circuits

1. Fundamentals
Of
Circuit Theory

2. ELECTRICAL CHARGE
 There are two types of charges:
positive and negative charge
 The Unit of Charge(Coulomb)
 Electrical charge (Q) is measured in
coulombs, abbreviated C.
 One coulomb is the total charge possessed by
6.25 x1018 electrons

3. VOLTAGE
A force of attraction exists between a positive and a
negative charge
The difference in potential energy of the charges is the
potential or voltage.
Voltage is the driving force in electric circuits and is
what establishes current.
The difference in potential energy in electrical terms is
called voltage
(V) and is expressed as energy or work (W) per unit
charge (Q).

4. The Unit of Voltage
The unit of voltage is the volt, symbolized by V.
One volt is the potential difference
(voltage) between two points when one
joule of energy is used to move one
coulomb of charge from one point to
the other.

5. SOURCES OF VOLTAGE
 A voltage source is a source of potential energy that is
also called electromotive
force (emf)
• Batteries : converts chemical energy into
electrical energy.
 primary cells: cannot be recharged
 secondary cells: can be charged

6. • Solar cells : light energy is converted directly
into electrical energy.
• Generator: Electrical generators convert
mechanical energy into electrical energy.
• Electronic Power Supply: Electronic power
supplies convert the ac voltage from the wall
outlet to a constant (dc) voltage

7. CURRENT
• movement of electrons is the electrical
current, symbolized by I.
• Electrical current is the rate of flow of
charges
• The Unit of current : Current is measure in a
unit is called the ampere or amp for short,
symbolized by A.

8. • One ampere (1 A) is the amount of
current that exists when a number of
electrons having a total charge of one
coulomb (1 C) move through a
given cross-sectional area in one second
(1 s).
• Direct Current (DC): current that remains
constant with time.
• Alternating Current (AC): current that varies
sinusoidal with time.

9. RESISTANCE
• The property of a material that restricts the
flow of electrons is called resistance.
• Resistance is the opposition to current flow
• The Unit of Resistance
• The unit for resistance is Ohm (Ω)
• One ohm (1Ω ) of resistance exists if there is
one ampere (1 A ) of current in a material
when one volt (1 V ) is applied across the
material.

10. Conductance
• The reciprocal of resistance is conductance,
symbolized by G.
Unit of Conductance: The unit of conductance
is the siemens, abbreviated S. (mho)

11. Resistors
• Components that are specifically designed to
have a certain amount of resistance are
called resistors.
 applications of resistors:-
to limit current.
to divide voltage.
to generate heat.
Resistors can be fixed or variable

12. OHM’S LAW
• Ohm’s law describes mathematically how
voltage, current, and resistance in a circuit
are related
. Three equivalent forms of OHM’S LAW
Formula for Current
Formula for Voltage
Formula for Resistance

13. Larger Units of Resistance (kΩ and
MΩ)

14. Ohm's Law: Graph of Current versus Voltage

15. Ohm's Law :Current and Resistance are Inversely
Related

16. Ohm’s law relationships

17. ENERGY AND POWER
• Energy is the ability to do work
• Power is the rate at which energy is used.
• Power, symbolized by P, is a certain amount of
energy used in a certain length of time.
• The Kilowatt-hour (kWh) Unit of Energy

18. POWER IN AN ELECTRIC CIRCUIT
Three Formula
P = V I
P = I2R
• P =V 2lR

19. RESISTOR POWER RATINGS
• The power rating is the maximum amount of
power that a resistor can dissipate without
being damaged.

20. • Example 1-10
• Choose an adequate power rating for each of
the carbon-composition resistors in Figure 1-
12
(0.125 W, 0.25 W, 0.5 W, 1 W and 2 W).

21. Figure1-12

22. Solution
In Figure 1-12(a), the actual power is
Select a resistor with a power rating higher than the
actual power. In this case, at least a 2 W resistor
should be used.

23. In Figure 1-12(b), the actual power is
At least a (0.125 W) resistor should be used in
this case.

24. CIRCUIT ELEMENTS
• An electrical circuit is made of components
• such as voltage sources, resistors and
capacitors which are called circuit
elements.

25. BRANCHES, NODES AND LOOPS
• A branch represents a single element
such as a voltage source or a resistor.
• A node is the point of intersection
between two or more branches.
A loop is any closed path in a circuit.

27. • A branch represents any two-terminal
element
• The circuit has five branches, namely, the
10V voltage source, the 2A current source
and the three resistors.

28. A node, which is the point of
• intersection between two or
• more branches, is usually
indicated by a dot in a circuit.
The circuit has three nodes a, b
and c.

30. A loop is a closed path formed
by starting at node, passing
through a set of nodes, and
returning to the starting node
without passing through any node
more than once.
• For example the closed path abca
containing the 2kΩ resistor in Figure
above is a loop.