Question 1

How do I calculate the force between two charged particles?

Accepted Answer

Use Coulomb's Law: F = kₑ × |q₁ × q₂| / r². Identify both charges (q₁ and q₂) in Coulombs, measure the distance (r) in meters, and multiply by Coulomb's constant (kₑ = 8.9876 × 10⁹ N·m²/C²). The result is the force magnitude in Newtons. If both charges have the same sign, the force is repulsive; if opposite signs, attractive.

Question 2

What is Coulomb's constant and why is it important?

Accepted Answer

Coulomb's constant (kₑ) equals 8.9875517873681764 × 10⁹ N·m²/C², defined as 1/(4πε₀) where ε₀ is the vacuum permittivity. It determines the strength of the electrostatic interaction. Without this constant, the formula wouldn't produce results in standard SI units.

Question 3

Is Coulomb's force attractive or repulsive?

Accepted Answer

It depends on the charge signs. Like charges (both positive or both negative) repel each other — the force pushes them apart. Unlike charges (one positive, one negative) attract each other — the force pulls them together. The magnitude of the force is the same in both cases; only the direction differs.

Question 4

Is Coulomb's Law an inverse square law?

Accepted Answer

Yes. The force is inversely proportional to the square of the distance between charges. This means doubling the distance reduces the force to one-quarter (1/4), tripling it reduces the force to one-ninth (1/9), and so on. This is the same relationship as gravitational force.

Question 5

How does the medium affect electrostatic force?

Accepted Answer

The medium between charges affects the force through its relative permittivity (εᵣ or dielectric constant). The force is divided by εᵣ: F = kₑ|q₁q₂|/(εᵣr²). In vacuum εᵣ = 1, in air εᵣ ≈ 1.0006, but in water εᵣ ≈ 78.5 — reducing the force by nearly 80 times. This is why ionic compounds dissolve in water: the electrostatic attraction between ions is drastically weakened.

Question 6

What units should I use for Coulomb's Law?

Accepted Answer

For direct substitution into the formula, use SI units: charges in Coulombs (C), distance in meters (m), and force in Newtons (N). In practice, charges are often given in microcoulombs (µC = 10⁻⁶ C), nanocoulombs (nC = 10⁻⁹ C), or elementary charges (e = 1.602 × 10⁻¹⁹ C). Always convert to Coulombs before calculating.

Question 7

How do I calculate the net force with 3 charges using Coulomb's Law?

Accepted Answer

Calculate the force on the target charge from each other charge separately using Coulomb's Law. Since force is a vector, you must consider both magnitude and direction. For charges in a line, add or subtract forces based on direction. For charges in 2D/3D, decompose into x and y components, sum each component separately, then use the Pythagorean theorem to find the resultant magnitude.

Question 8

What is the difference between Coulomb's Law and the electric field?

Accepted Answer

Coulomb's Law gives the force between two specific charges. The electric field (E = kₑq/r²) describes the force per unit charge at a point in space created by a single source charge. They're related: if you know the field E at a point, the force on a charge q placed there is F = qE. The electric field is more general — it exists even without a second charge present.

Question 9

Can Coulomb's Law be used to calculate energy?

Accepted Answer

Coulomb's Law directly gives force, not energy. However, the electric potential energy between two charges uses a related formula: PE = kₑq₁q₂/r (note: no square on r). The force is the negative derivative of potential energy with respect to distance. To calculate the work done moving a charge, integrate the force over the displacement.

Question 10

How strong is the Coulomb force compared to gravity?

Accepted Answer

Enormously stronger. Between a proton and electron in a hydrogen atom, the electrostatic force is about 2.3 × 10³⁹ times stronger than gravitational attraction. Gravity only dominates at large scales because matter is electrically neutral overall — positive and negative charges cancel out, but mass always adds up.

Medium	ε_r	Force Reduction
Vacuum	1.0000	None (reference)
Air (STP)	1.0006	~0.06%
Paper	3.5	3.5×
Glass	5.0	5×
Mica	6.0	6×
Rubber	7.0	7×
Silicon	11.7	11.7×
Water (25°C)	78.5	78.5×

Property	Coulomb's Law	Gravity
Formula	F = kₑq₁q₂/r²	F = Gm₁m₂/r²
Constant	8.988 × 10⁹	6.674 × 10⁻¹¹
Direction	Attractive or repulsive	Always attractive
Relative strength	~10³⁶ stronger	~10⁻³⁶ weaker
Shielding	Can be shielded	Cannot be shielded

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