Is Magnets Physics or Electrical Engineering? Core Theory Explained

Magnetism is a fundamental branch of physics describing the force exerted by moving electric charges, but in electrical work, we apply this physics to manage energy storage, filtering, and electromechanical conversion. If you are asking "is magnets physics," the short answer is yes—the underlying rules are pure physics, but the moment you wrap copper wire around an iron core to build a 120V contactor or a switch-mode power supply, it becomes electrical engineering. In a real circuit, magnetic fields change how current flows over time by introducing inductance and back-EMF, effectively resisting changes in current. What beginners most commonly confuse is magnetic flux (Webers) with magnetic flux density (Tesla); mixing these up on the bench means your transformer core saturates and your switching MOSFET blows up.

The Physics of Magnetism vs. Electrical Engineering Application

In a university physics lab, you study magnetostatics and Maxwell’s equations to understand how moving charges generate fields. You calculate the theoretical force between two point charges or the field lines around an infinite straight wire. According to the National Institute of Standards and Technology (NIST), the SI unit for magnetic flux density is the Tesla (T), defined as one Weber per square meter. In electrical engineering and DIY electronics, we do not care about infinite wires. We care about finite coils, specific core materials (like ferrite or silicon steel), and how those magnetic fields interact with alternating current (AC). The physics gives us the formula for inductance; the engineering tells us that a 100 µH power inductor will overheat and fail if we push 5 amps of DC through it without checking its DC resistance (DCR) and saturation current limits.

Worked Example: Calculating Inductor Impedance in an AC Circuit

To see how the physics of magnets translates to hard numbers on the bench, let us calculate the inductive reactance ($X_L$) of a choke in a circuit. The formula derived from Faraday’s Law of Induction is:

$X_L = 2 \pi f L$

Where:
$X_L$ = Inductive reactance in Ohms ($\Omega$)
$f$ = Frequency in Hertz (Hz)
$L$ = Inductance in Henrys (H)

Assume we have a 50 mH (0.050 H) inductor. Let us look at how this single physical component behaves in two entirely different engineering applications.

Scenario A: 60 Hz Mains AC Filter

If we place this inductor in a standard North American 120V/60Hz mains circuit to filter out line noise:

• $X_L = 2 \times 3.14159 \times 60 \times 0.050$
• $X_L = 18.85 \Omega$

At 60 Hz, the magnetic field expands and collapses relatively slowly. The inductor presents only about 19 ohms of opposition to the current.

Scenario B: 100 kHz Switch-Mode Power Supply (SMPS)

If we take that exact same 50 mH inductor and place it in the high-frequency switching node of a buck converter running at 100,000 Hz:

• $X_L = 2 \times 3.14159 \times 100,000 \times 0.050$
• $X_L = 31,415.9 \Omega$

The physics dictates that the faster the current tries to change, the harder the collapsing magnetic field pushes back. At 100 kHz, that same component acts as a massive 31.4 k$\Omega$ roadblock. This is why high-frequency SMPS designs use much smaller inductors (in the microhenry range)—if you used a 50 mH iron-core inductor at 100 kHz, the reactance would choke the circuit entirely, and the core losses (eddy currents and hysteresis) would melt the component.

For a deeper dive into the underlying physics equations governing these components, the Georgia State University HyperPhysics database provides excellent interactive breakdowns of inductor behavior and magnetic field calculations.

Where You Meet This in Practice (Bench and Jobsite)

You do not need to solve differential equations to wire a house or build an Arduino project, but you absolutely must respect the physical laws of magnetism to avoid destroying equipment or causing a fire.

1. Flyback Diodes and Relay Coils

When you energize a mechanical relay, current flows through a coil, creating a magnetic field that pulls the armature. When you cut the power, the magnetic field collapses. Physics dictates that this collapsing field will induce a voltage spike to keep current flowing ($V = -L \frac{di}{dt}$). In a 12V DC circuit controlling a heavy-duty automotive relay, this flyback spike can easily exceed 150V. If you are driving that relay with an ESP32 GPIO pin via a transistor, that spike will instantly punch through the transistor's junction and fry your microcontroller. The engineering fix is simple: wire a 1N4007 flyback diode in reverse parallel across the coil to give the induced current a safe path to dissipate.

2. Motor Back-EMF and Soft Starters

Every AC induction motor is also a generator. When a 240V well pump motor spins up, the rotating magnetic field induces a voltage that opposes the supply voltage (Back-EMF). At startup, before the rotor is spinning, there is no Back-EMF, resulting in Locked Rotor Amps (LRA) that can be 5 to 7 times higher than the running current. Understanding this magnetic interaction is why we size motor breakers using NEC Article 430 tables, which allow for higher instantaneous trip thresholds to accommodate the magnetic inrush without nuisance tripping.

3. Magnetic Circuit Breakers

Standard thermal-magnetic breakers in your home panel rely directly on electromagnetism. While the bimetallic strip handles slow overloads (thermal), a short circuit sends a massive, instantaneous surge of current through a small solenoid coil inside the breaker. The resulting magnetic field physically pulls a steel latch, tripping the contacts open in milliseconds to prevent a fire.

Frequently Asked Questions