Ohm’s Law: At constant temperature the current flowing through the conductor is directly

proportional to the potential difference across its ends

I ∝V

Or, V∝I

V= R I

Where

V = potential difference

I = current

R =Constant called Resistance

Resistance: The ratio of potential difference applied between the ends of a conductor and current
flowing through it.

Or

The ratio of potential difference to the current flowing through a wire is known as Resistance.

 It is represented by letter “R”.

 Its SI Unit is :
R= = = Ohm (Ω)

 1 Ω is defined as resistance of a conductor which allows 1 Ampere of current to flow

Conductors: Those substances which allow electricity to pass through them are called Conductors.

OR

Those substances which have excess of free electron in them are called Conductors.

Example: Gold, Silver, Copper, Aluminum, (Metal), etc.

Nichrome, Manganin, Constatan (Metal alloys), etc.

Human body, impure water, Graphite (Non-metal), etc.

Insulators: Those substances which do not allow electricity to pass through them are called
Insulators.

OR
Those substances which have very low or no free electron in them are called Insulators.

Example: Glass, ebonite, rubber, most plastic, paper, dry wood, cotton, Mica, dry air (Non-Metal),
etc.

Factors on which Resistance depend:

1. It depends on length of wire. Greater the length of wire higher resistance it provides.
2. It depends on thickness of wire. Greater the Cross-sectional area lower the resistance.
3. It depends on nature of material.
4. It depends on temperature.
Thus,

Resistivity: Resistivity is a property that describes the extent to which a material opposes the flow of
electric current through it. It is a property of the material itself (not the size or shape of the sample),
usually depends on temperature and may depend on other quantities such as pressure.

Superconductors: The materials exhibiting virtually zero resistance at very low temperature are
called Superconductors.

Example: Aluminium, niobium, magnesium diboride, cuprates such as yttrium barium copper oxide,
and iron pnictides .

Combination of Resistance: In order to control the current flow in the circuit the resistances are
combined in two ways:

1: Series

2: Parallel

Equivalence Resistance: A single resistance which can replace any combination of resistance so that
the current flow through the circuit remains same is called equivalence resistance.

Resistance in Series: When two or more resistances are connected end to end then are said to be
connected in series.
Law of Combination of Resistance in Series: Law of combination of resistance in series states that:
“When a number of resistances are connected in series there equivalence resistance is equal to sum
of individual resistance.”

R= R1 + R2 + R3 + …………….

Use Resistance in Series:

1. If we want to increase the total resistance then individual resistance is connected in series.

2. The Current flow ‘I’ remains same throughout the circuit.

Resistance in Parallel: When two or more resistances are connected between the same two points
then are said to be connected in parallel.

Law of Combination of Resistance in Parallel: Law of combination of resistance in parallel states

= + + + …………….

Use of Resistance in Parallel:

1. If we want to decrease the total resistance then individual resistance is connected in

2. The potential difference ‘V’ remains same throughout the circuit.

Domestic Electric Circuit: As per requirement Series or parallel both combinations are used in
households.

Disadvantage of Series Combination for Domestic Electric Circuit

1: If one appliance stops working, then others also stop.

2: Only 1 switch is available to control all the devices.

3: The overall resistance of the circuit increases too much due to which current flow from power
supply is low.

4: The appliances do not get same voltage. Due to which they do not work properly.

Advantage of Parallel Combination for Domestic Electric Circuit

2: Each appliance can have its own switch. Thus it is easy to control all the devices.

3: The overall resistance of the circuit is reduced due to which current flow from power supply is
high.

4: The appliance gets same voltage. Due to which they work properly.

Power: Electric power is the rate of transfer of electrical energy within a circuit.

• Its SI unit is the watt,

• The general unit of power, defined as one joule per second.

• P = W/T

Heating effect of Current

 The Electric Bulb: The filament bulb is such that the inner core filament (mostly
tungsten as it does not melt easily) is heated up to a certain temperature after which it
is illuminated.
 The Electric Iron: Bimetallic plates and applying the heating effect of current helps
the iron break the flow of electricity when a certain temperature is reached. You may
have noticed this when the red light on an iron box comes on and off.
 Electric Heater: Similar to that of an iron box but here, a circuit breaker is used
 The Electric Fuse: Copper or zinc can be used as the fuse element. When there is an
excess of current flow through the electrical circuit, the heat generated increases as
well and melts the fuse, thereby breaking the circuit.

Commercial Unit of Power:

1Kwh is known as 1 unit. 1 Unit is the commercial unit of Power in India.

In order to calculate the electricity bill the amount of 1 unit is multiplied with the amount of power
consumed in house hold.