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1. ELECTRICITY
1. Electric Current
2. Conventional Current
3. Electric Circuit
4. Electric Potential and Potential Difference
5. Symbols used in Circuit Diagram
6. Ohm’s Law
7. Resistance and Resistivity
8. Conductance and Conductivity
9. Series and Parallel combination of resistors
10. Heating Effect of Current
11. Joule’s Law of Heating
12. Electric Power and Electric Energy
13. Electric Fuse

2. Electric Current
The electric current is defined as the charge flowing through any section
of the conductor in one second.
Different types of current:
I
t
0
a
b c
d) Alternating current whose
magnitude varies continuously
and direction changes
periodically
a) Steady current which does not
vary with time
b) & c) Varying current whose
magnitude varies with time d
(if the rate of flow of charge is steady)
I =
Q
t

3. SI unit of electric charge is coulomb (C) and SI unit of time is second (s).
SI unit of electric current is coulomb / second or C/s.
Or SI unit of electric current is ampere (A).
(From the formula)
(Named after the French
scientist, Andre-Marie Ampere
(1775 – 1836))
Note:
➢ Q = ne ( Q – charge, n – no. of electrons and e – electronic charge)
➢ Charge of one electron is 1.6 x 10-19 coulomb.
➢ 1 coulomb is equivalent to the charge of 6.25 x 1018 electrons.
➢ 1 coulomb, in fact, is very high charge.
➢ Therefore, charge is expressed in smaller units like milli coulomb (mc) or
micro coulomb (µC).
➢ Similarly current is expressed in milli ampere (mA) or micro ampere (µA).

4. Conventional Current
Conventional current is the current
whose direction is along the direction of
the motion of positive charge under the
action of electric field.
+
+
+
+
-
-
-
-
Conventional current due to motion of
electrons is in the direction opposite to
that of motion of electrons.
+ + +
I
+
+
+
+
-
-
-
-
I
- - -

5. Electric Circuit
A continuous and closed path of an electric current is called an electric
circuit.
A simple electric circuit consists of a cell (or battery), switch (or key) and
load (or resistance).
An ammeter (current measuring instrument) is always connected in series
in an electric circuit.
A voltmeter (potential measuring instrument) is always connected in
parallel (shunt) across the load (or resistance).
A Simple Electric Circuit
E
I I
An Electric Circuit with
Ammeter and Voltmeter
E
A
V
I
I
R
K K

6. Electric Potential
Electric potential is a physical quantity which determines the flow of charges
from one body to another.
It is a physical quantity that determines the degree of electrification of a body.
Electric Potential at a point in the electric field is defined as the work done in
moving (without any acceleration) a unit positive charge from infinity to that
point against the electrostatic force irrespective of the path followed.
SI unit of electric potential is volt (V) or J C-1 or Nm C-1.
(named after Alessandro Volta (1745-1827), an Italian Physicist)
Electric potential at a point is one volt if one joule of work is done in moving
one coulomb charge from infinity to that point in the electric field.
V =
W
Q

7. Electric Potential Difference between any two points in the electric field is
defined as the work done in moving (without any acceleration) a unit positive
charge from one point to the other against the electrostatic force irrespective
of the path followed.
Electric Potential Difference
1. Electric potential and potential difference are scalar quantities.
2. Electric potential at infinity is zero.
3. Electric potential near an isolated positive charge (q > 0) is positive and that
near an isolated negative charge (q < 0) is negative.
4. cgs unit of electric potential is stat volt. 1 stat volt = 1 erg / stat coulomb
WAB
Q
VB - VA = ∆V =

8. Ohm’s Law
The electric current flowing through a conductor is directly
proportional to the potential difference across the two ends of the
conductor when physical conditions such as temperature, mechanical
strain, etc. remain the same.
I
I α V or V α I or V = R I
V
I
0
E
Ohm’s Law Experiment
Graph is linear (straight line)
V
R
E
A
K
I
I

9. Resistance
The resistance of conductor is the opposition offered by the
conductor to the flow of electric current through it.
Resistance in terms of physical features of the conductor
A
R = ρ
l where ρ is proportionality constant called ‘ resistivity’
or ‘specific resistance’, l is the length of the conductor
and A is the area of cross section of the conductor.
Resistance is directly proportional to length and inversely proportional
to cross-sectional area of the conductor and depends on nature of
material.
Resistivity depends upon nature of material and not on the geometrical
dimensions of the conductor.
R =
V
I
Both the resistance and resistivity vary with temperature.

10. SI unit of resistivitiy is ‘ohm-m’ or Ωm.
SI unit of resistance is ‘ohm’ and its symbol is ‘Ω’.
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 of the conductor is
said to be 1 ohm.
Conductance and conductivity
Conductance is the reciprocal of resistance. Its S.I unit is mho.
Conductivity is the reciprocal of resistivity. Its S.I unit is mho / m.

11. Resistance of a system of resistors
Resistors are said to be
in series combination
when they are connected
end to end.
Resistors in series
A
- +
R1 R2 R3
V
I I
I
I
V1 V2 V3
In series combination,
1. Current through each
resistor is same.
V = V1 + V2 + V3
IR = IR1 + IR2 + IR3
Or R = R1 + R2 + R3
The Effective or Equivalent Resistance of the resistors in a series
combination is the sum of the resistances of the individual resistors.
K
2. Potential difference across resistors of unequal resistance is different.
However, the total p.d. is the sum of the p.d. across the individual resistors.
Note:
The Effective or Equivalent Resistance of the resistors in a series
combination is greater than the greatest of them.
I I

12. Resistors are said to be in
parallel combination when
they are connected
together between two
points.
Resistors in parallel
A
- +
V
I I
V
In parallel combination,
1. Potential difference
across each resistor is
same.
I = I1 + I2 + I3
The reciprocal of Effective or Equivalent
Resistance of the resistors in a parallel
combination is the sum of the reciprocals
of resistances of the individual resistors.
R1
R3
R2
I I
K
I3
I1
I2
=
V
R
+
V
R1
V
R3
+
V
R2
=
1
R
+
1
R1
1
R3
+
1
R2
Or
2. Current through resistors of unequal resistance is different. However, the
total current supplied by the battery is the sum of the currents through
the individual resistors.
Note:
The Effective or Equivalent Resistance of the
resistors in a parallel combination is smaller
than the smallest of them.
I I

13. Joule’s Law:
When an electric current is passed through a conducting wire, the electrical
energy is converted into heat energy. This effect is called ‘heating effect of
electric current’.
It is also called ‘Joule Heating’.
Joule’s Law gives Q = I2 R t Q is in Joule, I in Amp, R in ohm and t in sec.
I2 R t
J
Q = Q is in Calorie, I in Amp, R in ohm and t in sec.
Cause of Heating Effect of Current:
When a battery is connected to the ends of a conductor, electric field is set
up. The free electrons move in the direction opposite to the field. They
undergo collision with the positive ions and atoms of the conductor. The
average kinetic energy of vibration of the ions and the atoms increases and
hence the temperature increases.
Note: The course of electron through resistor is like that of the stone falling
in water.
HEATING EFFECT OF CURRENT

14. Electric Power
Electric power is the rate at which work is done by an electric current.
P = V I P = I2 R P = V2 / R
SI unit of power is ‘watt’.
Other units are ‘kW’ and ‘hp’.
1 kW = 1000 W 1 hp = 746 W
Electric Energy
Electric energy is the total work done by an electric current in a given time.
E = V I t E = I2 R t E = V2 t / R
Commercial Unit of Electric Energy is kWh or B.O.T.U.
1 kWh = 1000 w x 3600 s
= 3600000 w-s or joule
= 3.6 x 106 joule

15. Electric Heating Appliances use Nichrome wire (alloy of Ni and Cr).
It is used because:
i) It has high specific resistance ii) It has high melting point
iii) It has high malleability iv) It is not easily oxidised.
For given V, P α I P α 1 / R
i.e. i) Higher the power of the appliance, more is the current drawn
ii) Higher the power of the appliance, less is the resistance.
Electric Fuse
Electric fuse is a protective device used in series with an electric circuit or
an electric appliance to save it from damage due to overheating produced
by strong current in the circuit or appliance.
Fuse is generally made of alloy of 63% tin and 37% lead.
It has high resistance and low melting point.
More of Electricity in Higher Class…