conductor which determines the direction of flow, charge when the two conductor are kept in contact., (c) Electric Potential Difference :, , * Consider a charge Q placed ata point P. Let A and B be, two other points (B being ‘closer to A) as shown in, figure., , q, , A --<4—o, , --- 4 -, , , From infinity, , The quantity V, — V, is called the potential difference, between points A and B in the electric field of charge Q., Mathematically we have,, , wy,, , 1h Mass, q 4, , Electric potential difference is also measured in volt., , ELECTRIC POTENTIAL ENERGY, , Consider a charge Q placed at a point P as.shown in, figure. If another charge q of the same sign is now, brought from a very far away distance (infinity) to point, O near P, then charge q will experience a force of, repulsion due to charge @. If charge q is still pushed, towards P, work is done. This work done is the potential, energy of the system of these two charges., , Q, , @ r a eA., , P O From infinity, , , , Thus, the electric potential energy of a system of, charges is defined as the amount of work done in, bringing the various charges.from infinite separation, to their present positions to form the required, system. It is denoted by U. For the system of two, charges separated by distance r as shown in figure,, the electric potential energy is given by :, , Mob :-9304684650, , The voltmeter is connected in parallel across the points, where the potential difference-is to be measured, A, voltmeter has a high resistance so that it takes a negligible, current from the circuit., , ELECTRIC CURRENT, , This rate of flow electric charge from one body to another, through a conductor such as, metal wire is called, electric current and its direction is opposite to direction, of flow of electrons., , Thus, if Q is the charge which flows through a, conductor. in time t, then the electric current is given, by, , , , Charge (Q), Current (I) = Time (), Q, or rr, or Q=lt, , Note : The electric current is a scalar quantity., (a) Unit of current :, , 1 coulomb, Tampere'= "4 second, or 1A=1Cs*, , (b) Direction of Electric Current :, , When electricity was invented a long time back, it was, known that there are two types of charges : positive, charges and negative charges, but the electron had, not been discovered at that time. So, electric current, was considered to be a flow of positive charges and, the direction of flow of the positive charges was taken, to be the direction of electric current. Thus, the conventional, direction of electric current is from positive terminal of, a cell (or battery) to the negative terminal through the, circuit., , When two charged bodies at different electric, potentials are connected by a metal wire, an electric, current will flow from the body at higher potential to the, one at lower potential till they both acquire the same, potential. Let two oppositely charged metal conductors, A and B are held on insulated stands., , Electric Current, , , , Positively 77 Negatively, kQq Charged Charged, ape Concer Conductor, , j<—Insulated Stand, Electric potential energy is the from of energy, therefore, it is measured in joule (J). The potential difference is, , measured by means of an instrument called voltmeter. The positively charged conductor A is said to be at, , higher potential and the negatively charged conductor, , , , Page 3, , VIVEK CLASSES PATNA

Page 4 :
B is.said to be at a lower potential. Thus, there isa, potential difference between the oppositely charged, conductors Aand B. So, when we join the positively charged, conductor A to negatively charged conductor.B by a metal, wire, then electric current starts flowing from Ato B., , (c) How the Current Flows in a Wire :, , As electric current is the flow of electrons in a metal, wire (or conductor) when a cell or battery is connected, across its ends. Ametal wire has plenty of free electrons, in it. When the metal wire has not been connected to a, source of electricity like a cell or a battery, then the, electrons present in it move at random in all the, directions between the atoms of the metal wire as, shown in figure below., , Metal Wire, , , , When a source of electricity like a cell or a battery is, connected between the ends of the metal wire, then, an electric force acts on the electrons present in the, wire. Since the electrons are negatively charged, they, start moving from negative end to the positive end of, the wire and this flow of electrons constitutes the, electric current in the wire., , +O <Q) <Q <Q\He) <@) <€) <@), , Direction of conventional Current, , , , , , , , , , , , , , , , +) =, ——_, , Cell, , , , LQHM Law.), , It states that the current passing through a conductor, is directly proportional to the potential difference across, its ends, provided the temperature and other physical, conditions (mechanical strain etc.), remain unchanged, ie., IV or VET or V=RI, , Where R is a constant called resistance of the, conductor., , The relation R = V/I is referred to as Ohm's law, after, , the German physicist George Simon. Ohm (1789 - 1854),, who discovered it., , , , It is quite ‘clear from the above equation that, , (i) The currentIis proportional to the potential difference, V between the ends of the resistor., , (ii) If V is constant, then current I is inversely, proportional to the resistance., , Now, plot a graph between the current and the potential, difference. we will get a straight line graph., , VIVEK CLASSES PATNA, , , , Mob :-9304684650, , lee, , Potential 3V, difference (V) 2V, Vv, , {,- 21. 3h ‘AL, Current (A) ——>, , Slope of graph, tan® =, , ELECTRICAL RES! TANCE ), , (a) Definition :, , The property of a conductor due to which it opposes, the flow of current through it, is called resistance. The, resistance of a conductor is numerically equal to the, ratio of potential difference across its ends.to the, current flowing through it., , Potential difference, , istance =, es Rel Current, , Vv, or =, , !, (b) Unit of Resistance :, , The S.1. unit of resistance is ohm, which is denoted by, the symbol Q ., , When a potential: difference of 1 volt is applied to the, ends of the conductor and a current of 1 ampere flows, through it, then resistance of the conductor will be 1, ohm., , (c) Factors affecting the Resistance of a, Conductor :, , Resistance depends upon the following factors:, (i) Length of the conductor., , (ii) Area of cross-section of the conductor (or thickness, of the conductor)., , (iii) Nature of the material of the conductor., (iv) Temperature of the conductor., , Mathematically : It has been found by experiments, that :, , (i) The resistance of a given conductor is directly, proportional to its length i.e., , ReL, , , , i), (ii) The resistance of a given conductor is inversely, proportional to its area of cross-section i.e., , Re +, A, , from (i) and (ii), , Ail), , , , , , Page 4

Page 5 :
rot 3 Re Pxh (iii), , A, , , , Where P (rho) is a constant known as resistivity of the, material of the conductor, Resistivity is also known as, , specific resistance., Effect of stretching of a wire on resistance:, , In stretching, the density of wire usually does not, change. Therefore, Volume before stretching = Volume after stretching, , £yAy = £2A2, , Re. 22 An, , ana, ey Ae, If information of lengths before and after stretching, , Ai _ fo, is given, then use Ap ee, 1, , 2, Re | fa:, R, (ey, , If information, of radius r, and r, is given then use, , eas, £4, A, , Re (Ar) _(n), R, (Ag B, , Dependency of resistance on temperature :, , IFR, is the resistance of the conductor at 0°C and R, is, the resistance of the conductor at °C then the relation, between R, and R, is given by,, , R=R, 1+aAt) [Here At=t-0=t], , , , Here, « =Temperature Coefficient of Resistance,, t = temperature in °C, , The length of a given cylindrical wire is increased by, 100%. Due to the consequent decrease in diameter, the change in the resistance of the wire., , Given, |, =| + 100% | = 21, , Initial volume = final volume, ie, me)=xr2l, , or r=, , , , VIVEK CLASSES PATNA, , Mob :-9304684650, , , , (d) Resistivity :, , Resistivity, p= RA secesuee(WV), , , , By using this formula, we will now obtain the definition, of resistivity. Let us take a conductor having a unit area, of cross-section of 1 m? and a unit length of 1 m. So,, putting A=1 and L = 1 in equation (iv), we get:, Resistivity, p =R, , (i) Definition of resistivity :, , The resistivity of a substance is numerically equal to, the resistance of a rod of that substance which ir, 1 metre long and 1 metre square in cross-section., Unit of resistivity,, , ohm x (metre)?, metre, , , , ‘p = ohm - metre, The S.I. unit of resistivity is ohm-metre which is written, in symbols as Q-m., , Resistivity of a substance does not depend on its, length or thickness. It depends only on the nature of, the substance. The resistivity of a substance is its, characteristic property. So, we can use the resistivity to, compare the resistances of two or more substances., , (ii) Importance of resistivity :, , A good conductor of electricity should have a low, resistivity and a. poor conductor of electricity should, have a high resistivity. The resistivity of alloy are much, more higher than those of the pure metals., , It is due to their high resistivities that manganin and, constantan alloys are used to make resistance wires, used in electronic appliances to reduce the current in, an electrical cirquit, , Nichrome alloy is used for making the -heating, elements -of electrical appliances like electric irons,, room-heaters, water-heaters and toasters etc., because it has very high resistivity and it does not, undergo oxidation (or burn) even when red-hot., , , , (iii) Factor affecting the resistivity :, It depends on:% Nature of-conductor :, , The resistivity is less. for a good conductor and is large, for a bad conductor., , , , Page 5