Introduction; Circuit Elements; Ohm's Law; KCL Dr. Holbert January 14, 2008

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Introduction; Circuit
Elements; Ohm's Law; KCL
Dr. Holbert
January 14, 2008
Lect1
EEE 202
1
Introduction
• Syllabus: textbook, homework, grading,
schedule, reading assignments, laboratory
• Lectures: let’s avoid “Death by
PowerPoint”
Lect1
EEE 202
2
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=IV
1 Watt = 1 Volt·Amp = 1 Joule/sec
Lect1
EEE 202
3
Current, I
• Normally we talk about the movement of positive
charges although we know that, in general, in
metallic conductors current results from electron
motion (conventionally positive flow)
• The sign of the current indicates the direction of
flow
I(t)
• Types of current:
– direct current (dc): batteries and some special
generators
– alternating current (ac): household current which
varies with time
Lect1
EEE 202
4
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)
+
Lect1
V(t)
EEE 202
–
5
Default Sign Convention
• Passive sign convention : current should
enter the positive voltage terminal
I
+
Circuit Element
–
• Consequence for P = I V
– Positive (+) Power: element absorbs power
– Negative (-) Power: element supplies power
Lect1
EEE 202
6
Electrical Analogies (Physical)
Base
quantity
Flow
variable
Potential
variable
Lect1
Electric
Hydraulic
Charge (q)
Mass (m)
Current (I)
Fluid flow (G)
Voltage (V)
Pressure (p)
EEE 202
7
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)
Lect1
EEE 202
8
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
Lect1
EEE 202
Current
Source
9
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)
Lect1
EEE 202
10
Ohm’s Law
v(t) = i(t) R
- or p(t) = i2(t) R = v2(t)/R
V=IR
[+ (absorbing)]
i(t)
The
Rest of
the
Circuit
Lect1
+
R
v(t)
–
EEE 202
11
Open Circuit
• What if R =  ?
i(t)=0
+
The
Rest of
the
Circuit
v(t)
–
i(t)=0
• i(t) = v(t)/R = 0
Lect1
EEE 202
12
Short Circuit
• What if R = 0 ?
i(t)
The
Rest of
the
Circuit
+
v(t)=0
–
• v(t) = R i(t) = 0
Lect1
EEE 202
13
Series
Two elements are in series if the current that
flows through one must also flow through
the other.
Series
R1
R2
Not Series
R1
Lect1
R2
EEE 202
14
Parallel
Two elements are in parallel if they are
connected between (share) the same two
(distinct) end nodes.
R1
R1
Lect1
R2
R2
Parallel
Not Parallel
EEE 202
15
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
Lect1
EEE 202
16
KCL (Kirchhoff’s Current Law)
i1(t)
i5(t)
i2(t)
i4(t)
i3(t)
The sum of currents entering the node is
zero:
n
 i (t )  0
j 1
j
Analogy: mass flow at pipe junction
Lect1
EEE 202
17
Class Examples
• Drill Problems P1-1, P1-2, P1-4
– While working these problems, we shall
define the term ‘node’ as a point where two or
more circuit elements are connected
Lect1
EEE 202
18
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