Physics Practice Test: Electrostatics and Coulomb’s Law

Rubbing transfers electrons, and the balloon gained extra electrons, so it is negative and the hair is left positive. Protons are locked in the nuclei and do not move during rubbing, and charge is never created.

Coulomb’s law has distance squared in the denominator, so doubling r divides the force by 2² = 4. Halving the force would be true only if force depended on 1/r.

Doubling both charges multiplies the force by 2 × 2 = 4, and doubling the distance divides it by 2² = 4, so the effects cancel. Counting only the charge change gives four times as large.

F = kq₁q₂/r² = 8.99 × 10⁹ × (2.0 × 10⁻⁶)² ÷ 0.090 ≈ 0.40 N. Forgetting to square the distance gives about 0.12 N.

E = F ÷ q = 0.0060 ÷ (3.0 × 10⁻⁶) = 2,000 N/C. Multiplying the force by the charge gives 1.8 × 10⁻⁸, which is the wrong operation.

Field direction is defined as the direction of the force on a positive charge. An electron is negative, so the force on it points the opposite way, to the west.

In metals, outer electrons are not tied to one atom and drift through the whole solid, carrying charge. Protons stay fixed in the nuclei, so they cannot carry a current in a wire.

The rod repels the free electrons in the sphere to the far side, leaving the near side short of electrons and positive. The sphere as a whole stays neutral because no charge was added.

Each electron carries 1.6 × 10⁻¹⁹ C, so 4.8 × 10⁻¹⁹ ÷ 1.6 × 10⁻¹⁹ = 3. Charge comes only in whole numbers of electrons, so a fraction like 0.33 is impossible.

Work equals charge times potential difference: W = qV = 2.0 × 12 = 24 J, since 1 V means 1 J per coulomb. Dividing 12 by 2.0 gives 6.0, which is the wrong operation.