Coulomb's Law: Force, Constants, and Superposition

All flashcards

Coulomb's law (magnitude form)

$F = k_e |q_1q_2|/r^2$, where $r$ is the separation between point charges.

Coulomb's constant

$k_e=1/(4\\pi\\varepsilon_0)$, approximately $8.99\\times10^9\\ \\mathrm{N\\cdot m^2/C^2}$ in vacuum.

Principle of superposition

The net electric force is the vector sum of the forces from individual source charges.

Source charge and test charge

A source charge produces a force; a test charge is the charge on which the force is calculated.

Force components

For non-collinear forces, add the horizontal components and vertical components separately.

Quiz questions

1. How is Coulomb's constant related to vacuum permittivity?Answer: $k_e=1/(4\pi\varepsilon_0)$
2. Which SI units should be used for charge and separation to obtain force in newtons from Coulomb's law?Answer: Coulombs for charge and metres for separation
3. Two point charges exert forces on each other. According to Newton's third law, how do these forces compare?Answer: They have equal magnitudes and opposite directions.
4. When several source charges exert forces on one test charge, how is the net force calculated?Answer: Add the individual forces as vectors.
5. A net force has $F_x<0$ and $F_y>0$. Which quadrant contains its direction?Answer: The second quadrant