Lecture 1 Circuit Variables

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Lecture 1Circuit Variables

• ECE 205
• Prof. Ali Keyhani

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Symbols and Units

• In this course international system (SI) of units
  is used
• SI system has six fundamental units
• Meter Shown as m
• Kilogram Shown as kg
• Second Shown as s
• Ampere Shown as A
• Kelvin Shown as K
• Candela Shown as cd
• Other units are derived from the six fundamental
  units

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• Symbols in Electrical Engineering

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• Numerical values in engineering range over many
  orders of magnitude.
• Example

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Circuit Variables

• Charge and Energy are two variables in study of
  electronic systems
• There are two kind of charges Positive and
  Negative
• Two important rules
• Like charges repel
• Unlike charges attract one another

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Charge Current

• In SI shown by q and measured in coulombs (C)
• Electrons charge
• It is not easy to measure the electrical charges
• The charges are moving all the time

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Charge Current

• By having the amount of charge passing a point
  current i is defined
• The unit of current in SI is ampere (A)
• 1 ampere1 coulomb/second
• The direction of current is defined as the
  direction of the net flow of positive charge

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Voltage

• Change in energy as a result of a charge passing
  through a circuit
• W energy (joules) or (J)
• V voltage (volt) or (V)
• 1 joule/coulomb 1 volt

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Power

• Power is defined as the time rate of change of
  energy
• Unit in SI watts
• 1 watt 1 joule/second
• Power is also defined as Volt. Amperes

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Passive Sign Convention

• Engineering problems are stated in terms of
  voltage, current and power.
• Voltage is not measured at a single point but is
  measured between two points in a circuit.

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• The arrows define the positive direction of the
  current with respect to the voltage across a
  device
• Ground A reference point for the voltage
• The voltage of all other points in the circuit
  are defined based on the common reference point
• The symbol for the ground

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Example 1

• The current through a circuit is 20mA. Find the
  total charge and the number of electrons
  transferred in 200ns.

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Example 2

• Power dissipation of an electrical device is
  200W. How much energy is delivered to this device
  in 20 sec?
• Solution
• ? W200204000J4kJ