What Is Magnetism in Electrical Circuits? A Maker's Guide

Magnetism is a fundamental physical force generated by moving electrical charges that creates a field capable of attracting ferromagnetic materials and inducing voltage in adjacent conductors. In a real circuit, magnetism changes how current flows over time by introducing inductance, storing energy in a magnetic field, and generating back-electromotive force (back-EMF) that opposes sudden changes in current. People frequently confuse magnetism with static electric fields; while an electric field exists around a stationary charge (like the static shock you get from a doorknob), a magnetic field only exists when those charges are actively moving through a conductor.

The Core Mechanics of Magnetic Fields

When DC current flows through a straight wire, it generates a cylindrical magnetic field around the wire. The direction of this field follows the right-hand rule: if you point your right thumb in the direction of conventional current flow (positive to negative), your fingers curl in the direction of the magnetic flux lines.

To make this field useful in electronics, we coil the wire. Coiling concentrates the magnetic flux lines into a dense, directional field, creating an electromagnet. The strength of this field depends on three variables: the number of wire turns, the magnitude of the current, and the magnetic permeability of the core material inside the coil. This is the foundational principle behind every relay, transformer, and inductor on your workbench.

Worked Numeric Example: Sizing a Solenoid Coil

Let's calculate the actual magnetic field strength ($B$) inside a DIY solenoid lock to see how core materials change the math. The formula for the magnetic field inside a long solenoid is $B = \mu \cdot n \cdot I$, where $\mu$ is the permeability of the core, $n$ is the turn density (turns per meter), and $I$ is the current in amps.

Our Build Parameters:

• Turns ($N$): 500 wraps of 24 AWG magnet wire
• Length ($L$): 0.05 meters (5 cm)
• Turn density ($n$): $500 / 0.05 = 10,000$ turns/meter
• Current ($I$): 0.5 Amps (driven by a 12V supply and a 24-ohm coil)

Scenario A: Air Core
The permeability of free space ($\mu_0$) is $4\pi \times 10^{-7}$ T·m/A (approx $1.257 \times 10^{-6}$).
$B = (1.257 \times 10^{-6}) \times 10,000 \times 0.5 = 0.006285$ Tesla.

Scenario B: Soft Iron Core
Now, we slide a soft iron rod with a relative permeability ($\mu_r$) of 200 into the coil. The new permeability is $\mu_0 \times 200$.
$B = 6.28 \text{ mT} \times 200 = 1.256$ Tesla.

Where You Meet Magnetism in Practice

Understanding magnetic induction and inductance is non-negotiable for circuit design. Here is where magnetism dictates your component choices:

• Inductors and Chokes: Components like the Würth Elektronik WE-PD series store energy in their magnetic fields to smooth out current in buck/boost converters. When the switching MOSFET turns off, the collapsing magnetic field keeps current flowing, preventing voltage dropouts.
• Transformers: Mutual inductance allows energy to jump from a primary coil to a secondary coil via a shared magnetic flux path in the ferrite core, enabling isolated AC-DC power supplies.
• Brushless DC (BLDC) Motors: Drones and 3D printers rely on the physical repulsion and attraction between stator electromagnets and rotor permanent magnets (usually N52-grade neodymium) to generate torque.
• Parasitic Inductance: This is the unwanted magnetism. A standard 10mm PCB trace has roughly 10nH of parasitic inductance. If you switch a 5A load with an IRLZ44N MOSFET in 10 nanoseconds, the collapsing magnetic field in that tiny trace generates a voltage spike ($V = L \cdot di/dt$) of 5 volts. In high-speed logic, this parasitic magnetism causes ringing and EMI failures.

Differentiating Magnetism, Inductance, and Induction

Makers often use these terms interchangeably, but they represent distinct physical and electrical concepts. Refer to this matrix when debugging power circuits:

Frequently Asked Questions