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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