Key Equations

19.1 Ohm's law

electric current I is the charge $\Delta Q$ that passes a plane per unit time $\Delta t$	$I=\frac{\Delta Q}{\Delta t}$
an ampere is the coulombs per unit time that pass a plane	$1\text{A}=1\text{C/s}$
Ohm’s law: the current I is proportional to the voltage V, with the resistance R being the constant of proportionality	$V=IR$

19.2 Series Circuits

the equivalent resistance of N resistors connected in series

$R_{\text{equiv}}=R_1+R_2+\cdots +R_N$

19.3 Parallel Circuits

the equivalent resistance of N resistors connected in parallel

$R_{\text{equiv}}=\frac{1}{1/R_1+1/R_2+\cdots +1/R_N}$

19.4 Electric Power

for a given current I flowing through a potential difference V, the electric power dissipated	$P=IV$
for a given current I flowing through a resistance R, the electric power dissipated	$P=I^{2}R$
for a given voltage difference V across a resistor R, the electric power dissipated	$P=\frac{V^2}{R}$