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:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left|\vec{F}_{coulomb}\right| = k\frac{q_1q_2}{{r_{12}}^2}\hat{r}_{12}}

• ${\hat {r}}_{12}$ is a directional (unit) vector pointing from Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_1} to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_2} depending on whether the charges are the same or different.
• Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle k=9 \times 10^9 \ \mathrm{Nm^2/c^2}}

Example

Suppose we have 2 point charges (Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_1} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_2} ), each 1 µC, separated by a distance Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle d} of 1 m along the x axis:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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!!