# Current Electricity Formula

## Ohm’s Law Formula:

V = I×R

where,

• V = voltage (in volts)
• I = current (in amperes)
• R = resistance (in ohms)

## Power Formula:

P = V×I

where,

• P = power (in watts)
• V = voltage (in volts)
• I = current (in amperes)

## Resistivity Formula:

R = ρ × {l\over a}

where,

• R = resistance (in ohms)
• ρ = resistivity (in ohm-meter)
• l = length of the conductor (in meters)
• a = cross-sectional area of the conductor (in square meters)

## Conductance:

G = 1\over R

• G: Conductance (in siemens or mho)
• R: Resistance (in ohms)

## Drift velocity:

I = nevdA

vd = I \over neA

vd = at = eEt \over m

• vd: Drift velocity (in meters per second)
• I: Current (in amperes)
• n: Number density of charge carriers (in per cubic meter)
• e: Charge of an electron (in coulombs)
• A: Cross-sectional area of the conductor (in square meters)
• t: time interval

## Current Density:

J = I \over A

J = nqvd   [since I = nevdA]

• J: Current density (in amperes per square meter)
• I: Current (in amperes)
• A: Cross-sectional area of the conductor (in square meters)

## Series Resistance Formula:

R= R1 + R2 + R3

where,

• R = total resistance in a series circuit
• R1, R2, R3 = individual resistances in the circuit

## Parallel Resistance Formula:

{1\over R} = {1\over R_1} + {1\over R_2} + {1\over R_3}
• R = total resistance in a parallel circuit
• R1, R2, R3 = individual resistances in the circuit

## EMF and Internal Resistance of a Cell:

V = ε – Ir

I = ε\over R + r

• V = Terminal voltage of the cell (in volts)
• ε = EMF of the cell (in volts)
• I = Current flowing through the circuit (in amperes)
• r = Internal resistance of the cell (in ohms)

## Kirchhoff’s Laws:

a) Kirchhoff’s Current Law (KCL):

ΣIin = ΣIout

The sum of currents entering a junction is equal to the sum of currents leaving the junction.

b) Kirchhoff’s Voltage Law (KVL):

ΣIR = V

The sum of voltage drops around any closed loop in a circuit is equal to zero.

## Wheatstone Bridge Formula:

{R_1\over R_2} = {R_3\over R_4}

where,

R1, R2, R3, R4 = resistances in a Wheatstone bridge circuit

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