Electricity class 10 notes chapter 12

Electric Charge

There are two type of electric charge
i) Positive charge
ii) Negetive charge

Opposite charge attract each other

Unlike charge repel each other

SI unit of electric charge(Q): The SI unit of electric charge is coulomb(C).
An electron possessse a negetive charge of 1.6 x 10^-19C
A Coulomb is equal to the charge contained in 6.25 x 10¹⁸ electron

Conductor: The Substance through which electricity can flow are called conductor

Insulator: The Substance through which electricity can not flow are called Insulator

Electric Current: The flow of electric charge in a conductor in time t is called electric current
Current(I)=
SI UNIT: the SI unit of current is Ampere (A)
Define 1A : When 1 coulomb of charge flows through any cross-section of a conductor in 1 second the current flowing through it is said to be 1 ampere

Ammeter: An instrument called ammeter measures electric current in a circuit

Direction of electric current:
Conventionally, in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons, which are negative charges.

Electric circuit:


A continuous and closed path consisting of wire bulb and switch between the two terminal of a cell along which an electric current flow called an electric circuit

ELECTRIC POTENTIAL:
An electric potential is the amount of work needed to move a unit of electric charge from a point to a specific point in an electric field without producing an acceleration.

Potential Difference:
The amount of work require for moiving a unit charge particle from one point to another point in electric circuit is called Potential Difference

Potential difference (V) between two points
V=

SI unit: The SI unit of electric potential difference is volt (V)

Define 1 volt: One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to the other

1 Volt =
1 V = 1 J C^–1

Voltmeter: The potential difference is measured by means of an instrument called the voltmeter

CIRCUIT DIAGRAM:
It is often convenient to draw a schematic diagram, in which different components of the circuit are represented by the symbols conveniently used.

OHM’S LAW:
It's stated that the electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm’s law

V ∝ I
or V/I = constant
= R
or V = IR Where R is a constant for the given metallic wire at a given temperature and is called its resistance.

Resistance: It is the property of a conductor to resist the flow of charges through it

SI Unit of Resistance: Its SI unit is ohm(Ω)
, R = V/I

Define 1 ohm: If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω

1 ohm =

1 volt / 1 ampere

here I=

V / R

Note the current through a resistor is inversely proportional to its resistance. If the resistance is doubled the current gets halved.

variable resistance: A component used to regulate current without changing the voltage source is called variable resistance

Factor effecting the resistance of a conductor:
The resistance of the conductor depends on following:
(i) on its length
(ii) on its area of cross-section,
(iii) on the nature of its material
a) Resistance of a given conductor is directly proportional to its lenght

R ∝ l
b) Resistance of a given conductor is inversely proportional to the area of cross-section (A)
R ∝

1 / A

Combining above we ge
R ∝

l / A

R = ρ

l / A

where ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor

Resistivity: A measure of the resisting power of a specified material to the flow of an electric current.

Resistivity ρ=

R x A / l

SI Unit of Resistivity: The SI unit of resistivity is Ω m.

Combination of resistances:
There are two type of combination of resistances
1) Series:
ii) Parallel:

1) Resistors in Series:

The number of resistances are joined end to end. Here the resistors are said to be connected in series.

As show in fig Three resistance R₁,R₂,R₃ With potential difference across resistance is V₁,V₂,V₃ respectily connecting in series with battery of V volts has been applied to the end of this series combination
Now
That is the total potential difference across a combination of resistors in series is equal to the sum of potential difference across the individual resistors
V = V₁ + V₂ + V₃ ..............(1)
According to ohm's law

R=

V / I

Or, V =I x R .................(2)
So ,
V₁ = I x R₁ ....................(3)
V₂ = I x R₂....................(4)
V₃ = I x R₃....................(5)
On putting equation 2,3,4 and 5 in equation 1
I x R =I x R₁ + I x R₂ + R₃
= I x ( R₁ + R₂ + R₃)
We get
R= R₁ + R₂ + R₃
the resistance of the combination R equals the sum of their individual resistances R₁,R₂,R₃

Resistors in Parallel:

As show in fig Three resistance R₁,R₂,R₃ are connecting parallel to another between the same two point A and B battary of V volt applied across the end of this combination.
the total current I, is equal to the sum of the separate currents through each branch of the combination. I = I₁ + I₂ + I₃
Let R be the equivalent resistance of the parallel combination of resistors.
By applying Ohm’s law

I= ----------------------1
I₁ = , I₂ = , I₃ = ------------------------2
On putting Equation 2 in 1
= + +
we get
= + +

Note the reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances.

1) Disadvantage of a series :
i) when one electrical appliance fails the circuit is broken and none of the electrical appliance works
ii) In series circuit all electric appliance connected only one switch So we can not turn of separately.
iii) Do not get eqaul volt of each electric appliance

Advantages of a parallel :
i) a parallel circuit divides the current through the electrical gadgets
ii)Each electric appliance get eqaul volt

HEATING EFFECT OF ELECTRIC CURRENT :

When an electric current passed through a high resistance wire its becomes hot and produce heat. this Known as Heating effect of current

. The work done in moving the charge Q through a potential difference V is VQ. Therefore, the source must supply energy equal to VQ in time t.

SO , W = V x Q -----1
here Q = I x t ---------(2)
from Ohm's law

R =
So, V= R x I -------------(3)

From equation 2 and 3 in 1 ,we get

W = I x t x I x R
So, Work done W = I² x R x t
for a steady current I, the amount of heat H produced in time t is
H =I² x R x t This is known as Joule’s law of heating.

The law implies that heat produced in a resistor is
i)) directly proportional to the square of current for a given resistance
(ii) directly proportional to resistance for a given current
(iii) directly proportional to the time for which the current flows through the resistor

Applications of Heating Effect of Electric Current :
i)The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater are some of the familiar devices
ii)The electric heating is also used to produce light, as in an electric bulb
iii) The heating effect of current is utilised in electric fuse for protecting household wiring and electric appliance

tungsten is used for making bulb filaments because it has high melting point

ELECTRIC POWER :
the rate of doing work is power This is also the rate of consumption of energy.

Power =
or,
P =
Or, P = I²R =

SI UNIT ELECTRIC POWER : The SI unit of electric power is watt (W).

Define one watt: The power of 1 watt is a rate of working of 1 joule per second

Watt =

The commercial unit of electric energy: The commercial unit of electric energy is kilowatt hour (kW h), commonly known as ‘unit’.

1 kW h = 1000 watt × 3600 second
= 3.6 × 106 watt second
= 3.6 × 106 joule (J)