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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 .
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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.
