Electricity (Main)

What is charge?
Types of charges
What is electricity? Types of electricity
Difference between conductor and insulator.
What is Electric potential?
What is current?
How does current flow?
Ohm’s law
Resistivity-Factors Affecting Resistance.

Content of electricity is known as Charge.
• Charge is denoted by Q or q. It’s SI unit is coulomb
(C).
Properties of charges:
Opposite or unlike charges attract each
other.
Similar or like charges repel each other.

Charge
Positive Negative
Every Conductor has large no of
free electrons.
They are those electrons which

become free from outermost orbit
of atoms of a conductor.
These electrons are free to move within the conductor and responsible for
carrying current.
Which metal is better conductor or which material is an insulator depends

on availability of these free electrons.
No free electron- Insulators Very Less - Metalloids Very Large - Conductors
Those substances through which electricity can flow are called conductors.
Those substances through which electricity cannot flow are called insulators
https://www.youtube.com/watch?v=QJIJVB9o
FP8
Electricity is a controllable and convenient form of energy
caused by the flow of charges.
ELECTRIC CIRCUIT
A continuous and closed path of electric
current is called electric circuit
ELECTRIC CURRENT
Electric Current is defined as the
amount of charge flowing through a
particular area of conductor per unit
time The total no of free electrons present in any conductor
constitutes charge (Q) responsible for the flow of
current.
Q = no. of electrons(n) x charge on one electron (e)
OR
𝒆 = 𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗 𝑪
It is defined as the rate of flow of In one coulomb Q=1 C of charge there are
electric charge 𝒏 = 𝟔 × 𝟏𝟎𝟏𝟖 electrons (nearly)
Electric Current is defined as the amount of charge flowing through a
particular area of conductor per unit time
Current is denoted by I.
If a charge of Q coulomb flows through a conductor in time t
seconds, then the magnitude of the electric current is given by:
𝑄 SI unit of current is Ampere (A). Named after
𝐼=
𝑡 Andre-Marie Ampere a
1 AMPERE
If Q = 1 C t=1s then I = 1 A
When 1 Coulomb of charge flows through any cross-section of
a conductor in 1 second, the current flowing is said to be 1 Ampere.
Example 12.1
A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes.
Find the amount of electric charge that flows through the circuit.
1. What does an electric circuit mean?
2. Define the unit of current.
3. Calculate the number of electrons constituting one coulomb of charge.

What makes the electric charge to flow?
For flow of charges in a conducting metallic wire, the electrons move only if
there is a difference of electric pressure – called the potential difference – along
the conductor (Circuit).
This Potential Difference is produced by a battery or cell
When a cell is connected across

a conductor the charges start
moving and produce electric
current.
Electric current always flows from
the positive terminal of a battery
to the negative terminal.
This is called conventional

current and always taken in
the direction of movement of +
-
+ive Charges.
The electric potential difference between two points in an electric
circuit is defined as the work done in moving a unit charge from one
point to another point.
If W joules of work is done to move

Q coulomb of charge from one point
(A) to another (B), then the potential
difference ‘V’ is:
𝑊
𝑉=
𝑄
1 VOLT
The potential difference between 𝐼𝑓 𝑊 = 1 𝐽 ,

two points is said to be 1 volt if 1 𝑄 = 1 𝐶 𝑡ℎ𝑒𝑛
joule of work is done in moving 1
coulomb of charge from one point to 1𝐽
another point in a conductor. 1𝑉 =
1𝐶
Q. 1 An amount of 200 J of work is done in transferring a
charge of 10 C from one terminal of a battery to the other. Find
the potential difference across the terminals of the battery.
Will you be able to answer following Questions-

Current is measured by a device known
as ammeter.
The ammeter is connected in series with the circuit
in which the current is to be measured.
 An ammeter should have very low resistance so

that it may not change the value of the current
flowing in the circuit
The device used to measure the potential difference
is known as voltmeter.
The voltmeter is always connected in parallel

across the two points where the potential difference
is to be measured.
A voltmeter has a high resistance so that it takes

an negligible current from the circuit.
Ohm’s Law
At constant temperature, the current flowing through a conductor is
directly proportional to the potential difference across its ends.
VI (at constant temperature)
V=IR R is called Resistance of the

Conductor
https://www.youtube.com/watch?v=ldNPl67x-E8
𝑉 𝑈𝑛𝑖𝑡 𝑜𝑓 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 "𝑂ℎ𝑚“
Or R=
𝐼
𝑆𝑦𝑚𝑏𝑜𝑙 "Ω”
Ohms Law Circuit V-I Graph

Potential Difference V/S Current Graph-
 Slope Of V-I graph gives Resistance
B 𝐴𝐵 (𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟) 𝑉𝑜𝑙𝑡
𝑆𝑙𝑜𝑝𝑒 = = = 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑂𝐴(𝑏𝑎𝑠𝑒) 𝐴𝑚𝑝𝑒𝑟𝑒
A
O
Q.1 Graphs between electric current and potential difference

across two conductors A and B are plotted as shown in the
figure. Which of them has more resistance.
Q.2 Calculate the resistance of a conductor if the current flowing through

it is 0.2 A. When the applied potential difference is 0.8 V.
Q.3 When a 12 V battery is connected across an unknown resistor, 2.5mA

current flows in the circuit. Find the resistance of the resistor.
Factors on which the resistance of a conductor depends :-
The resistance of a conductor depends upon its:-
i) Length (l)
ii) Area of cross section(A)
iii) Material of the conductor.
iv) Temperature
Resistance is directly proportional to the length of the conductor and
inversely proportional to the area of cross section of the conductor.
𝑹 ∝ 𝒍
𝑹 ∝A
𝑙
𝑅∝
𝐴
𝒍
𝑹= 𝝆
𝑨
Where ρ (rho) is a constant of proportionality called Resistivity of

the material of the conductor.
The SI unit of resistivity is ohm meter ( Ωm).
If L = 1m , A = 1m2 then ρ=R
Resistivity of a material is the resistance offered by a piece made up of

that material of length 1 m and area of cross-section 1m2 .
PROBLEMS
1. A wire is 1m long, 0.2 mm diameter has resistance of 20 Ω.

Calculate the resistivity of material. (CBSE 2010)
2. A copper wire has a diameter of 0.5 mm and a resistivity of

1.6 × 10−6 Ω − 𝑐𝑚 of this wire would be necessary to make a resistance of
10 Ω.
Example 12.6
A 4 Ω resistance wire is doubled on it. Calculate the new resistance
of the wire.

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Electricity (Main)

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What is charge?

What is electricity? Types of electricity

Difference between conductor and insulator.

What is Electric potential?

How does current flow?

Resistivity-Factors Affecting Resistance.

Similar or like charges repel each other.

They are those electrons which

Which metal is better conductor or which material is an insulator depends

3. Calculate the number of electrons constituting one coulomb of charge.

This Potential Difference is produced by a battery or cell

When a cell is connected across

This is called conventional

If W joules of work is done to move

The potential difference between 𝐼𝑓 𝑊 = 1 𝐽 ,

Will you be able to answer following Questions-

 An ammeter should have very low resistance so

The voltmeter is always connected in parallel

A voltmeter has a high resistance so that it takes

VI (at constant temperature)

V=IR R is called Resistance of the

Ohms Law Circuit V-I Graph

 Slope Of V-I graph gives Resistance

Q.1 Graphs between electric current and potential difference

Q.2 Calculate the resistance of a conductor if the current flowing through

Q.3 When a 12 V battery is connected across an unknown resistor, 2.5mA

Where ρ (rho) is a constant of proportionality called Resistivity of

Resistivity of a material is the resistance offered by a piece made up of

1. A wire is 1m long, 0.2 mm diameter has resistance of 20 Ω.

2. A copper wire has a diameter of 0.5 mm and a resistivity of

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