UNIT I
DC CIRCUITS
Syllabus :
Electrical circuit elements (R, L and C), voltage and current sources, Kirchoff current and
voltage laws, analysis of simple circuits with dc excitation. Superposition, Thevenin and
Norton Theorems.
Notes :
BASIC DEFINITIONS
Electric Current
It is defined as rate of flow of electric charge. The unit of current is ampere.
Fig. 1.2 Movement of free electron in a conducting material
before applying electric potential
, Fig. 1.3 Movement of free electron in a conducting material
after applying electric potential
Usually the flow of direct current is assumed to be from positive terminal (Anode)
to negative terminal (Cathode). This current is said to be conventional current flow. If the
direction of current is from cathode to anode, then it is called electron current.
Electric Potential
It is the work done in moving a charge of one coulomb between two points. Its unit
is volt.
Electric Resistance
The resistance of a circuit is the property by which it opposes the flow of current.
The unit of resistance is ohms (Ω).
Fig. 1.3 Collision of free electron with metal atoms
When an voltage is applied, the free electron moves and they collide with other
atoms. The collision between the free electron and the atoms is reflected as an
‘opposition’ to the flow of free electron. That means, this collision process ‘resists’ the
flow of free electron. Hence it is referred to as the resistance of the material.
Resistance in terms of physical quantities
The resistance of a conductor depends on
, a. its length (l)
b. Cross-sectional area (a)
c. material of the conductor
d. temperature
The resistance of a material is given by,
where,
ρ – Specific resistance or resistivity in ohm-m
Electrical Conductance
The reciprocal of resistance is called conductance. Its unit is mho ( )
Similarly the reciprocal of resistivity is called conductivity (σ ).
Its unit is Siemen / metre.
Power
It is defined as rate of doing work. Unit of power is Watts.
P = VI
where,
V – Potential difference in volts
I – Electric current in ampere
Energy
It is the amount of work done for ‘t’ seconds.
Energy = P x t
, = VI x t
Unit of energy is watt-hour.
Types of elements
There are 2 types of elements found in electrical circuits.
1) Active elements (is capable of generating energy), e.g., generators, batteries, and
operational amplifiers (Op-amp).
2) Passive element (is capable of absorbing energy), e.g., resistors, capacitors and
inductors.
OHM’S LAW
Ohms law, sometimes more correctly called Ohm’s Law, named after Mr. Georg
Ohm, mathematician and physicist born 1789 and died 1854 in Bavaria, defines the
relationship between power, voltage, current and resistance. These basic electrical units
apply to direct current, or alternating current. Ohm’s Law is the foundation of electronics
and electricity. These formulae are very easy to learn and are used extensively by
electricians. Without a thorough understanding of “Ohm’s Law” an electrician would
either design or troubleshoot even the simplest of electronic or electrical circuits.
When a voltage is applied to a closed circuit, it causes a flow of electrons and
consequently a current in the circuit. The resistance of the circuit opposes this flow. The
actual value of the current flowing depends on the applied emf and resistance in the
circuit, the relationship being given by Ohm’s law.
Statement
When the temperature remains constant, current flowing through a circuit is directly
proportional to potential difference across the conductor.
Mathematically, we may write
E ∞ I (or) V∞I
Therefore, V = Constant x I
This constant is the resistance (R) of the material. Hence,
V= IR
Taking the above equation into account, the formula for power can be written as
DC CIRCUITS
Syllabus :
Electrical circuit elements (R, L and C), voltage and current sources, Kirchoff current and
voltage laws, analysis of simple circuits with dc excitation. Superposition, Thevenin and
Norton Theorems.
Notes :
BASIC DEFINITIONS
Electric Current
It is defined as rate of flow of electric charge. The unit of current is ampere.
Fig. 1.2 Movement of free electron in a conducting material
before applying electric potential
, Fig. 1.3 Movement of free electron in a conducting material
after applying electric potential
Usually the flow of direct current is assumed to be from positive terminal (Anode)
to negative terminal (Cathode). This current is said to be conventional current flow. If the
direction of current is from cathode to anode, then it is called electron current.
Electric Potential
It is the work done in moving a charge of one coulomb between two points. Its unit
is volt.
Electric Resistance
The resistance of a circuit is the property by which it opposes the flow of current.
The unit of resistance is ohms (Ω).
Fig. 1.3 Collision of free electron with metal atoms
When an voltage is applied, the free electron moves and they collide with other
atoms. The collision between the free electron and the atoms is reflected as an
‘opposition’ to the flow of free electron. That means, this collision process ‘resists’ the
flow of free electron. Hence it is referred to as the resistance of the material.
Resistance in terms of physical quantities
The resistance of a conductor depends on
, a. its length (l)
b. Cross-sectional area (a)
c. material of the conductor
d. temperature
The resistance of a material is given by,
where,
ρ – Specific resistance or resistivity in ohm-m
Electrical Conductance
The reciprocal of resistance is called conductance. Its unit is mho ( )
Similarly the reciprocal of resistivity is called conductivity (σ ).
Its unit is Siemen / metre.
Power
It is defined as rate of doing work. Unit of power is Watts.
P = VI
where,
V – Potential difference in volts
I – Electric current in ampere
Energy
It is the amount of work done for ‘t’ seconds.
Energy = P x t
, = VI x t
Unit of energy is watt-hour.
Types of elements
There are 2 types of elements found in electrical circuits.
1) Active elements (is capable of generating energy), e.g., generators, batteries, and
operational amplifiers (Op-amp).
2) Passive element (is capable of absorbing energy), e.g., resistors, capacitors and
inductors.
OHM’S LAW
Ohms law, sometimes more correctly called Ohm’s Law, named after Mr. Georg
Ohm, mathematician and physicist born 1789 and died 1854 in Bavaria, defines the
relationship between power, voltage, current and resistance. These basic electrical units
apply to direct current, or alternating current. Ohm’s Law is the foundation of electronics
and electricity. These formulae are very easy to learn and are used extensively by
electricians. Without a thorough understanding of “Ohm’s Law” an electrician would
either design or troubleshoot even the simplest of electronic or electrical circuits.
When a voltage is applied to a closed circuit, it causes a flow of electrons and
consequently a current in the circuit. The resistance of the circuit opposes this flow. The
actual value of the current flowing depends on the applied emf and resistance in the
circuit, the relationship being given by Ohm’s law.
Statement
When the temperature remains constant, current flowing through a circuit is directly
proportional to potential difference across the conductor.
Mathematically, we may write
E ∞ I (or) V∞I
Therefore, V = Constant x I
This constant is the resistance (R) of the material. Hence,
V= IR
Taking the above equation into account, the formula for power can be written as
