Conduction Current vs. Displacement Current

When people think of current, they typically think of the flow of electrons through a wire. That’s a fair starting point, but it’s not all encompassing. In physics, current is really anything that produces a magnetic field. There are two ways of achieving this:

1. Through the movement of charges in a material (known as conduction current)

2. Through a time-varying electric field (known as displacement current)

Conduction Current

Conduction current is what most people mean when they say “current.” It is the flow of electric charge through a conductor (typically a wire) due to a voltage (or electric field). An easier way to visualize this is with the hollow tube analogy: Imagine a hollow tube with a rope inside. The tube is the conductor, the rope is the electric charge. The rope doesn’t move by itself – it needs to be pulled or pushed. That force is the voltage. Anything that makes it harder for the rope to slide, like a narrow tube or obstacles is the resistance, and the amount of rope that passes through the tube over time is the electric current.

How much rope moves (current) depends on Ohm’s Law:

I = V/R

Where: I= current (A), V = voltage (V), R = resistance (Ω)

So, the more the voltage, the more the current, and more resistance means less current. 

Displacement Current

Unlike conduction current, displacement current doesn’t involve charges actually moving across a material. Instead, it is generated by a changing electric field which creates a magnetic field just like a conduction current would. So, even though no charges move through a medium, this still counts as current in terms of its magnetic effects.

Capacitors are a classic example of displacement current. A capacitor consists of two metal plates separated by a dielectric. When an electric field or voltage is applied, the charges separate and build up along the plates (negative on one side, positive on the other). As the charges build up, the electric field between the plates changes. Even though charges don't move across the dielectric, this changing electric field generates a magnetic field, acting like a current. This is displacement current in action!

An important caveat to highlight about displacement current is that it only occurs in an electric field that varies with time. This is because in a constant, non-changing electric field, no magnetic field is generated, so the displacement current is zero.

Relating this back to capacitors, once the capacitor plates accumulate charge to a point where the voltage across the capacitor equals the applied voltage, the plates stop accumulating more charge and the electric field becomes constant. When this happens, displacement current is no longer generated.

Ampère-Maxwell Law

The Ampère-Maxwell law is an evolved version of the Ampère law and includes a term for displacement current in the mathematical relation of electric current to the circulation of a magnetic field. In doing so, it shows that magnetic fields are produced both by electric currents and time-varying electric fields, and ties everything together.

The Ampère-Maxwell law is written as: 

Where: ∇×H = curl of the magnetic field intensity (A/m²), Jc​ = conduction current density (A/m²), ∂D/∂t = displacement current density (A/m²)