Magnetism is a type of fundamental physical force—specifically, a macroscopic manifestation of the electromagnetic force generated by the motion of electric charges. In a real circuit or installation, magnetism fundamentally changes how current behaves by introducing inductance; it stores energy in a localized field, causes alternating current to lag behind voltage, and generates back-electromotive force (back-EMF) that actively opposes any sudden change in current flow. Beginners and intermediate hobbyists commonly confuse magnetism with static electric fields (the effect of stationary charge separation), or they mistakenly use the terms magnetic flux density (measured in Tesla) and total magnetic flux (measured in Webers) interchangeably when sizing transformer cores or inductors.
The Physics: Magnetism as an Electromagnetic Force
To answer the question directly: magnetism is not a standalone phenomenon, but rather one half of the electromagnetic force, which is one of the four fundamental forces of nature (alongside gravity, the strong nuclear force, and the weak nuclear force). According to Georgia State University's HyperPhysics, you cannot have a magnetic field without moving charges or the intrinsic quantum spin of electrons.
In practical electrical engineering, we treat electric fields and magnetic fields as two sides of the same coin. An electric field (measured in Volts per meter) exerts force on charges regardless of whether they are moving. A magnetic field (measured in Tesla) only exerts force on charges that are in motion relative to the field. This is why a static DC voltage across a capacitor creates an electric field but no magnetic field, while DC current flowing through an inductor creates a magnetic field but drops zero steady-state voltage.
To visualize this, use the water hammer analogy—but only once. Think of an electric field like the static pressure in a water pipe, while the magnetic field represents the kinetic momentum of the water actually flowing. When you try to shut a physical valve instantly, the water's momentum causes a destructive pressure spike (water hammer). In an electrical circuit, the magnetic field's "momentum" is inductance; when you open a switch, the collapsing magnetic field induces a massive voltage spike to keep the current moving.
Worked Numeric Example: Energy Storage and Inductive Kickback
Let us look at exactly what magnetism does in a circuit by calculating the energy stored in a magnetic field and the resulting voltage spike when that field collapses. This is a critical calculation when driving relays or solenoids with microcontrollers like an Arduino or ESP32.
Assume we are switching a standard 12V DC automotive relay coil using a MOSFET. The coil has the following measured properties:
- Inductance (L): 150 mH (0.15 H)
- DC Resistance (R): 120 Ω
- Supply Voltage (V): 12V DC
First, we find the steady-state current using Ohm's Law:
I = V / R = 12V / 120Ω = 0.1 A (100 mA)
Next, we calculate the energy stored in the magnetic field using the inductor energy formula (E = 0.5 * L * I²):
E = 0.5 * 0.15 H * (0.1 A)² = 0.00075 Joules (0.75 mJ)
While 0.75 mJ sounds tiny, the danger lies in how fast that energy is released. When your MOSFET turns off, the current drops from 100 mA to 0 A. If the MOSFET switches off in 1 microsecond (1 µs), we calculate the induced back-EMF voltage using Faraday's Law (V = -L * (di/dt)):
Without a flyback diode (like a standard 1N4007) to provide a recirculation path for that collapsing magnetic field, that 15,000V spike will instantly punch through the drain-source junction of your MOSFET or arc across a mechanical switch, destroying your driver circuit. This numeric reality is why magnetism must be treated as an active energy storage mechanism, not just a passive side effect.
Where You Meet This in Practice
You interact with the electromagnetic force constantly in both DIY electronics and residential wiring. Here is where magnetic theory dictates physical hardware choices:
| Application | How Magnetism Dictates the Design | Practical Hardware Example |
|---|---|---|
| Transformers | Alternating magnetic flux in a ferromagnetic core transfers energy between isolated windings without a direct electrical connection. | Stepping down 120V AC mains to 12V AC for halogen lighting using a laminated silicon-steel core. |
| AC Wire Grouping | Alternating current creates an alternating magnetic field. If the hot and neutral wires of an AC circuit are separated, the uncanceled magnetic field induces eddy currents in surrounding metal, causing severe heating. | NEC 300.3(B) requires all conductors of the same circuit to be routed in the same conduit to ensure their magnetic fields cancel each other out. |
| Brushless DC (BLDC) Motors | Electronic speed controllers (ESCs) sequentially energize stator coils to create a rotating magnetic field that drags the permanent magnet rotor along. | Gimbal motors in drones or the spindle motor in a CNC router, requiring precise 3-phase PWM timing. |
| Inductive Proximity Sensors | A high-frequency oscillating magnetic field is disrupted by the presence of conductive metals, changing the impedance of the sensor coil. | M18 barrel sensors used in 3D printers and industrial automation to detect metal limits without physical contact. |
Frequently Asked Questions
Is magnetism a type of energy or a force?
Strictly speaking, magnetism is a type of force (a fundamental interaction between moving charges). However, a magnetic field contains potential energy. When you energize an inductor, you do work against the induced back-EMF to build the magnetic field, storing energy in the space around the conductor. When the field collapses, that stored magnetic potential energy is converted back into electrical energy (often as a high-voltage spike or heat).
Why is magnetism considered a relativistic effect of electricity?
This is one of the most profound concepts in physics, heavily detailed in The Feynman Lectures on Physics. Magnetism is literally the electric field viewed from a different frame of reference. When electrons move through a wire, special relativity dictates that the space between the moving charges undergoes length contraction relative to a stationary observer. This slight shift in charge density creates what we measure as a magnetic field. Without Einstein's theory of special relativity, the existence of the magnetic force cannot be mathematically reconciled with Coulomb's law of electrostatics.
What type of magnetism do copper and aluminum wiring exhibit?
Copper and aluminum are not ferromagnetic (like iron or nickel); they do not retain a permanent magnetic field. Copper is diamagnetic, meaning it slightly repels magnetic fields, while aluminum is paramagnetic, meaning it is very weakly attracted to them. In practical electrical work, this means you do not have to worry about your NM-B copper cable or THHN aluminum feeder wires becoming permanently magnetized or sticking to steel enclosures. However, their interaction with alternating magnetic fields is what causes the "skin effect," forcing high-frequency AC current to travel only on the outer surface of the wire.
How does magnetism affect wire sizing and ampacity in AC circuits?
In AC circuits, the constantly reversing current generates an expanding and collapsing magnetic field around the wire. If multiple current-carrying conductors are bundled tightly together in a conduit, their overlapping magnetic fields cause the proximity effect, which forces current to crowd into narrow bands on the wire's surface. This effectively reduces the usable cross-sectional area of the copper, increasing AC resistance and generating excess heat. This is why the National Electrical Code (NEC) mandates strict ampacity derating factors (NEC Table 310.15(C)(1)) when you pull more than three current-carrying conductors through a single raceway.
