Charge & Current

Electric Charge

Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric field. Charge is measured in coulombs (C).

There are two types of charge: positive and negative. In metals, the charge carriers are electrons, which carry a negative charge and are free to move through the metal.

Charge is conserved, meaning the total charge in an isolated system remains constant. Charge cannot be created or destroyed, only transferred from one object to another.

Electric Current

Electric current is the rate of flow of electric charge. It tells us how much charge passes through a point in a circuit every second.

Current is measured in amperes (A), where 1 ampere equals 1 coulomb of charge passing per second.

The direction of conventional current is defined as the direction positive charge would flow, which is opposite to the actual flow of electrons in metals. This convention was established before the discovery of electrons, so conventional current direction is opposite to electron flow.

In metals, current is caused by electrons moving through the metal lattice from the negative terminal to the positive terminal of a power supply.

For instance, if 3 coulombs of charge pass through a wire in 2 seconds, the current is:

$\text{Current} = \frac{\text{Charge}}{\text{Time}} = \frac{3\, \text{C}}{2\, \text{s}} = 1.5\, \text{A}$

Relationship Between Charge and Current

The relationship between charge ($Q$) and current ($I$) is given by the formula:

$$I = \frac{Q}{t}$$

where:

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

This means current is the amount of charge flowing per second.

You can rearrange the formula to calculate the charge flow if current and time are known:

$$Q = I \times t$$

For example, if a current of 0.5 A flows for 10 seconds, the total charge that has passed through the circuit is:

$$Q = 0.5 \times 10 = 5\, \text{C}$$