Electric Current and Ohm's Law

Electric current is the rate of flow of charge: $I = Q/t$. The SI unit is the ampere (1 A = 1 C/s). Rearranged, the charge transported in time $t$ is $Q = It$. Conventional current is taken in the direction a POSITIVE charge would move — opposite to the actual electron drift in a metal.

In a metallic conductor, current is carried by free (conduction) electrons — not ions, not protons, not bound electrons. When a potential difference is applied, these electrons drift opposite to the field; the conventional current points the other way (direction of positive-charge flow).

Ohm's law: at constant temperature, the current through a conductor is directly proportional to the potential difference across it, $V = IR$, where $R$ is constant. It is an empirical law, not a universal one — it holds for metals in a normal range, but NOT for all materials or arbitrarily strong fields. On an I–V plot (I vertical, V horizontal) an ohmic conductor is a straight line through the origin with slope $1/R$ — so a steeper line means a smaller resistance.

• Ohmic — obeys $V = IR$ with constant $R$; I–V graph is a straight line through the origin. Examples: copper wire, nichrome heating coil, rheostat (a variable resistor, but ohmic).
• Non-ohmic — $R$ depends on voltage/current/direction; I–V graph is curved or asymmetric. Example: a semiconductor diode (conducts one way, blocks the other).

DC flows in one direction (battery, cell). AC periodically reverses direction (mains supply). Indian mains frequency is 50 Hz — 50 complete cycles per second. Because each cycle reverses direction twice, the current changes direction every $\tfrac{1}{2\times50} = \tfrac{1}{100}$ second.