Charge, Coulomb's law and electric-field evidence

Charge, Coulomb's law and electric-field evidence

Electric charge can be positive or negative. Like charges repel and unlike charges attract. Coulomb's law gives the magnitude of electrostatic force between point charges: F = k|q₁q₂|/r². The force acts along the line joining the charges, and its direction is then decided by the signs. Use charge in coulombs and separation in metres; micro means 10⁻⁶. The inverse-square relationship is important: doubling separation makes the force one quarter, not one half. Electric field is a way to describe the force per unit positive test charge at a point; field direction is the direction a positive test charge would be pushed.

Work it through

Two charges of +2.0 μC and +3.0 μC are 0.30 m apart. F = 8.99 × 10⁹ × (2.0 × 10⁻⁶)(3.0 × 10⁻⁶)/(0.30)² ≈ 0.60 N. Both are positive, so the force is repulsive. State magnitude and direction separately: each charge feels a 0.60 N force away from the other.

Mastery target

Calculate electrostatic force with SI units, use charge signs to state direction and connect a force description to electric-field direction.