Electric Current and Potential Difference
Electric Current (I) is the rate of flow of electric charge through a conductor. It is mathematically expressed as I = Q / t, where Q is charge and t is time. The SI unit of current is the Ampere (A). Current is measured by an ammeter, which is always connected in series.
Potential Difference (V), or voltage, is the work done to move a unit charge from one point to another in an electric circuit. V = W / Q. Its SI unit is the Volt (V). Potential difference is measured by a voltmeter, which is always connected in parallel.
Ohm's Law and Resistance
In 1827, Georg Simon Ohm found a relationship between current and voltage. Ohm's Law states that the potential difference across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it, provided its temperature remains the same. V ∝ I or V = IR.
The constant of proportionality, R, is called Resistance. It is the property of a conductor to resist the flow of charges. Its unit is the Ohm (Ω). Resistance depends on:
- Length of the conductor (R ∝ L)
- Area of cross-section (R ∝ 1/A)
- Nature of the material and Temperature
This is combined into the equation R = ρ (L/A), where ρ (rho) is the electrical resistivity of the material.
Resistors in Series and Parallel
When two or more resistors are connected end to end consecutively, they are in Series. The current remains the same throughout, but voltage is divided. Equivalent resistance: Rs = R₁ + R₂ + R₃ + ...
When resistors are connected between two common points, they are in Parallel. The voltage remains the same across each resistor, but current is divided. Equivalent resistance: 1/Rp = 1/R₁ + 1/R₂ + 1/R₃ + ...
Heating Effect of Electric Current
When an electric current flows through a high-resistance wire (like nichrome), the electrical energy is converted into heat energy. This is governed by Joule's Law of Heating, which states that the heat produced (H) is directly proportional to the square of current (I²), resistance (R), and time (t). Formula: H = I²Rt. Applications include electric irons, toasters, and the filament of an electric bulb (usually made of tungsten due to its high melting point).
Electric Power
Electric Power (P) is the rate at which electrical energy is consumed in a circuit. P = VI = I²R = V²/R. The SI unit of power is the Watt (W). The commercial unit of electrical energy is the kilowatt-hour (kWh), commonly known as one "Unit" of electricity. 1 kWh = 3.6 × 10⁶ Joules.