PHYS 208 Lecture 2

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Electric Charge

Charge on an electron is 1.6E-19 C, and all charges are integral multiples of this value

Net charge of the Universe is 0 (matter is neutral and laws of conservation of electric charge apply) and must have been that way since the Big Bang.

Conduction and Induction

Conduction Building up charge by taking it from a source and by making contact with a conductor.

Induction Bringing an excess charge close to an object, pushing the opposite charge away.

Coulomb's Law

Electric force between two objects is inversely proportional to the distance between them:

$\left|{\vec {F}}_{coulomb}\right|=k{\frac {q_{1}q_{2}}{{r_{12}}^{2}}}{\hat {r}}_{12}$

• ${\hat {r}}_{12}$ is a directional (unit) vector pointing from $q_{1}$ to $q_{2}$ depending on whether the charges are the same or different.
• $k=9\times 10^{9}\ \mathrm {Nm^{2}/c^{2}}$

Example

Suppose we have 2 point charges ($q_{1}$ and $q_{2}$), each 1 µC, separated by a distance $d$ of 1 m along the x axis:

$\left|{\vec {F}}\right|=\left(9\times 10^{9}\right)\,\left({\frac {\left(1\times 10^{-6}\right)\,\left(1\times 10^{-6}\right)}{1^{2}}}\right)=9\times 10^{9}\ \mathrm {N}$ ← HUGE FORCE!!