ELECTRICITY Chapter-16

DEFINITIONS COULOMB It is SI unit of electric charge. One coulomb (1C) of charge being that quantity of charge which when placed one meter from an identical charge in vacuums repels it with a force of 8.99 x 10 9 N. INSULATORS Insulators are those materials, which do not allow electric charges to pass through them. In other words, insulators are materials that do not allow electrical current to pass. In insulators electrons are tightly bounded to their atoms. Insulators do not have free electrons. Examples Plastic, rubber, wood, glass etc. CONDUCTOR Conductors are those materials, which allow electric charges to pass through them. In other words, conductors are materials that allow electric current to pass. In conductors electrons are loosely bounded to atoms. Conductors have free electrons. Examples: Copper, Gold, Aluminum, Silver etc.

ELECTRIC FIELD Space or region surrounding a charge or charged body within which another charge experiences some electrostatic force of attraction or repulsion when placed at a point is called Electric Field.

ELECTRIC INTENSITY It is the strength of electric field at a point. Electric intensity at a point is defined as the force experienced per unit positive charge at a point placed in the electric field. Mathematically, E=F/q It is a vector quantity. It has the same direction as that of force. Units N/C or Volt/m E=1/4pe x q/r 2

ELECTRICAL POTENTIAL Electric potential at a point is defined as the amount of work done in moving unit positive charge against the direction of electric field from a point to that point. Electrical potential = work done/charge or U=work/q unit of electric potential in SI system is Volt. 1 volt = 1 Joule/coulomb

VOLT Unit of electric potential and potential difference in SI system is called Volt. It is defined as "in an electric field potential b/w two points is 1 volt if the amount of work done in moving 1 Coulomb charge from one point to another point is 1 Joule."

POTENTIAL DIFFERENCE Potential difference b/w two points A and B is equal to the amount of work done by moving a unit positive charge from point A to point B against the electric field V B -V A =V AB or V AB = (work) AB /q Unit Volt or Joule/Coulomb

ELECTRIC CURRENT The rate of flow of electric charge through a cross section of a conductor is called Electric Current or Electric charge passes through a cross section of a conductor is called Electric Current. It is denoted by I. FORMULA I = Q/t UNITS Ampere in SI system. OTHER UNITS mA (milli Ampere) = A m A (micro Ampere) = A

AMPERE If one coulomb of electric charge passes through a cross section of a conductor in one second, the amount of current passes through it is called Ampere. 1A = 1c/1sec. RESISTANCE opposition offered by the atoms of a conductor in the flow of electric current is called Resistance. It is a hurdle in the flow of electric current. Different substances have different resistance. Resistance of a conductor increases with the increase in temperature. SYMBOL It is denoted by R. UNIT Oh

CAPACITOR Capacitor is an electronic device, which is used to store electric charge or electrical energy. A capacitor stores electric charge on its plates. There are a number of types of capacitors available. STRUCTURE OF CAPACITOR A capacitor consists of two identical conducting plates which are placed in front of each other. One plate of capacitor is connected to the positive terminal of power supply and the other plate is connected to negative terminal. The plate, which is connected to positive terminal acquired positive charge, and the other plate connected to negative terminal. Separation between the plates in very small. The space between the plates is field with air or any suitable dielectric material

Capacitor

PRINCIPLE OF CAPACITOR Electric charge stored between the plates of a capacitor is directly proportional to the potential difference between the plates. Let the potential difference between the plates is V and the charge stored on any one of the plates of capacitor is Q then, Q directly prpportional V Q = CV where C= Capacitance of the capacitor

CAPACITANCE Charge storing capability of a capacitor is called capacitance of capacitor. Definition: Capacitance of a capacitor is defined as the ratio of the charge stored on any of the plates of capacitor to the potential between the plates.

COMBINATION OF RESISTORS Resistance can be joined to each other by two ways: 1. Series combination 2. Parallel combination SERIES COMBINATION Characteristics: 1. If different resistances are joined with each other such that there is only one path for the flow of electric current then the combination of such resistances is called Series Combination. 2. In series combination current through each resistor is constant. 3. In series combination Potential difference across each resistor is different depending upon the value of resistance. 4. Equivalent resistance of circuit is equal to the sum of individual resistances.

Series combination Re = R 1 + R 2 + R 3 + R 4 + …………….. R n DISADVANTAGE If one component is fused, then the other components of circuit will not function.

EQUIVALENT RESISTANCE IN SERIES COMBINATION Consider three resistances R 1, R 2, & R 3 connected in series combination with a power supply of voltage. Potential difference of each resistor is V 1, V 2, & V 3 respectively. Let electric current I is passing through the circuit. Now V = V 1 + V 2 + V 3 According to Ohm’s law V = IR thus IRe = IR 1 + IR 2 + IR 3 IRe = I(R 1 + R 2 + R 3 ) IRe/I = R 1 + R 2 + R 3 Re = R 1 + R 2 + R 3 This shows that in series combination equivalent resistance of circuit is always greater than individual resistances.

PARALLEL COMBINATION Characteristics: 1. If there are more than one path for the flow of current in a circuit then the combination of resistances is called Parallel Combination. 2. In parallel combination current through each resistor is different. 3. Potential difference across each resistor is constant. 4. Equivalent resistance of circuit is always less than either of the resistances included in the circuit.

Parallel Combination ADVANTAGE In parallel combination of resistors, if one component of circuit (resistor) is damaged then rest of the component of the circuit will perform their work without any disturbance. It is due to the presence of more than paths for the flow of electric current.

EQUIVALENT RESISTANCE IN PARALLEL COMBINATION Consider three resistances R 1, R 2 & R 3 connected in parallel combination with a power supply of voltage V. Now I = I 1 + I 2 + I 3 according to Ohm’s law V/R = I Therefore, V/Re = V/R 1 + V/R 2 + V/R 3 V/Re = V(1/R 1 + 1/R 2 + 1/R 3 ) V/ReV = 1/R 1 + 1/R 2 + 1/R 3 OR

JOULE'S LAW INTRODUCTION: When an electric current passes through a wire heat energy is produced. It is due to the collision of electrons with the atoms. In order to continue steady current, work has to be done on electric charges. STATEMENT: Amount of work done on electric charge on steady current is directly proportional to amount of heat. Work directly proportional to Heat

PROOF Consider a conductor through which electric current q is passing in time t let the potential difference between two ends of wire is V. We know that v = W/q or W = q x V_(i) According to Ohm’s law V = IR putting the value of V in equation (i) W = q x IR_______(ii) But I = q/t Or Q = It putting the value of q in equation (ii) W = It. IR W = I 2 Rt

OHM'S LAW INTRODUCTION Ohm’s law is a quantitative relation b/w potential difference and electric current. STATEMENT According to Ohm’s law, "The electric current passes through a conductor is directly proportional to the potential differences between the ends of conductor, if physical conditions of conductor remain constant." i.e. I directly proportional to V I = kV K =constant and is called "conductivity of material" I/K = V or V = I/K or V = I x 1/K so, Let [1/K = resistance] V = I x R

GRAPHICAL REPRESENTATION