Electrostatics

Electric Charge

Electrostatics studies electric charges at rest and their field. There are two kinds of charges: positive and negative. Charges of the same sign repel each other, and charges of opposite signs attract.

• Elementary charge e = 1,6·10⁻¹⁹ C. Any charge is an integer multiple of e.
• Law of conservation of charge: in a closed system, the algebraic sum of charges does not change.

Coulomb's Law

The force between two point charges is found by the formula F = k·q₁q₂/r², where k = 9·10⁹ N·m²/C².

The force is inversely proportional to the square of the distance. If the distance increases 2 times, the force decreases 4 times, not 2 times.

Field

• Electric field strength: E = F/q (N/C or V/m). For a point charge, E = kq/r².
• Fields obey the principle of superposition: the field of several charges is equal to the vector sum of the individual fields.
• Potential: φ = W/q = kq/r. Voltage: U = φ₁ − φ₂.
• In a uniform field, U = E·d.

Quantity	Formula	Unit
Coulomb force	F = k·q₁q₂/r²	N
Field strength	E = F/q	N/C or V/m
Potential	φ = kq/r	V
Voltage	U = E·d	V