Electrical Engineering: Basic Concepts in Electric Circuits, Lecture notes of Circuit Theory

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Outline

• Introduction
• System Units
• Charge and Current
• Voltage
• Power and Energy
• Circuit Elements

A Simple Electric Circuit

Connecting wires electrical element (^) electrical element

A Complicated Circuit:

Radio Receiver

Units in Circuit

Term Unit Symbol Charge Coulomb C Current Ampere A Voltage Volt V Power Watt W Energy Joule J Resistance Ohm Ω Capacitance Farad F Inductance Henry H

Charge

• The concept of electric charge is the basis for

describing all electrical phenomena.

• Charge
  • An electrical property of the atomic particles of which matter consists, measured in coulombs (C).
• The charge e of an electron
  • sign: negative
  • magnitude: 1.602 × 10 - 19

C.

• In 1 C charge, there are 6.24 × 10 18

electrons.

Charge ( cont. )

• Law of conservation of charge
  • Charge can neither be created nor destroyed,

only transferred.

• The separation of charge creates an electric

force or voltage ( v ).

• The motion of charge creates an electric fluid

or current ( i ).

Current

Electric current due to flow of electronic charge in a conductor.

Current ( cont. )

• Although current is made up of discrete

moving electrons, we consider i to be a

continuous variable as there are so many of

them.

• Conventionally, the current is always assumed

to be the rate of flow of positive charge.

Current ( cont. )

• Direct current (dc)
  • A current that remains constant with time.
• Alternating current (ac)
  • A current that varies sinusoidally with time.

Example 1

Q: How much charge is represented by 4,

electrons?

Sol:

• Each electron has - 1.602× 10
  • 19 C.
• 4600 electrons have
  • 1.602× 10
    • 19 × 4600 = - 7.369× 10 - 16 C

Example 2

5 sin 4

(5 sin 4 )

(5sin 4 20 cos 4 )

At 0.5 ,

5sin 2 10 cos 2

C/s

A

mC

m

m

s

0 10 mA

q t t

d

t t

dt

t t t

t

i

dq

i

dt

Q: The total charge entering a terminal is

q ( t ) =5t∙ sin( 4 πt ) mC. Calculate the current at t=0.5 s.

Sol:

q ( t )

Voltage

• Voltage
  • The voltage between two points a and b in an electric circuit is the energy (work) required per unit of charge against a static electric field to move the charge from a to b , measured in volts (V). v = voltage in volts (V), w = energy in joules (J) q = charge in coulombs (C)
• Note: Net charge (sum of positive and negative charges) is always zero for any element in a circuit. ab dw v dq 

Voltage ( cont. )

• dc voltage
  • A voltage that remains constant with time

(as produced by a battery).

• ac voltage
  • A voltage that varies sinusoidally with time

(as produced by a generator).