Electromagnitism

(1) Coulumb’s Law:
$$ F = K {m_1 m_2}/{d^2} $$

(2) Magnetic Flux Density:

$ B = ϕ / A $

1 Weber = $10^8$ lines of force

(3) Magnetic Field Strength (H):

$$ H = {\text"Ampere Turns"}/{\text"length in metre"} $$

Multiple conductors: $H = {N I}/{2πr} $ A/m

Solenoid: $H = {N I}/{d}$ AT/m

(4) Permeability:

Absolute Permeability (μ) = B/H henry per metre

Permeability of free space ($μ_0$) = B/H in vaccum or air

$μ_0$ is 4π x $10^{-7}$ H/m

Relative permeability ($μ_r$) = $B/{B_0}$ H is same

$ μ = μ_0 μ_r $ H/m

$B = μ_0 μ_r H$

(5) Magnetomotive Force (m.m.f):
m.m.f = N x I ampere turns
where, N = No. of turns of magnetizing coil
I = Current through coil

(6) Reluctance (S):us
$$ S = {K l}/{a} $$

where, K = Constant of proportionality
l = Length of magnetic circuit in metre

a = Area of cross section in $m^2$

K = 1/μ

$S = {l}/{μ_0 μ_r a}$

S = m.m.f / Flux

S = N I/ϕ AT/Wb

(7) Leakage flux:

$$ \text"Leakage Factor (λ)" = {\text"Total flux" (ϕ_T)}/{\text"Useful flux" (ϕ_u)} $$