Magnetism is the physical force generated by moving electrical charges that attracts or repels magnetic materials and exerts mechanical torque on conductors. When searching for a practical description of magnetism, most textbooks bury the lede in historical anecdotes about lodestones and compass needles. On the workbench, we don't care about 12th-century navigation; we care about what happens when you push 5 amps through a coil wrapped around a ferrite core, and why your MOSFET just vented magic smoke. This guide strips away the abstract physics and focuses on how magnetic fields store energy, saturate cores, and destroy poorly protected silicon.
What Magnetism Actually Changes in a Circuit
In a purely resistive DC circuit, current flows instantaneously when voltage is applied. Magnetism changes this fundamental behavior by introducing inductance. When current flows through a conductor, it generates a magnetic field. If that conductor is coiled, the fields of adjacent turns intersect, concentrating the magnetic flux.
This concentrated flux does three specific things to your circuit:
- Opposes changes in current: According to Faraday's Law, a changing magnetic field induces a voltage that opposes the change in current that created it. This is why inductors resist AC but pass DC.
- Stores energy: The magnetic field acts as a temporary energy reservoir. The energy stored is defined by the inline-data-highlight: E = 0.5 × L × I² (where E is Joules, L is Henries, and I is Amps).
- Generates back-EMF: In motors and transformers, the collapsing or rotating magnetic field induces a voltage in adjacent conductors, which is the foundational principle of all electromechanical power conversion.
Where You Meet This in Practice
You interact with engineered magnetic fields every time you power up a modern electronics project. Here is where it physically manifests on your bench:
- Buck/Boost Converters: The inductor in an LM2596 or TPS5430 circuit stores energy in its magnetic field during the switch's ON time and dumps it into the load during the OFF time.
- Relays and Contactors: A 12V coil generates a magnetic field that pulls a steel armature against a spring, physically closing high-current contacts.
- Brushless DC (BLDC) Motors: The ESC (Electronic Speed Controller) sequentially energizes stator coils, creating a rotating magnetic field that drags the permanent magnet rotor along with it.
- Transformers: Mains voltage drives a primary coil, creating an alternating magnetic flux in a laminated iron core, which induces a scaled voltage in the secondary coil.
Worked Numeric Example: Calculating Inductor Core Saturation
Magnetic materials can only support a maximum amount of magnetic flux density before they 'saturate.' Once saturated, the core loses its permeability, the inductance drops to near zero, and the component acts like a short piece of wire. Let's run the numbers on a real-world scenario.
The Setup: You are winding a custom inductor for a 12V-to-5V buck converter. You choose an EE16 ferrite core (Mix 43 material) and wind 30 turns of 22 AWG magnet wire. Your design requires an inductance of 100 µH, and the peak switch current is 3.0 A.
The Math:
We calculate the peak flux density (B) using the formula:
B = (L × I_peak) / (N × A_e)
- L (Inductance) = 100 µH = 100 × 10⁻⁶ H
- I_peak (Peak Current) = 3.0 A
- N (Number of turns) = 30
- A_e (Effective cross-sectional area of EE16 core) ≈ 0.19 cm² = 0.19 × 10⁻⁴ m²
The Calculation:
B = (100 × 10⁻⁶ × 3.0) / (30 × 0.19 × 10⁻⁴)
B = 0.0003 / 0.00057
B ≈ 0.526 Tesla
The Verdict: Mix 43 ferrite saturates at approximately 0.35 Tesla at 100°C. Your calculated flux density of 0.526 T is deep into saturation. During the peak current spike, your 100 µH inductor will temporarily become a ~2 µH piece of wire, causing a massive current spike that will likely destroy your switching MOSFET. The fix? Add an air gap to the core or increase the number of turns to lower the flux density.
Real-World Scenario Walkthrough: The Burnt-Out H-Bridge
To understand what magnetism changes in a real installation, let's look at a classic bench failure involving inductive kickback.
Setup: A hobbyist is driving a 12V brushed DC winch motor using a discrete H-bridge built with IRFZ44N MOSFETs (rated for 55V V_DS). The motor has an internal coil inductance of 50 mH and draws a steady 2.0 A while running. The builder assumes that because the supply is only 12V, 55V MOSFETs are more than adequate. They omit flyback diodes to save board space.
Numbers: The microcontroller sends a PWM signal to turn off the high-side MOSFET. The MOSFET's gate charge dissipates, and the channel closes in roughly 100 nanoseconds (dt = 100 × 10⁻⁹ s). The current (di) drops from 2.0 A to 0 A.
Outcome: The moment the MOSFET turns off, there is a loud 'pop'. The IRFZ44N splits its casing, and the 12V fuse blows. The motor never engages.
What Went Wrong: The collapsing magnetic field inside the motor's coils must dissipate its stored energy. According to the inductor voltage equation V = L × (di / dt), the induced voltage spike is:
V = 0.050 H × (2.0 A / 100 × 10⁻⁹ s)
V = 0.050 × 20,000,000
V = 1,000,000 Volts (theoretical)
In reality, the voltage rises until it finds a path to ground. It hits the 55V avalanche breakdown limit of the IRFZ44N's body diode. The MOSFET attempts to clamp 1,000,000V worth of magnetic energy at 55V, instantly exceeding its thermal junction limits and vaporizing the silicon die. The fix: Always place 1N5819 Schottky flyback diodes across the motor terminals to give the collapsing magnetic field a safe, low-resistance recirculation path.
Common Confusions: Magnetism vs. Inductance vs. Eddy Currents
When reading datasheets and application notes (such as the excellent Texas Instruments Magnetics Design Handbook), it is easy to conflate related terms. Here is how they differ on the bench:
| Concept | What it is | How you measure/observe it |
|---|---|---|
| Magnetism (Flux) | The actual physical field and force generated by moving charges. | Measured in Teslas (B) or Webers (Φ) using a gaussmeter. |
| Inductance (L) | A circuit property defining the ratio of magnetic flux linkage to the current producing it. | Measured in Henries using an LCR meter. |
| Eddy Currents | Parasitic, localized loops of electrical current induced inside the magnetic core material itself by changing flux. | Observed as unwanted heat in the core; mitigated by using laminated steel or high-resistivity ferrite. |
FAQ: Bench Questions on Magnetic Fields
Why do we use ferrite cores instead of solid iron for high-frequency SMPS inductors?
Solid iron is highly conductive. At high switching frequencies (e.g., 500 kHz in a modern buck converter), the rapidly changing magnetic field induces massive eddy currents inside a solid iron core, turning it into a literal heater. Ferrite is a ceramic material containing iron oxide; it provides high magnetic permeability but has very high electrical resistance, effectively choking off eddy currents while still guiding the magnetic flux. For deeper theory on core materials, Georgia State University's HyperPhysics provides excellent baseline models on inductance geometries.
Does using thicker wire increase the magnetic strength of an electromagnet?
Indirectly, yes. The magnetic force of an electromagnet is determined by the Ampere-turns (Current × Number of Turns). Thicker wire has lower DC resistance (R). If your power supply is voltage-limited (e.g., a 12V battery), lower resistance allows more current to flow (I = V/R), which increases the Ampere-turns and strengthens the magnetic field. However, if you are driving the coil with a constant-current source, wire thickness will not change the magnetic strength at all.
What is the 'skin effect' and how does it relate to magnetic fields in wire?
When alternating current (AC) flows through a wire, it generates a changing magnetic field inside the conductor itself. This internal magnetic field induces eddy currents that oppose the main current flow in the center of the wire, forcing the electrons to travel only along the outer 'skin'. At 60 Hz mains frequency, the skin depth in copper is about 8.5 mm. At 100 kHz (typical SMPS frequency), the skin depth shrinks to roughly 0.2 mm. This is why high-frequency transformer windings use Litz wire (many individually insulated thin strands) rather than a single thick solid core.
