Current Electricity: Summary

SUMMARY

·        The current, I flowing in a conductor I = dQ/dt ,where dQ is the charge that flows through a cross-section in a time interval dt. SI unit of current is ampere (A). 1A = 1 C s^-1.

·        The current density J in a conductor is the current flowing per unit area. J = /AI

·        Current is a scalar but current density is a vector.

·        The general form of Ohm’s law 

·        Practical form of Ohm’s law states that V ∝ I, or V = IR where I is the current.

·        The resistance R of a conductor is R = V/I .  SI unit of resistance is ohm (Ω) and 1 Ω = 1V/1A

·        The resistance of a material R = ρ l/A where l is length of the material and A is the area of cross section.The resistivity of a material determines how much resistance it offers to the flow of current.

·        The equivalent resistance (R_S) of several resistances (R₁, R₂, R₃……..) connected in series combination is R_S = (R₁+R₂ +R₃……..)

·        The equivalent resistance (RP) of several resistances (R₁, R₂, R₃……..) connected in parallel combination is  1/R_P = 1/R₁ + 1/R₂ + 1/R₃ ………………, 

·        Kirchoff’s first rule (Current rule or junction rule): The algebraic sum of the currents at any junction is zero.

·        Kirchoff’s second rule (Voltage rule or loop rule): In a closed circuit the algebraic sum of the products of the current and resistance of each part of the circuit is equal to the total emf included in the circuit.

·        Electric power is the rate at which energy is transformed.

·        If a current I flows across a potential difference V, the power delivered to the circuit is P = IV.

·        In a resistor R, the electrical power converted to heat is P = I²R =V²/R

·        The energy equivalent of one kilowatt-hour (kWh) is 1kWh = 3.6 X 10⁶ J.

·        Metre bridge is one form of Wheatstone’s bridge.

·        Potentiometer is used to compare potential differences.

·        Joule’s law of heating is H = VIt (or) H = I2Rt.

·        Thermoelectric effect: Conversion of temperature differences into electrical voltage and vice versa.