Units (1.1) Basic Quantities (1.2) Circuit Elements (1.3)

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Units (1.1)Basic Quantities (1.2)Circuit
Elements (1.3)
Introduction

• Prof. Phillips
• January 22, 2003

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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 Nm/coulomb
• Think charge (-e_) PRESSURE
• Power (dissipated) P I V
• 1 Watt 1 VoltAmp 1 Joule/sec

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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 (dc) batteries and some special
  generators (constant)
• alternating current (ac) household current which
  varies with time (sine wave)

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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

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Sign Convention

• Passive sign convention current should enter
  the positive voltage terminal (flows through the
  device from to -, downhill)
• Consequence for P I V
• Positive () Power element absorbs (dissipates)
• Negative (-) Power element supplies power

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Electrical Analogies (Physical)
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Class Examples
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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)

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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. The dependent source magnitude is a
function of another voltage or current in the
circuit.

24V
6 cos(t) A
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Dependent Voltage Sources
6Vx
6000Ix


Voltage Controlled Voltage Source
Current Controlled Voltage Source
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Dependent Current Sources
0.006Vx
6Ix
Voltage Controlled Current Source
Current Controlled Current Source
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Class Examples