Transcript Voltage and Current - Prescott Campus, Arizona

Voltage and Current
Conventions
It’s 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.
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.
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.
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).
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. It’s just like
raising a weight in a gravitational field, or
compressing a spring.
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.
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.