Conduction Current vs. Displacement Current

Conduction Current

Conduction current is a more specific descriptor of what is typically referred to as "current". I'll start off with it's definition, give an analogy, and then link the two.

Definition: Conduction current refers to the flow of electric charge through a conducting material in response to an electric field (or voltage) applied across it. Let's break this down with an analogy.

The Hollow Tube Analogy: Imagine a hollow tube with a rope inside it. The tube represents the conductor, and the rope represents electric charge. While the tube is a conduit for the rope, the rope won’t move on its own—someone needs to pull or push it from one end. This pulling or pushing force represents the electric field or applied voltage. The amount of rope that moves through the tube over a given period is analogous to the electric current.

So, to summarize, conduction current can be thought of as the movement of charge (rope) through a conductor (tube), driven by an applied electric field (someone pulling/pushing the rope). If the force on the rope from one side is weaker or nonexistent, the rope moves in the direction of the stronger pull/push.

Ohm’s Law Connection: The magnitude of conduction current is described by Ohm’s Law: I = V/R. Meaning, current (I) is proportional to the voltage (V) (the pulling force), and inversely proportional to resistance (R), (the ease with which the rope moves through the tube). Resistance could come from a tight fit, kinks, or obstacles inside the tube - anything that makes it harder for the rope to pass through.

Displacement Current

Displacement current is a bit more abstract. Unlike conduction current, which involves the continuous movement of charge, displacement current refers to the build-up of electric charge in a fixed location. Despite this difference, it's still considered a type of current because it has the ability to change a magnetic field—a key characteristic of current.

The Physics Behind It: According to Ampere’s Law, a changing electric field generates a magnetic field and vice versa. Therefore, anything that causes a change in the magnetic field can be considered a form of current. A common example of displacement current is seen in capacitors. As charge accumulates between the two plates on a capacitor, the changing electric field between them gives rise to a changing magnetic field, even though no charges are physically crossing the gap. This phenomenon is termed as displacement current.

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