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What Is a Coulomb?

A coulomb (C) is the unit of electric charge. It equals the charge of 6.242×10¹⁸ electrons, the amount of charge that flows when one ampere of current passes for one second.

Key Properties of Coulombs

• One coulomb = 6.242×10¹⁸ elementary charges
• Current (in amperes) is the rate of coulombs per second
• Voltage (in volts) represents the energy per coulomb, measured in joules

Calculating Electrical Charge

To find the total charge that passes through a circuit, multiply the current by the time the current flows:

Q (C) = I (A) × t (s)

Example: 20 A for 40 s → Q = 20 × 40 = 800 C.

Determining Energy Transferred

When you know the charge and the potential difference, the energy delivered is:

E (J) = V (V) × Q (C)

Example: 100 V and 3 C → E = 100 × 3 = 300 J.

Using Coulomb’s Law

Coulomb’s law describes the electrostatic force between two point charges:

F = k q₁ q₂ / r²

Where F is force (N), q₁ and q₂ are charges (C), r is separation (m), and k is Coulomb’s constant. In air, k ≈ 8.99 × 10⁹ N·m²/C².

Like charges repel; opposite charges attract. The force decreases with the square of the distance.

For detailed calculations or advanced topics, consult physics textbooks or reputable scientific resources.