Current Electricity | Class 12 Physics Notes

3.2 ELECTRIC CURRENT

Electric current is defined as the net amount of electric charge passing through a given area per unit time. Consider a small area placed normal to the direction of charge flow. Both positive and negative charges can move across this area, possibly in both forward and backward directions. Let q+ be the net amount of positive charge flowing forward (forward minus backward) and q- be the net amount of negative charge flowing forward. The net charge q flowing forward across the area in time t is q = q+ - q-. For steady currents, q is proportional to t, and the current I is defined as I = q/t. If I is negative, it indicates current in the backward direction. More generally, for time-varying currents, the current at time t is defined as the limit of ΔQ/Δt as Δt approaches zero, where ΔQ is the net charge crossing the area in time Δt. The SI unit of current is the ampere (A), which is defined based on magnetic effects of currents. Typical currents in household appliances are of the order of amperes, while lightning currents can be tens of thousands of amperes, and nerve currents are in microamperes.

🔗 Connection: This section leads to the understanding of how currents arise in conductors, discussed in the next section.