State that charge is measured in coulombs

4.2.1 Electric Charge

Objective

State that electric charge is measured in coulombs (C).

What is Electric Charge?

Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric or magnetic field. Like charges repel each other, unlike charges attract.

Unit of Charge

The SI unit of electric charge is the coulomb, symbolised by C. One coulomb is defined as the amount of charge transferred by a constant current of one ampere flowing for one second.

Mathematically,

\$Q = I \times t\$

where

• \$Q\$ is the charge in coulombs (C)
• \$I\$ is the current in amperes (A)
• \$t\$ is the time in seconds (s)

Relation to Elementary Charge

The charge of a single electron (or proton) is the elementary charge \$e\$, where

\$e = 1.602 \times 10^{-19}\ \text{C}\$

Thus,

\$1\ \text{C} = \frac{1}{e} \approx 6.242 \times 10^{18}\ \text{electrons (or protons)}\$

Summary Table

Example Problem

Calculate the charge transferred when a current of 2.5 A flows for 8 seconds.

1. Write down the formula: \$Q = I \times t\$.
2. Substitute the values: \$Q = 2.5\ \text{A} \times 8\ \text{s}\$.
3. Calculate: \$Q = 20\ \text{C}\$.

Therefore, 20 coulombs of charge have been transferred.

Suggested Diagram

Key Point to Remember

All electric charge is measured in coulombs (C). This unit links the concepts of current (amperes) and time (seconds) through the relationship \$Q = I t\$.