Could somebody explain the following excerpt from my Physics book? I don’t understand how, if the voltage of the balloon is so high, the charge can be so low? Maybe it’s just worded confusingly:
“Rub a balloon on your hair, and the balloon becomes negatively charged–perhaps to several thousand volts! That would be several thousand joules of energy, if the charge were 1 coulomb. However, 1 coulomb is a fairly respectable amount of charge. The charge on a balloon rubbed on hair is typically much less than a millionth of a coulomb. Therefore, the amount of energy associated w/ the charged balloon is very, very small. a high voltage means a lot of energy only if a lot of charge is involved. Electrical potential energy differs from electric potential (or voltage).”
Best Answer
Say that you build a charge of 1 $\mu C$ on the balloon rubbing it. A normal rubber balloon has a capacitance of approx 100 pF (spherical capacitor). Thus you have the voltage on the balloon
$C=Q/U$
$U= Q/C = 10^{-6}/10^{-10} = 10000 Volts$
$E = CU^2/2 = 10^{-10}*10^8 =0.01 J$
This is true, when you discharge the balloon a spark happens. This is similar to taking off a sweater when small sparks appear.