Fundamentals of Electrostatics and Coulomb's Law

Electric Charge and Electrostatics

Electrostatics is the branch of physics that studies phenomena involving electrical charges at rest. The measure of the electric charge within bodies can be determined through this science.

Electric charge (Q) is a property of quantified matter manifested by repulsions and attractions between particles. Its unit of measurement is the Coulomb (C), and it can be negative or positive.

All substances are formed by atoms and molecules. Atoms consist of a positively charged nucleus (protons) and a negatively charged orbit (electrons). When an atom has an equal number of electrons and protons, it is considered neutral.

An electron’s charge is: An atom is negatively charged when it has an excess of electrons; conversely, it is positively charged when it has a deficit.

Principles of Electrostatic Interaction

The First Law of Electrostatics states that charges with different signs attract, while charges with similar signs repel.

Metals are examples of conductive materials, while plastic is a good insulating material.

An object in a neutral state can be charged by induction without direct contact. For example, if two neutral spheres are placed near a negatively charged magnetic bar, the charges within the spheres redistribute, causing the sphere nearest the bar to become positively charged, while the other becomes negatively charged.

An electroscope is a device that detects the presence of an electric charge in an object using the induction phenomenon.

Coulomb’s Law

Coulomb’s Law states: “The force of attraction or repulsion between two punctual charges is directly proportional to the product of both charges, and inversely proportional to the square of the distance that separates them.” (Tippens, 2007)

Variables in Coulomb's Law

• F: The force of attraction or repulsion in Newtons (N). It is a vector quantity; its direction is determined by the First Law of Electrostatics.
• k: The proportionality constant:
• q1, q2: The punctual charges expressed in Coulombs (C).
• r: The distance between both charges measured in meters (m).

Example 5.1

Two charges are separated by 120 mm. One charge is 4µC and the other is 3µC. Calculate the resulting force and determine if it is repulsive or attractive.

Answer: Using the Coulomb Law formula:

Because both charges have a positive sign, the force is one of repulsion.

Example 5.2

Given 3 charges as shown in the drawing, obtain the resulting force and its direction.

We define F1 as the force on q2 caused by q1, and F2 as the force on q2 caused by q3. The resulting force is the vector sum of these forces.

Since both F1 and F2 are directed to the right, the resulting force is also directed to the right.