Electric Current

Understanding Electric Current

Electric current is fundamentally about the net flow of charge across a given area. It is not simply the total charge moving, but the net amount—the difference between charge flowing forward and backward.

Imagine a small area placed perpendicular to the direction of charge flow. Over a time interval $t$:

• Let $q_+$ be the net positive charge flowing forward across the area.
• Let $q_-$ be the net negative charge flowing forward across the area.

The net charge $q$ flowing forward in time $t$ is:

If the flow is steady (constant over time), the current $I$ is defined as the constant ratio:

If $I$ turns out negative, it simply means the net current is in the backward direction.

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General Definition (for varying currents)

Currents are not always steady. For a more general definition, consider a small time interval $\Delta t$ (from $t$ to $t + \Delta t$). Let $\Delta Q$ be the net charge flowing across a cross-section during $\Delta t$.

The instantaneous current $I(t)$ at time $t$ is defined as the limit of this ratio as $\Delta t$ approaches zero:

This is the derivative of charge with respect to time: $I = \frac{dQ}{dt}$.

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SI Unit: The Ampere

• The SI unit of current is the ampere (symbol: A). …