Electrostatics

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🌐 Version en français : Électrostatique

Textbook

University Physics Volume 2: Chapter 5

Theory

Charge, Conductors, and Insulators: Introduction to Electrostatics

Electric Charge (q)

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

Conductors

• Definition: Materials where electric charges (electrons or ions) can move freely.
• Examples: Metals like copper and silver. Salt-Solutions.

Insulators

• Definition: Materials where charges can’t move freely.
• Examples: Glass, rubber, and plastic.

Coulomb's Law without vectors

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 given by:

$F_{\mathrm{t}\mathrm{s}}=k\cdot \frac{|q_{\mathrm{s}}|\cdot |q_{\mathrm{t}}|}{r^2}$ where $k\approx 8.99\cdot 10^9\mathrm{N}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}/{\mathrm{C}}^{\mathrm{2}}$ is the electrostatic constant.

Coulomb's Law in Vector Form

The electrostatic force, as a vector, $\overrightarrow{F_{ts}}$ between two point charges $q_s$ (source charge) and $q_t$ (test charge), separated by a distance $r$, is given by:

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

where:

• $\overrightarrow{F_{ts}}$: Force (vector) on the test charge by the source charge
• $k_e$: Electrostatic constant, approximately $8.99\times 10^9{\text{N m}}^2/{\text{C}}^2$
• $q_s$: Source charge (the charge exerting the force)
• $q_t$: Test charge (the charge the force is exerted on)
• $r$: Distance between the charges
• $\hat{r}$: Unit vector (a vector of length one) pointing from the source charge to the test charge

Triboelectric effect/series or triboelectricity

The triboelectric effect is a phenomenon where certain materials become electrically charged after they come into contact and are then separated. This effect occurs due to friction, causing electrons to transfer between the materials. As a result, one material gains electrons (becoming negatively charged) and the other loses electrons (becoming positively charged).

Different materials have varying tendencies to gain or lose electrons, which are organized in the triboelectric series. Materials higher on the series (such as glass or hair) tend to lose electrons and become positively charged, while materials lower on the series (such as rubber or Teflon) tend to gain electrons and become negatively charged.

The triboelectric effect is commonly observed in everyday life, for example:

• When a balloon rubbed on hair makes the hair stand up.
• When synthetic clothing generates static cling.

Electrostatics Simulations

Check out these links for playing with charges:

• Be careful John Travolta!
  

• Charge up a balloon
  

These and more links can be found at:

http://www.thephysicsteacher.ie/lcphysics19staticelectricity.html

See how lightning strikes:

Lightning applet

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