Table of Contents

Toggle## 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 = nev_{d}A

v_{d} = I \over neA

v_{d} = at = eEt \over m

- v
_{d}: 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 = nqv_{d} [since I = nev_{d}A]

- 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= R_{1} + R_{2} + R_{3}

where,

- R = total resistance in a series circuit
- R
_{1}, R_{2}, R_{3}= 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
- R
_{1}, R_{2}, R_{3}= 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):

ΣI_{in} = ΣI_{out}

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,

R_{1}, R_{2}, R_{3}, R_{4} = resistances in a Wheatstone bridge circuit