Chapter 1
Basic Concepts
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Electric Circuits
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Chapter 1
1.1 Introduction
• This chapter introduces the concept of voltage and
current.
• The concept of a circuit will be introduced.
• Sources will be introduced.
• These can provide either a specified voltage or
current.
• Dependent and independent sources will be
discussed.
• Also a strategy for solving problems will be
introduced.
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What is a circuit?
• An electric circuit is an interconnection of electrical
elements.
• It may consist of only two elements or many more:
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1.2 System of Units
• We use the International Systems of Units (SI for short)
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1.3 Charge and Current
• Charge is an electrical property of the atomic
particles of which matter consists, measured in
Coulombs (C).
• Charge of single electron is 1.602 x 10-19 C.
• In 1 C of charge, there are 6.24 x 1018 electrons.
• The law of conservation of Charge
Charge can neither be created nor destroyed. Thus the
algebraic sum of the electric charges in a system does not
change.
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Current
• The movement of charge is called a current
• Historically the moving charges were thought to be
positive
• Thus we always note the direction of the equivalent
positive charges. That is, opposite to the flow of
negative charges. (By Benjamin Franklin)
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• Current, i, is measured as charge moved per unit
time through an element.
i
dq
dt
• 1 ampere = 1 coulomb/second
t
Q   idt
t0
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DC vs. AC
• A current that remains constant
with time is called Direct Current
(DC)
• Such current is represented by the
capital I, time varying current uses
the lowercase, i.
• A common source of DC is a battery.
• A current that varies sinusoidally
with time is called Alternating
Current (AC)
• Mains power is an example of AC
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Direction of current
• The sign of the current indicates the direction in
which the charge is moving with reference to the
direction of interest we define.
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1.4 Voltage
• Electrons move when there is a difference in charge
between two locations.
• This difference is expressed at the potential
difference, or voltage (V).
• It is always expressed with reference to two
locations
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• It is equal to the energy needed to move a unit
charge between the locations.
• Positive charge moving from a higher potential to a
lower yields energy.
v ab 
dw
dq
• 1 volt = 1 joule/coulomb
 Vab 
a
I ab
b
 Vab 
a
b
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Passive Sign Convention
 Positive voltage means : (ex) v=+5[V]
1) Voltage drop from terminal 1 to 2.
2) Voltage rise from terminal 2 to 1.
Fig. 1.5
 Positive current means : (ex) i=+2[A]
1) Positive charge flowing from terminal 1 to 2.
2) Negative charge flowing from terminal 2 to 1.
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1.5 Power and Energy
• Power is the time rate of expending or absorbing
energy, measured in Watts (W)
p
dw dw dq

 vi
dt
dq dt
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The algebraic sign of power
 if p > 0, power is being delivered to the circuit inside the
box. (Power absorbed)
 if p < 0, power is being extracted from the circuit inside
the box. (Power supplied)
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Conservation of Energy
• In a circuit, energy cannot be created or
destroyed.
• Thus power also must be conserved
• The sum of all power supplied must be absorbed
by the other elements.
• +Power absorbed = - Power supplied
p  0
Energy
t
t
t0
t0
W   pdt   vidt
• measured in joules (J)
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p(mW)
300
0
2
4
t(s)
-300
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Example : Find the power associated with each component.
a
a
2A
b
b
Example : Find V1 and I.
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Example : Find the power associated with each component.
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Example : Find the power associated with each component and the Vo
using the power .
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1.6 Circuit Elements
• Two types:
– Active
– Passive
• Active elements can generate energy
– Generators
– Batteries
– Operational Amplifiers
• Passives absorb energy
– Resistors
– Capacitors
– Inductors
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• Ideal Voltage Source
a circuit element that maintains a prescribed voltage
across its terminals regardless of the current
flowing in those terminals.
• Ideal Current Source
a circuit element that maintains a prescribed current
through its terminals regardless of the voltage
across in those terminals.
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Independent & Dependent Sources
• Independent Source:
The voltage and current sources are not influenced
by any other current or voltage in the circuit.
• Dependent Source :
It is determined by some other current or voltage in
the circuit.
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ideal independent
voltage source
Chapter 2
ideal independent
cuurent source
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(a) Ideal dependent voltatgecontrolled voltage source
(b) Ideal dependent currentcontrolled voltage source
(c) Ideal dependent voltatgecontrolled current source
(d) Ideal dependent currentcontrolled current source
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1.8 Problem Solving
1. Carefully define the problem.
2. Present everything you know about the problem.
3. Establish a set of alternative solutions and
determine the one that promises the greatest
likelihood of success.
4. Attempt a problem solution.
5. Evaluate the solution and check for accuracy.
6. Has the problem been solved satisfactorily? If so,
present the solution; if not, then return to step 3 and
continue through the process again.
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