# Useful Formulae

44 ELECTRICITYANDCHARGE tuning the circuit For a simple electrical circuit,the voltageEin volts across a resistance R in ohms, produces a currentI in amps defined by Ohms Law, E IR. The powerPin watts in the circuit is then P EI I2R Resistors in series give resistance Rs R1 Rn ohms. Capacitors in parallel give capacitance Cp C1 Cn faradays. Resistors in parallel and capacitors in series combine to give Rp Cs Equations for circuits involving inductors are shown opposite. All electrical effects are the result of charge measured in coulombs. The charge of an electron is 1.6 x 1019 C. Coulombs Lawstates that the force Fbetween two point charges Q1 andQ2 separated by a distance of rmetres is given by F where 0 is the permittivity of empty space,8.85 x 1012 faradm. In the microcosm,Coulomb forces bind electrons to nuclei to form atoms, atoms to atoms to form molecules and molecules to molecules to form solids and liquids. 1 1 R1 1 Rn 1 1 C1 1 Cn Q1Q2 40 r2 45