ELECTRICITY
In this chapter, we will learn
• What is electric current and circuit
• Electric potential and Potential difference
• Circuit diagrams and symbols
• Ohm’s law
• Factors affecting resistance
• Series and Parallel connection
• Heating effect of electric current
• Electric Power
How does the bulb work when we switch on?
Electrons
Electric Current
The amount of charge flowing
through a particular area in unit time
The rate of flow of electric charges which
are moving in a specified direction
The rate of flow of electric charges which are
moving in a specified direction is called Electric
Current
If a net charge Q, flows across any cross-section of a conductor in time t, then
the current I through cross-section is
𝑸
𝑰=
𝒕
S.I unit of electric current is ampere or
Note:
𝟏𝑪
Charge of an electron = 𝟏 𝑨=
𝟏𝒔
Electric Potential
The work done to bring an electron into an
electric field is called Electric Potential
The work done to move a unit charge
from one point to the other is called
Electric Potential Difference
𝒘𝒐𝒓𝒌 𝒅𝒐𝒏𝒆(𝑾 )
𝑷𝒐𝒕𝒆𝒏𝒕𝒊𝒂𝒍 𝒅𝒊𝒇𝒇𝒆𝒓𝒆𝒏𝒄𝒆 (𝑽 ) =
𝒄𝒉𝒂𝒓𝒈𝒆(𝑸)
1 𝑗𝑜𝑢𝑙𝑒
𝑆 . 𝐼 𝑢𝑛𝑖𝑡 ⇒ 1 𝑣𝑜𝑙𝑡 =
1 𝑐𝑜𝑢𝑙𝑜𝑚𝑏
In a circuit, The ammeter should be connected in series and
the Voltmeter should be connected in parallel
Numerical
A current of 0.5A is drawn by a filament of an electric bulb for 10
minutes. Find the amount of electric charge that flows through the
circuit.
Find the current drawn by a bulb for 2 minutes if the amount of electric
charge is 1200C
How much work is done in moving a charge of 2C across two points
having a potential difference of 12V?
Circuit diagram
The schematics diagram
of a circuit represented
with symbols
Brainstormer
Draw a circuit diagram having
3- cell battery, a closed switch
and a bulb where the ammeter
and voltmeter are connected to
the bulb.
Ohm’s Law
Georg Simon Ohm found the relationship
between current(I) flowing in a metallic wire
and potential differences(V) across its
terminals
Current Potential
Number through the difference
Sno of cells nichrome wire across the
used (I) nichrome
(ampere) (volt)
1 1
2 2
3 3
4 4
Ohm’s Law Statement
At constant temperature, the potential difference is
directly proportional to the current
If the potential difference across the two 𝑉
ends of a conductor is 1V and the 𝑅=
current through it is 1A, then the 𝐼
resistance R, of a conductor is
1𝑉
1 Ω=
1𝐴
Resistance(R) is the property which opposes the flow
of current in a circuit.
𝑉
I=
𝑅
Current through a resistor is inversely proportional to its resistance.
If the resistance is doubled the current gets halved.
Note:
• Ohm’s law applies only to conductors or metals but does not apply to
semiconductors.
• Resistors are the devices which make oppose the flow of electrons.
• Rheostat is the component used for varying the resistance.
Factors on which the Resistance of a conductor depends
We use the following materials
i. Nichrome wire with a length (l)
ii. Nichrome wire with length (2l)
iii. Nichrome wire with a double thickness
of the same length (l)
iv. Copper wire with the (i) parameters
We check the current readings for all the
above parameters and calculate the
resistance
From the experiment, the observations are as follows
1. Length is directly proportional to the resistance of the
wire
2. The area of the cross-section is inversely
proportional to the resistance of the wire
Note:
Therefore 𝒍
𝑹=𝝆
𝑨 Resistance of the wire depends
on
𝑹𝑨 i. Length and area of cross-
R – Resistance
𝝆= A – area of cross section section of wire
𝒍 l – length of the wire ii. Nature of the material
iii. Temperature of the material
More length – More Resistance More Area – Less Resistance
Less length – Less Resistance Less Area – More Resistance
𝑹𝑨 R – Resistance
𝝆=
𝒍 A – area of cross section
l – length of the wire
S.I unit of resistivity is
Note:
• Resistivity or specific resistance depends on the nature of the material
and temperature.
• Metals and alloys have very low resistivity in the range of
• Insulators like rubber and glass have very high resistivity in the range
of
Brainstormer
• Generally, we use alloys like nichrome as heating elements in
electric heaters. toasters and electric iron. Why?
• Why do we use Tungsten filament in the electric bulb?
• Silver is more efficient and has less resistivity than copper.
But why are we using copper in electric wire and aluminium
for electrical transmission lines?
Numerical
1. How much current will be an electric bulb draw from a 220V source, if
the resistance of the bulb filament is 1200 ?
2. How much current will an electric heater coil draw from a 220V
source, if the resistance of the heater coil is 100
3. A wire of given material having length l and cross-sections section A
has a resistance of 4 What would be the resistance of another wire of the
same material having length and an area of cross-section 2A?
Resistance of a system of resistors
Series connection of resistors
In a circuit, if all the resistors are
connected one after the other, then its
connection is said to be in a Series
connection
In a series connection, the current is same in all the resistors, but the voltage differs.
So total voltage in the circuit
⇒ 𝐼𝑅 =𝐼𝑅 1+ 𝐼𝑅 2+ 𝐼𝑅3
⇒ 𝑹= 𝑹𝟏 + 𝑹 𝟐+ 𝑹𝟑
⇒ 𝑹 𝒔 =𝑹 𝟏+ 𝑹𝟐 + 𝑹𝟑
Note:
Advantage of series connection
• Cells connected in series give a greater resultant voltage than
individual cells.
• Voltage increases if the number of cells increases.
• Series circuits do not overheat easily.
Disadvantage of series connection
• If one component in a series circuit fails, then all the components in the
circuit fail because the circuit has been broken.
• The more components there are in a series circuit, the greater the
circuit’s resistance.
Parallel connection of resistors
In a circuit, if all the ends of the resistors are
connected to one point, then the connection
is said to be in a Parallel connection.
In a parallel connection, the voltage is same in all the resistors, but the current differs.
So total current in the circuit
𝑉 𝑉 𝑉 𝑉
⇒ = + +
𝑅 𝑅1 𝑅 2 𝑅3
𝟏 𝟏 𝟏 𝟏
⇒ = + +
𝑹 𝑹𝟏 𝑹𝟐 𝑹𝟑
𝟏 𝟏 𝟏 𝟏
⇒ = + +
𝑹 𝒑 𝑹𝟏 𝑹𝟐 𝑹 𝟑
Advantages of parallel connection
• Every unit that is connected in a parallel circuit gets an equal amount
of voltage.
• It becomes easy to connect or disconnect a new element without
affecting the working of other elements.
• If any fault happens to the circuit, then also the current can pass
through the circuit through different paths.
Disadvantages of parallel connection
• It requires the use of a lot of wires.
• We cannot increase or multiply the voltage in a parallel circuit.
Parallel connection fails at the time when it is required to pass exactly
the same amount of current through the unit
𝑺𝒆𝒓𝒊𝒆𝒔 𝒄𝒐𝒏𝒏𝒆𝒄𝒕𝒊𝒐𝒏⇒ 𝑹 𝒔 =𝑹 𝟏+ 𝑹𝟐 +𝑹𝟑
𝟏 𝟏 𝟏 𝟏
𝑷𝒂𝒓𝒂𝒍𝒍𝒆𝒍 𝒄𝒐𝒏𝒏𝒆𝒄𝒕𝒊𝒐𝒏 ⇒ = + +
𝑹 𝒑 𝑹𝟏 𝑹 𝟐 𝑹𝟑
Brainstormer
Are the car headlights
connected in series or
How are these
parallel
lights
connected?
Give some applications of Series and
parallel combinations in our daily life
Numerical
1. Find the current in the given circuit
2. Find the voltage in the given circuit.
Numerical
3. Observe the given circuit and
answer the following
i. Identify the type of circuit given
ii. Calculate the total voltage in the
circuit
iii. Calculate the current passing in
each resistor.
Electric Power
The rate of doing work is called Power.
The rate at which electric energy is dissipated or consumed in an
electric circuit is termed Electric Power(P)
The S.I unit of electric power is watt(W).
1 W is the power consumed by a device that
carries 1A current when operated at a potential
difference of 1V.
Note:
• The unit watt is very small. Therefore, in actual
practice, we use a much larger unit called ‘kilowatt’
• Electric energy is also measured as KWH (kilowatt
hour) – [Energy = power x time]
Heating Effect of Electric Current
The electric energy supplied to the device converts into heat energy. This
effect is called the heating effect of electric current.
This is known as Joule’s heating effect.
The law implies that heat produced in a resistor is
(i) Directly proportional to the square of the current
(ii) Directly proportional to the resistance of the current
(iii)Directly proportional to the time for which current flow through the resistor.
Applications of Heating Effect of Electric Current
1. Tungsten is used for making bulb filament because
Tungsten has high melting point and low resistivity
2. The fuse (connected in series) which we use in our home is used to
protect from short circuits and overloading. A thin wire of less
resistivity (for example Al, Cu, Fe, or Pb) is used as fuse wire.
Whenever it is overloaded, the fuse wire melts and breaks the circuit.
3. Nichrome wire (an alloy of Ni, Cr, Mn, and Fe) is used as a heating filament
in electric heaters, electric irons, and toasters. It has high melting point (2210
and high resistivity compare to the metals.
Numerical
1. 100J of heat is produced each second in a 4 resistance. Find the
potential difference across the resistor
2. An electric iron of resistance 20 takes a current of 5A. Calculate the heat
developed in 30s.
3. An electric refrigerator rated 400W operates 8 hours/day. What is the cost of
the energy to operate it for 30 days at Rs. 3.00 per kWh?
THANK YOU
