Current, Voltage, and Resistance

Objective: Define current and distinguish conventional direction from electron motion.

Electric current is the rate of charge crossing a chosen surface, I = ΔQ/Δt, measured in amperes (A = C/s). Conventional current direction is the direction positive charge would flow; in an ordinary metal wire, drifting electrons move opposite that direction. A complete conducting path and an energy source or potential difference can maintain current. Charges are not consumed as they traverse a simple steady circuit; energy is transferred to circuit components. Current is a rate rather than a stock of charge, and its local value can depend on branches. A closed switch permits a conducting path, while an open switch interrupts it.

Objective: Relate potential difference to energy per charge in circuit components.

A battery or other source maintains a potential difference by transferring energy to charges through internal processes. Voltage between two points is energy change per unit charge, ΔV = ΔE/q, with units J/C. A 9 V source can supply approximately 9 J per coulomb in an idealized account. A resistor or lamp transfers electrical energy into thermal, light, or other outputs as charge passes through its voltage drop. Voltage is measured across two points, not “through” one wire like current. Real sources have internal resistance and voltage can change under load; terminal voltage is not always the printed rating.

Objective: Use V = IR for an ohmic component within its operating range.

Electrical resistance is the ratio of potential difference across a component to current through it under stated conditions, R = V/I, measured in ohms (Ω = V/A). For an ohmic resistor at approximately constant temperature, V = IR. A 12 V drop across a 4 Ω resistor gives 3 A. Doubling voltage across the same ohmic resistor doubles current, while doubling resistance at fixed voltage halves it. Not every component is ohmic: filament lamps, diodes, and changing-temperature materials may have nonlinear V–I behavior. Resistance depends on material, geometry, and temperature. Apply the equation to a specific component or equivalent circuit, not to mismatched points and branches.