Electrostatics

 🌐 Version en français : Électrostatique

 
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General Resources

📖 Reference Textbook:
University Physics Volume 2: Chapter 5 – Electrostatics

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Theory

⚡ Charge, Conductors, and Insulators

Electric Charge (q)

• Basic property: Objects can have a positive or negative charge.
• Types: Positive (more protons than electrons), Negative (more electrons than protons).
• Conservation: Charge is neither created nor destroyed, only transferred.
• Quantized: $Q=n\cdot e$, with $e=1.6\times 10^{-19}\text{C}$.

Conductors

• Charges move freely.
• Examples: copper, silver, salt solutions.

Insulators

• Charges can’t move freely.
• Examples: glass, rubber, plastic.

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⚖️ Coulomb’s Law (Scalar Form)

The magnitude of the electrostatic force $F_{\mathrm{t}\mathrm{s}}$ between two point charges $q_{\mathrm{s}}$ (source charge) and $q_{\mathrm{t}}$ (test charge), separated by a distance $r$, is: $F_{\mathrm{t}\mathrm{s}}=k\cdot \frac{|q_{\mathrm{s}}|\cdot |q_{\mathrm{t}}|}{r^2}$, with $k\approx 8.99\times 10^9\mathrm{N}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}/{\mathrm{C}}^{\mathrm{2}}$.

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➡️ Coulomb’s Law (Vector Form)

$\overrightarrow{F_{ts}}=k_e\frac{q_s{q}_t}{r^2}\hat{r}$

Where:

• $\overrightarrow{F_{ts}}$ = force on test charge
• $k_e\approx 8.99\times 10^9\mathrm{N}{\mathrm{m}}^{\mathrm{2}}/{\mathrm{C}}^{\mathrm{2}}$
• $q_s,q_t$ = charges
• $r$ = distance
• $\hat{r}$ = unit vector from source to test charge

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✋ Triboelectric Effect

When two materials rub and separate, electrons transfer → one becomes negative, the other positive.

• Higher in series (glass, hair) → lose electrons → positive
• Lower (rubber, Teflon) → gain electrons → negative

Daily life examples: balloon on hair; static cling in clothes.

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⚙️ Simulations

• PhET: Be careful, John Travolta!
• PhET: Charge up a balloon
• The Physics Teacher: Static Electricity resources
• Lightning applet

 
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