Current Electricity

Current electricity is the flow of electric charge through a conductor. This section covers the fundamental principles governing the behavior of electric circuits, essential for the JAMB syllabus.

1. Basic Concepts: EMF, P.D., and Current

• Electric Current (I): The rate of flow of charge. Measured in Amperes (A). I = Q/t.
• Potential Difference (V): The work done in moving a unit positive charge from one point to another in a circuit. Measured in Volts (V).
• Electromotive Force (EMF): The total work done by a cell in moving a unit charge around a complete circuit (including the cell itself). It is the p.d across the terminals of a cell when no current is being drawn (open circuit).
• Internal Resistance (r): The opposition to the flow of current within the cell itself due to its chemicals and electrodes.
• Lost Volt (v): The potential difference dropped across the internal resistance of the cell. v = Ir.

2. Ohm's Law

Ohm's law states that the current (I) flowing through a metallic conductor is directly proportional to the potential difference (V) across its ends, provided temperature and other physical conditions remain constant. V = IR.

3. Resistivity and Conductivity

The resistance (R) of a conductor depends on its length (L), cross-sectional area (A), and material. R = ρL/A, where ρ (rho) is the resistivity. Conductivity (σ) is the reciprocal of resistivity (1/ρ).

4. Arrangement of Resistors

• Series: Total resistance R_T = R₁ + R₂ + .... The current is the same through all resistors.
• Parallel: 1/R_T = 1/R₁ + 1/R₂ + .... The potential difference is the same across all resistors.

5. Measuring Instruments

• Meter Bridge: Used for measuring unknown resistance based on the principle of the Wheatstone bridge. At balance point: R/L₁ = X/L₂ (where L₁ + L₂ = 100cm).
• Potentiometer: An instrument used to measure EMF and internal resistance without drawing current from the source, making it more accurate than a voltmeter. E₁/E₂ = L₁/L₂.

6. Kirchhoff's Laws

• First Law (Junction Law): The sum of currents entering a junction equals the sum of currents leaving it (Conservation of Charge).
• Second Law (Loop Law): In any closed loop, the sum of EMFs is equal to the sum of the p.d. drops (Conservation of Energy).