Basic definitions and circuit laws

1. BASIC DEFINITIONS & LAWS
Syed Hasan Saeed
1
Syed Hasan Saeed, Integral University,
Lucknow

2. CURRENT: The movement of electric charge along a definite path is
known as current.
S.I. Unit of current is Ampere (A)
VOLT: If 1 joule of work is required to put 1 coulomb of charge on a
charged body, the body is said to have a potential of 1 VOLT. Consider
the following diagram
Syed Hasan Saeed, Integral University,
Lucknow
2
BASIC DEFINITIONS & LAWS
5 Volts
1 Volts
1 coulomb charge
A
B
Work done required to raise 1 coulomb
charge up to point A =1 joule
Potential of A =1 Volt
Work done required to raise 1 coulomb
charge up to point B = 5 joule
Potential of B = 5 Volt

3. BASIC DEFINITIONS & LAWS
Let,
W = Energy required to move positive charge from one point to other
(joules)
Q = Positive charge (coulomb)
Then V= W/Q J/C
V is known as the voltage or potential and its unit is volt.
POTENTIAL DIFFERENCE:
Electrical energy or current flows from higher potential to lower potential.
Therefore for flow of current a potential difference is necessary.
This can be easily understand by the diagram of two water tanks.
Fig. (a) : when two water tanks are not connected, then no water flows.
Fig. (b) : when two water tanks having water of same level and they are
placed at same level then no water flows from either tank to other.
Syed Hasan Saeed, Integral University,
Lucknow
3

4. Water tanks
Syed Hasan Saeed, Integral University,
Lucknow
4
BASIC DEFINITIONS & LAWS
Tank A Tank B
Fig. (a)
No water flows
Fig. (b)
No water flows
Tank A Tank B

5. BASIC DEFINITIONS & LAWS
Water tanks
Tank A is at higher level and tank B is at lower level, the water flows from
tank A to Tank B i.e from higher level to lower level. Similarly, if two
bodies are at different potentials, no current flows between them, but
when two bodies are connected by a wire, the current will flow and try to
equalize the potentials.
Syed Hasan Saeed, Integral University,
Lucknow
5
Tank A
Tank B

6. BASIC DEFINITIONS & LAWS
Syed Hasan Saeed, Integral University,
Lucknow
6
Potential
100 volts
Potential
50 volts
Body “A”
Body “B”
Potential
100 volts
Potential
50 volts
Current
Body “A”
Body “B”
Two bodies A and B
are at different
potentials but no
current will flow
Two bodies A and B
are at different
potentials current
will flow from Body A
to Body B

7. BASIC DEFINITIONS & LAWS
Therefore for flow of current from one body to other or from one point to
other in a circuit, not only must there be a potential difference but also an
electrical connection between them.
Therefore the potential difference (P.D) between two points is the energy
required to move one coulomb of charge from one to the other. Its unit is
volt.
Thus if two bodies have a P.D. of 50 Volts, 50 joule of work done will
require to move 1 coulomb from one to the other body.
VOLTAGE DROP: The voltage drop between two points of an element is a
decrease in energy in transferring a charge from one point to other point.
Syed Hasan Saeed, Integral University,
Lucknow
7

8. BASIC DEFINITIONS & LAWS
RESISTNCE: Resistance is property of material due which it opposes the
flow of current through it. It is denoted by R and unit is Ohm (Ω).
INDUCTANCE: Inductance is property of material due which it opposes
any change in current passing through it. It is denoted by L and unit is
Henry (H).
CAPACITANCE: Capacitance is property of an element due which it
opposes any change in voltage across it. It is denoted by C and unit is
Farad (F).
Syed Hasan Saeed, Integral University,
Lucknow
8
R
L
C

9. BASIC DEFINITIONS & LAWS
OHM’S LAW: Ohm’s law states that the current through any conductor is
directly proportional to the potential difference between its terminals. It is
assumed that the physical conditions of the conductor do not change.
I α V
voltage
I = (1/R) V
Or, V = IR
Where “R” is known as resistance of the conductor.
EXAMPLE: An Electric iron takes a current of 3.7 A from 230 V supply.
What is its resistance?
SOLUTION: According to Ohm’s Law
R =
Syed Hasan Saeed, Integral University,
Lucknow
9
Current
= Constant
V
I
= = 62.16 ohms230
3.7

10. BASIC DEFINITIONS & LAWS
POWER: Rate of doing work is known as power. Also defined as rate of
change of energy.
Also,
Power = rate of change of energy
But I = Q/t
Therefore, P = VI Also, I = V/R
Then
P = V2 / R Also. V = IR Then P = I2R
Syed Hasan Saeed, Integral University,
Lucknow
10
Power =
Energy
Time
Unit : J/s i.e Watt
Power =
Work
Time
=
W
t
=
VQ
t

11. hasansaeedcontrol@gmail.com
https://syedhasansaeed.gnomio.com/
https://shasansaeed.yolasite.com/
Syed Hasan Saeed, Integral University,
Lucknow
11