Resistance is the opposition to the flow of current. The symbol for resistance is R, and the basic unit of measure is the ohm Ω.
Conductance is the reciprocal of resistance. The symbol for conductance is G, and the basic unit of measure is the siemen (S).
R = 1/G and G = 1/R
An electric circuit is a closed path for current flow. A voltage must be connected across a circuit to produce current flow. In the external circuit outside the voltage source, electrons flow from the negative terminal toward the positive terminal.
A motion of positive charges, in the opposite direction of electron flow, is considered conventional current.
Voltage can exist without current, but current cannot exist without voltage.
Direct current has just one direction because a dc voltage source has fixed polarity. Alternating current periodically reverses in direction as the ac voltage source periodically reverses in polarity.
Table 1–6 summarizes the main features of electric circuits.
A digital multimeter is used to measure the voltage, current, or resistance in a circuit.
