Voltage and Current Conventions

1
Voltage and Current Conventions
2
Its All About Charge

• Periodic Table is arrangement of The Elements by
  Electric Charge
• Occupancy of space by electrons about the nucleus
  determines the chemical properties of The
  Elements.
• All chemical reactions are the result of charge
  exchange/sharing between atoms.
• All physical sensations are the results of
  electrical currents in the brain and nerves.
• Likewise for thoughts, ideas, and emotions.

3
Current Conventions

• Current is the passage of electrical charge
  through a specified region.
• We envision an imaginary plane dividing the
  specified region, and measure the amount of
  charge passing through the plane per unit time
• amperes coulombs/second.

4
Current (cont.)

• We must arbitrarily associate a direction with
  the imaginary plane (front and back
  surfaces). Positive charge passing through the
  plane from back to front constitutes positive
  valued current. If we change the direction, or
  the charge polarity, then we have negative valued
  current.
• The passage of charge through the imaginary plane
  is often confined within a closed contour (e.g.
  the circular cross section of a wire or neuron or
  circuit element). This constitutes current
  through the wire or device.

5
Applying Current Conventions

• We always speak of current as passing through a
  device or region of space, with a specified
  polarity.
• A symbol representing a current on a schematic
  diagram must always include an indication of
  positive direction (arrow).

6
Voltage Conventions

• Voltage is a consequence of the separation of
  opposite charges, which requires Energy.
• Voltage is a relative measure of the Energy of a
  charged body at point A with respect to its
  energy at point B.
• If it requires Energy of amount U to move a body
  having charge Q from point B to point A, then we
  say that there is a potential difference
  between points A and B of U/Q VAB volts.
• The energy expended in moving Q from B to A can
  be recovered and used to perform Work as the
  charge is allowed to return from A to B. Its
  just like raising a weight in a gravitational
  field, or compressing a spring.

7
Potential Difference

• The word difference implies subtraction, thus
  the ordering of the two points is critical in
  determining polarity.
• The voltage at point B with respect to point A is
  the negative of the voltage at point A with
  respect to point B
• VBA -VAB
• If the two points are the terminals of a circuit
  element, we may speak of the voltage across the
  device, positive or negative, depending upon the
  ordering of the terminals.
• We often choose one node in a circuit as a common
  reference point or ground, and determine the
  voltage of other nodes with respect to the chosen
  reference node.

8
Applying Voltage Conventions

• We always speak of voltage as appearing across a
  device, or between two points in space, with a
  specified polarity.
• A symbol representing a voltage across a circuit
  element on a schematic diagram must always
  include an indication of polarity ( and -
  signs).
• When referring to the voltage at a point or
  node, it is assumed that the other point is a
  defined reference or ground node.