Coulomb's law describes the electric force between two charged particles. It is one of the basic rules for understanding electricity, atoms, circuits, and many forces inside matter. The law shows that larger charges create stronger forces, while greater separation makes the force weaker.
It matters because electric forces hold atoms together and shape the behavior of charged objects in everyday life and technology.
The force acts along the straight line joining the two charges. Like charges repel, so two positive charges or two negative charges push away from each other, while opposite charges attract. The size of the force is proportional to the product of the charges and inversely proportional to the square of the distance between them.
This inverse square relationship means doubling the distance makes the force one fourth as large.
Key Facts
- Coulomb's law: F = k|q1q2|/r^2
- Coulomb constant: k = 8.99 x 10^9 N m^2/C^2
- Like charges repel: positive-positive or negative-negative charges push apart.
- Opposite charges attract: positive-negative charges pull together.
- If one charge doubles, the electric force doubles.
- If distance doubles, the electric force becomes 1/4 as large because F is proportional to 1/r^2.
Vocabulary
- Electric charge
- A property of matter that can be positive or negative and causes objects to exert electric forces on each other.
- Coulomb
- The SI unit of electric charge, abbreviated C.
- Electric force
- The push or pull between charged objects due to their electric charges.
- Inverse square law
- A relationship where a quantity decreases in proportion to the square of the distance, such as F proportional to 1/r^2.
- Point charge
- An idealized charged object treated as if all of its charge is concentrated at one point.
Common Mistakes to Avoid
- Forgetting to square the distance, which is wrong because Coulomb's law uses r^2 in the denominator, not just r.
- Using centimeters instead of meters, which is wrong because the SI units in F = k|q1q2|/r^2 require distance in meters.
- Ignoring the signs of the charges when deciding direction, which is wrong because the sign tells whether the force is attractive or repulsive.
- Thinking the larger charge feels a larger force, which is wrong because Newton's third law says the two charges exert equal magnitude forces on each other in opposite directions.
Practice Questions
- 1 Two charges, q1 = 3.0 x 10^-6 C and q2 = 2.0 x 10^-6 C, are separated by 0.50 m. What is the magnitude of the electric force between them?
- 2 A pair of charges exerts a force of 12 N on each other at a distance of 0.20 m. What is the force if the distance is increased to 0.40 m, with the charges unchanged?
- 3 Two equal positive charges are placed near each other, then one charge is replaced by an equal negative charge. Explain how the direction of the force changes and why the magnitude may stay the same.