Formulas 1

Electric field and potential  
  • Coulomb's law:
F12 = (keq1q2/r122) (r12/r12).
  • Electrostatic field of a point charge:
E = (keq/r2) (r/r).
  • Gauss' law:
Φe(through closed surface) = Qinside0.
  • Electrostatic potential energy:
∆U = UB - UA = -q ΣAB E∙r.
  • Electrostatic potential difference:
∆V = ∆U/q
  • The potential of a point charge:
V(r) = kq/r  (convention:  V = 0 at infinity.)
  • Field and potential:
Ex = -∆V/∆x, Ey = -∆V/∆y, Ez = -∆V/∆z
Capacitors  
  • Capacitance:
C = Q/V
  • Parallel plate capacitor:
C = εA/d
  • Energy stored in a capacitor:
U = ˝(Q2/C) = ˝CV2
  • Capacitors in series
1/C = (1/C1) + (1/C2) + (1/C3)
  • Parallel capacitors:
C = C1 + C2 + C3
Currents and circuits  
  • Current:
I = ∆Qnet/∆t = j∙Aj = current density
  • Resistance:
R = ∆V/I
  • Resistance of a straight wire:
R = ρL/A
  • Power:
P = I∆V = I2R = (∆V)2/R
  • Resistors in series:
R = R1 + R2 + R3
  • Parallel Resistors:
1/R = (1/R1) + (1/R2) + (1/R3)
  • RC circuits time constant:
τ = RC

Magnetostatics

 
  • Magnetic force on a moving charge:
F = qv × B
  • Magnetic force on a long straight wire:
F = IL × B
  • Charged particle in a magnetic field:
r = mv/(qB)
  • The magnetic field of a long straight wire:
B = μ0I/(2πr)
  • The magnetic field inside a solenoid:
B = μ0nI
Current loops
  • Magnetic moment:
μ = IAn
  • Torque:
τ = μ × B,  τ = μB sinθ
  • Potential energy:
PEμ = -μB cosθ