Chapter

3
Current Electricity
Electric current

Electric current is defined as the charge flowing through any section of the conductor
in one second.
Note:
Current flows from higher potential (+ve) to lower potential (

ve).
Electro Motive force [emf]

It is the amount of work required to move a unit
–
ve charge once around
the complete electric circuit or the amount of energy supplied by the source
to
move a unit +ve charge
once around the circuit.
Ohm’s Law

The electric cu
rrent passing through
a
conductor is directly proportional to the potential
difference applied across it.
Resistance
–
The constant R in the above equation is called
resistance
and
is the opposition offered by
the
conductor to the flow of electric current through it.
1.
It is directly proportional to length of the conductor
2.
It
is inversely proportional to the area of cross section.
3.
Directly proportional to the temperature
.
4.
It also depends on the material to the co
nductor.
Resistivity is equal to the resistance of the conductor of unit length and unit cross

sectional area.
Series Combination of
Resistors
–
Parallel
Combination of Resisitors

Current density
⃗
⃗
⃗
I
t is the amount of current flowing per second through unit area of the conductor.
Conductance:
It is the reciprocal of resistance
Conductivity:
It is the reciprocal of resistivity.
Terminal Voltage (V)

Terminal voltage is the potential difference between the two terminals of a cell
when
the cell is in closed circuit.
Internal Resistance

Internal resistance is the obstruction in the path of ions (current) through
an
electrolyte
of
a cell
.
It depends on the following factors:
1.
Concentration of the electrolyte.
2.
A
rea of the electrodes. It is inversely proportional to it.
3.
D
irectly proportional to the distance between the two electrodes.
4.
Q
uantity of current drawn from it.
Terminal voltage
is the
between two terminals when current drawn from the cell.
When the circuit is open, no current is drawn and so no internal re
sistance will be there.
When the circuit is closed, there is potential loss and internal resistance comes into force.
Potential drop is due to the work done against the internal resistance.
According to Ohm’s law,
(
)
(
)
Series Combination of
Cells
–
Parallel Combination of
Cells

Total internal resistance,
(
)
(
)
Superconductor and Superconductivity

When the temperature of
a
conductor decreases
,
resistance
also decreases. At a particular temperature called critical temperature or below it, the resistance of the
conductor disappears.
This conductor is called super
conductor and t
he phenomenon is known as
super
conductivity.
Advantages of Superconduc
tors

1.
P
ower can be transmitted without any loss.
2.
Very high
magnetic field can be produced.
Heating Effect of Current

Current flows through a conductor when potential difference is applied. When the
is applied across
a conductor
experience for
ce
Under the influence of force
get accelerated. During the
motion
collide will the ions of conductor. Thus, conductor gets heated.
Work done to move the charge through the conductor
Electric Power

It is the rate at which electrical energy is consumed by a device or amount of energy
used by a device in 1 second.
[
]
Kirchhoff’s Laws
Law 1

The algebraic sum of electric currents at a junction in any electrical network is always zero.
Law 2

The algebraic sum of all the pote
ntial drops and emf’s along any
closed path in an
electrical
network is always zero.
Wheatstone Bridge

Wheatstone bridge consists of 4 resistors arranged in the form of a
quadrilateral. It is used
to find the unknown resistence comparing with the known resistences.
𝑔
啮歮k睮
G
A
P
B
𝑔
Q
C
S
D
R
1.
2.
Potentiometer
(
to compare the emfs of 2 primary cells
)
Leclanche
cell &
D
aniel cell.
Principle of potentiometer

When steady current flows through potiometer wire of uniform cross

sectional area, potential difference across
portion of the wire is directly proportional to the length of the
portion
K is called potential gradient,
the potential difference per unit length.
Problems
1.
Find the current in each branch of the circuit sh
own below:
A

+
𝐸
G
R.B
B
B
𝑅
ℎ
uniform
wire
A
C
0
50kg
1
3
2
𝐸
(to protect the
galvanometer
B
A
1m
2.
Find the equivalent resista
nce and current.
3.
Resistance of a wire is 8Ω.
It is stretched 4 times of its length. Find the new resistance.
4.
Two cells
0
are
connecte
d
as shown in diagram. Find the
current in each resist
a
nce. Calculate current in each resist
a
nce?
0
Ω
0
Ω
0
Ω
0
Ω
0
Ω
0
𝑉
Ω
A
Ω
B
E
㕖
Ω
2
Ω
C
2
Ω
1
Ω
5.
A potentiometer wire of length 5m through steady current flows from a cell of 5V. Two primary
cells
are joined in series. The combination is connected to a galvanometer and jockey. A
balancing length with sam
e polarity
0
& opposite polarity is 50cm. Find
the value of emfs.
Ω
6
Ω
Ω
𝐸
𝐸
𝑉
0
Ω
8
𝑉
Ω
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