Transcript Introduction Units (1.1) Basic Quantities (1.2) Circuit Elements (1.3)
Introduction
Units (1.1) Basic Quantities (1.2) Circuit Elements (1.3) Prof. Phillips January 22, 2003 ece201 lecture 1 1
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 Think “FLOW of charges (-e _ )” –
Voltage
: electromotive force or potential,
V
1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb Think “charge (-e _ ) PRESSURE” –
Power (dissipated)
:
P
=
I V
1 Watt = 1 Volt·Amp = 1 Joule/sec ece201 lecture 1 2
Current,
I
• Current is the movement of positive charges (flow) • But usually, in metallic conductors current results from electron motion • The sign of the current indicates the direction of flow of positive charges • Types of current: –
direct current
(constant) (dc): batteries and some special generators –
alternating current
(ac): household current which varies with time (sine wave) ece201 lecture 1 3
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 • A good analogy is pressure for fluids ece201 lecture 1 4
Sign Convention
•
Passive sign convention
: current should enter the positive voltage terminal (flows through the device from + to -, “downhill”)
I
+
Circuit Element
–
• Consequence for
P
=
I V
– Positive (+) Power: element absorbs (dissipates) – Negative (-) Power: element supplies power ece201 lecture 1 5
Electrical Analogies (Physical)
Base Flow Potential Power Electrical Charge (
q
) Current (
I
) Voltage (
V
)
P
=
I V
Hydraulic Mass (
m
) Fluid flow (
G
) Pressure (
p
)
P
=
G p
ece201 lecture 1 6
Class Examples ece201 lecture 1 7
Active vs. Passive Elements
•
Active elements
can generate energy – Batteries – Voltage and current sources •
Passive elements
cannot generate energy – Resistors – Capacitors and Inductors (but CAN store energy) ece201 lecture 1 8
Independent vs. Dependent Sources
An
independent source
(voltage or current) may be DC (constant) or time-varying, but does not depend on other voltages or currents in the circuit.
24V
+ –
6 cos(t) A The
dependent source
magnitude is a function of another voltage or current in the circuit.
ece201 lecture 1 9
Dependent Voltage Sources
+ – 6
V x
+ – 6000
I x
Voltage Controlled Voltage Source ece201 lecture 1 Current Controlled Voltage Source 10
Dependent Current Sources
0.006
V x
6
I x
Voltage Controlled Current Source ece201 lecture 1 Current Controlled Current Source 11
Class Examples ece201 lecture 1 12