Current Electricity

3.1 INTRODUCTION

In the first chapter of the NCERT Physics Class 12 textbook, the concept of electric charges at rest was introduced, which is the domain of electrostatics. However, when charges are in motion, they constitute an electric current, which is the foundation of current electricity. This chapter begins by contrasting electrostatics with current electricity, emphasizing that the latter involves charges flowing steadily through conductors. Natural phenomena such as lightning demonstrate the flow of charges, albeit in a transient and intense manner. In everyday applications, devices like torches and cell-driven clocks exhibit steady currents, where charges flow smoothly and continuously, analogous to water flowing in a river. This chapter aims to explore the fundamental laws governing such steady electric currents, their behavior in conductors, and their practical applications in circuits.

• Electric current arises from charges in motion, unlike electrostatics where charges are stationary.
• Lightning is a natural example of transient electric current.
• Steady currents are observed in many household devices like torches and clocks.
• The chapter focuses on laws and principles of steady electric currents.
• Understanding current electricity is essential for analyzing electric circuits and devices.
• 📌 Electric current: Flow of electric charges constituting a current.
• 📌 Electrostatics: Study of electric charges at rest.

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.

• Electric current I = net charge q passing per unit time t, I = q/t.
• Both positive and negative charges contribute to current; net charge flow is q+ - q-.
• Current direction is indicated by the sign of I; negative I means current opposite to chosen direction.
• For time-varying currents, I(t) = limit of ΔQ/Δt as Δt → 0.
• SI unit of current is ampere (A), defined via magnetic effects.
• Currents vary widely: lightning (~10⁴ A), household (~1 A), nerves (~μA).
• 📌 Electric current (I): Rate of flow of net charge through a cross-sectional area.
• 📌 Ampere (A): SI unit of current.