electricity

electric charges

electric charges

when an atom has an unequal amount of protons and electrons, it is said to have a charge

protons > electrons [positive]

electrons > protons [negative]

SI unit of electric charge

coulomb (C)

definition: one coulomb is that quantity of electric charge which exerts a force of 9 × 10^9N on an equal charge at a distance of 1 m from it

1 proton has positive charge of 1.6 × 10^-19C

1 electron has negative charge of 1.6 × 10^-19C

electric current

it is the flow of electric charge

explanation: when electrons are weakly bounded to the nucleus (free electrons), it breaks free and jumps from atom to atom. this results in the flow of electric charge. however, as electrons were not discovered at the time, we take electricity as the flow of positive charge, which is in the opposite direction

conductors

substances through which electricity can flow. this is because of the presence of free electrons

eg: all metals, graphite, human body

insulators

subtances through which electricity cannot flow. this is because of the absence of free electrons

eg: glass, ebonite, rubber, paper, wood, cotton, mica, porcelain, dry air

types of elecricity

there are two types of electricity

static electricity: the electric charges remain at rest (glass rod rubbed with silk cloth, lightning)

current electricity: the electric charges are in motion (electricity used in homes)

electric potential

the electrical potential of a point is defined as the work done to move a unit positive charge from infinity to that point

potential difference/ voltage

it is the difference in electrical potential between two points

SI unit: volt (V)

1V = W/Q (W = work done, Q = quantity of electric charge)

definition of 1V: the potential difference between two points is said to be 1V if 1J of work is done in moving 1 coulomb of charge from one point to the other

voltmeter

it is a device used to measure potential difference

it is connected in parallel across the two points where the p.d has to be measured

it has high resistance

cause for electric current

it is the potential difference between the ends of a wire which makes the electric charges flow in the wire (till they both acquire the same potential)

SI unit of electric current

ampere (A)

def: the magnitude of electric current in a conductor is the amount of electric charge passing through a given point of a conductor in a given amount of time

I = Q/t [where I = current, Q = quantity of electric charge, t = time]

1A = 1C/1s

ammeter

it is a device used to measure electric current

it is always connected in series with the circuit

it has very low resistance

simplest way to get a continuous flow of current

the simplest way to maintain a potential difference between the two ends of a conductor is to connect the two ends to the terminals of a battery/cell

due to the chemical reactions happening in a battery, a potential difference is maintained between its terminals

circuit

it is a continuous conducting path consisting of wires, resistances, and a switch, between the two terminals of a cell/battery along which an electric current flows

ohms law

at constant temperature, the current flowing through the conductor is directly proportional to the potential difference across its ends

I is directly proportional to V

or, V = R x I [where R, resistance, is a constant]

or, R = V/I

or, I = V/R

resistance

when electrons move through conductors, they collide with other electrons/atoms/ions, which results in some obstructions/oppositions to the flow of electric current. this is called resistance

resistance of a conductor depends on length, thickness, nature of material and temperature, etc.

SI unit of resistance is ohm

def of 1ohm: the resistance of a conductor is said to be 1ohm when a potential difference of 1V is applied to its ends and a current of 1A flows through it

effect of length of the conductor

the resistance of a conductor is directly proportional to its length

R is directly proportional to L

when length gets doubled, resistance is doubled, when length is halved, resistance is halved

effect of area of cross-section of the conductor

the resistance of a conductor is inversely proportional to its area of cross-section

R is directly proportional to 1/A

when the area is doubled, the resistance is halved, when the area is halved, the resistance is doubled

or,

we can also say that the resistance of a conductor is inversely proportional to the square of its diameter

R is inversely proportional to 1/D²

if the diameter is doubled, the resistance is ¼th, if the diameter is halved, the resistance is 4x

effect of temperature of the conductor

the resistance of all pure metals increases on raising the temperature and decreases on lowering the temperature

exceptions: alloys like manganin, constanan, nichrome

resistivity

also known as specific resistance, it is the resistance of a specific material

p(rho) = R x A/L (p = resistivity)

SI unit of resistivity is ohm meter

why alloys are used for electric heating appliances instead of pure metals

they have high resistivity

they do not undergo oxidation easily

eg: nichrome

resistors connected in series

the combined resistance of any number of resistances connected in series is equal to the sum of the individual resistances

R = R1 + R2 + R3……

the total potential difference across the ends of all the resistances in series is equal to the voltage of the battery

the same current flows through each resistance when connected in series

resistors connected in parallel

the reciprocal of the combined resistance of a number of resistances connected in parallel is equal to the sum of the reciprocals of all the individual resistances

1/R = 1/R1 + 1/R2 + 1/R3…….

the potential difference across each end is equal to the voltage of the battery

the sum of currents flowing through all of the resistances is equal to the total current flowing in the circuit

advantages of parallel circuits in domestic wiring

if one electrical appliance stops working due to some defect, the other appliances keep working normally

each electrical appliance has its own switch due to which it can be turned on or off independently without affecting the other appliances

each electrical appliance gets the same voltage (220V) as that of the power supply line

the overall resistance of the circuit is low, due to which the current from the power supply is high

electric power

it is the rate at which electrical power is consumed

P = W/t (P = power, W = work done, t = time taken in seconds)

or,

P = VI

or,

P = I²R

or,

P = V²/R

SI unit of power is watt (W)

heating effect of electric current

when electric current is passed through a high resistance wire, like nichrome, the wire becomes very hot and produces heat energy. this is due to the conversion of electrical energy into heat energy

H = I²Rt (joules law of heating)

applications of heating effect of current

it is used in the working of electrical heating appliances like iron, kettle, toaster etc.

it is used in electric bulbs for producing light (tungsten, a high resistance metal)

it is used in electric fuse for protecting household wiring and electrical appliances