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Electrical Circuits I
Emam Fathy
Department of Electrical and Control Engineering
email: [email protected]
http://www.aast.edu/cv.php?disp_unit=346&ser=68525
Lecture 1
Introduction; Circuit
Elements; Ohm's Law; KCL
Definition 1: An interconnection of electrical elements
linked together in a closed path so that an electric current
flow continuously
Battery Resistor
Wire
A Simple Circuit
Electric Circuit
Definition 2: A mathematical model that approximates the
behavior of an actual electrical system.
Definition 3: An interconnection between components or
electrical devices for the purpose of communicating or
transferring energy from one point to another. The
components of electric circuit are always referred to as
circuit elements.
Electric Circuit
Basic Electrical Quantities
• Basic quantities: current, voltage and
power
– Current: time rate of change of electric charge
I = dq/dt
1 Amp = 1 Coulomb/sec
– Voltage: electromotive force or potential, V
1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb
– Power: P = I V
1 Watt = 1 Volt·Amp = 1 Joule/sec
Current, I
• The movement of positive charges although we know that, in general, in metallic conductors current results from electron motion (conventionally positive flow)
• Types of current:– direct current (dc): batteries and some special
generators
– alternating current (ac): household current which varies with time
Direct current (DC) is a current
that remains constant with
time
Alternating current (AC) is a
current that varies sinusoidally
with time
i
t
Direct current
(DC)
Alternating current
(AC)
Current, I
Voltage, V
Voltage is the difference in energy level of a
unit charge located at each of two points in a
circuit, and therefore, represents the energy
required to move the unit charge from one
point to the other
Circuit Element(s)
+ –V(t)
Active vs. Passive Elements
• Active elements can generate energy
– Voltage and current sources
– Batteries
• Passive elements cannot generate energy
– Resistors
– Capacitors and Inductors (but CAN store
energy)
Independent Sources
An independent source (voltage or current)
may be DC (constant) or time-varying (AC),
but does not depend on other voltages or
currents in the circuit
+
–
Voltage
Source
Current
Source
40
Ideal voltage source connected
in series
Independent Sources
41
Ideal current source connected
in parallel
Independent Sources
xs Vi xs iV
39
DEPENDENT SOURCES
voltage current
Active element in which the source quantity is
controlled by another voltage or current
Resistors
• A resistor is a circuit element that
dissipates electrical energy (usually as
heat)
• Real-world devices that are modeled by
resistors: incandescent light bulbs, heating
elements (stoves, heaters, etc.), long
wires
• Resistance is measured in Ohms (Ω)
43
Ohm’s Law
The current in a circuit is directly
proportional to the applied voltage and
inversely proportional to the resistance of
the circuit.”
IRV
44
8 V
100
Determine the current in figure below :
AV
R
VI
IRV
08.0100
8
Example
Ohm’s Law
The current in a circuit is directly
proportional to the applied voltage and
inversely proportional to the resistance of
the circuit.”
IRV
RIP 2
From Ohm’s Law, we can get:
R
VP
2
and
28
Power is the rate of doing work, or the rate of
transfer energy
Measured in Watts (W)
1 hp = 746 watts
1 W = 1 J/s
Power, P
t
WP
Energy (J)
Time (s)
RIP 2R
VP
2
and
Power is the rate of using energy or
doing work.
POWER
Work (W)
consists of a force
moving through a
distance.
Energy (W)
is the capacity to
do work.
Joule (J)is the base unit for both energy and work.
32
Energy, W Work consists of a force moving through a distance
Energy is the capacity to do work.
Energy = Power × time
Units are joules = watt-seconds, watt-hours, or
more commonly, kilowatt-hours.
The electric power utility companies measure
energy in watt-hours (Wh).
PtW t
WP
Energy (J)
Time (s)
from equation
of ‘Power’
46
Short Circuit
Short circuit is a circuit element with resistance
approaching zero
VIIRV
AV
R
VI
0)0(
0
47
Open Circuit
Open circuit is a circuit element with resistance
approaching infinity
AV
R
VI 0
Series
Two elements are in series if the current that
flows through one must also flow through
the other.
R1 R2
Series
Not SeriesR1 R2
R3
Parallel
Two elements are in parallel if they are
connected between (share) the same two
(distinct) end nodes.
(R1&R2) Parallel (R1&R2) Not Parallel
R1
R2
R3R1
R2
Kirchhoff’s Laws
• Kirchhoff’s Current Law (KCL)
– sum of all currents entering a node is zero
– sum of currents entering node is equal to sum
of currents leaving node
• Kirchhoff’s Voltage Law (KVL)
– sum of voltages around any loop in a circuit is
zero
KCL (Kirchhoff’s Current Law)
The sum of currents entering the node is
zero:
i1(t)
i2(t) i4(t)
i5(t)
i3(t)
n
j
j ti1
0)(
KVL (Kirchhoff’s Voltage Law)
The sum of voltages around a loop is zero:
0)(1
n
j
j tv
v1(t)
++
–
–
v2(t)
v3(t)+
–
KVL (Kirchhoff’s Voltage Law)
The sum of voltages around a loop is zero:
0)(1
n
j
j tv
v1(t)
++
–
–
v2(t)
v3(t)+
–
Single Loop Circuit
• The same current flows through each element of the circuit—the elements are in series
• We will consider circuits consisting of voltage sources and resistors
+
–VS
R
R
R
I
Solve for I
• In terms of I,
what is the
voltage across
each resistor?
Make sure you
get the polarity
right!
• To solve for I,
apply KVL
around the loop
+
–VS
R
R
R
I + –
I R
+
–
I R
I R
+
–
N Total
Resistors
IR + IR + … + IR – VS = 0
I = VS / (N R)
In General: Single Loop
• The current i(t) is:
• This approach works for any single loop
circuit with voltage sources and resistors
• Resistors in series
sresistanceofsum
sourcesvoltageofsum
R
Vti
j
Si
)(
jNseries RRRRR 21
Voltage Divider
Consider two resistors in series with a
voltage v(t) across them:
R1
R2
–
v1(t)
+
+
–
v2(t)
+
–
v(t)21
11 )()(
RR
Rtvtv
21
22 )()(
RR
Rtvtv
In General: Voltage Division
• Consider N resistors in series:
• Source voltage(s) are divided between the
resistors in direct proportion to their
resistances
j
iSR
R
RtVtV
ki)()(
Example
Example
Applying the KVL equation for the circuit of the figure below.
va-v1-vb-v2-v3 = 0
V1 = IR1 v2 = IR2
v3 = IR3
va-vb = I(R1 + R2 + R3)
321 RRR
vvI ba
Current Divider
1R 2R
1I
V
2I
_
I
21
212211
RR
RRIRIRIV
Current Divider
1R 2R
1I
V
2I
_
I
IRR
RI
IRR
RI
21
12
21
21
2121
21
11
RRV
R
V
R
VIII
I R1 R2 V
+
–
I1 I2
21
21
21
11
1
RR
RRI
RR
IV
21
2
1
21
21
1
1RR
RI
R
RR
RRI
R
VI
Current Divider
Three Resistors in Parallel
I= I1 + I2 + I3
1
1R
VI
2
2R
VI
3
3R
VI
I R2 V
+
–
R1
I1 I2
R3
I3
Solve for V
321321
111
RRRV
R
V
R
V
R
VI
eqRI
RRR
IV
321
111
1
j
par
SRR
RII
j
1
1R
VI
2
2R
VI
3
3R
VI
I1 R1= I Req
Three Resistors in Parallel
I= I1 + I2 + I3
1
1R
VI
2
2R
VI
3
3R
VI
I R2 V
+
–
R1
I1 I2
R3
I3
j
par
SRR
RII
j
2121
21
11
RRV
R
V
R
VIII
I R1 R2 V
+
–
I1 I2
21
21
21
11
1
RR
RRI
RR
IV
21
2
1
21
21
1
1RR
RI
R
RR
RRI
R
VI
Current Divider
j
par
SRR
RII
j
Example
Is1 Is2 VR1 R2
+
–
I1 I2
Example
21212121
11
RRV
R
V
R
VIIII ss
21
2121
RR
RRIIV ss
Is1 Is2 VR1 R2
+
–
I1 I2
End of Lec