Electrostatics

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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*e. Charge exists in packets of e = 1.6 × 10⁻¹⁹ 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 in Vector Form

The electrostatic force ${\vec {F_{ts}}}$ between two point charges $q_{s}$ (source charge) and $q_{t}$ (test charge), separated by a distance $r$, is given by:

${\vec {F_{ts}}}=k_{e}{\frac {q_{s}q_{t}}{r^{2}}}{\hat {r}}$

where:

• ${\vec {F_{ts}}}$: 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

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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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