What Is the Relationship Between Electricity and Magnetism Called? Electromagnetism Explained

The relationship between electricity and magnetism is called electromagnetism, a fundamental physical interaction where moving electric charges generate magnetic fields, and changing magnetic fields induce electrical voltage in a conductor. In a real circuit or installation, this relationship is not just theoretical; it dictates inductive reactance in AC systems, causes destructive voltage spikes when relay coils de-energize, and governs how transformers step voltage up or down across your main service panel.

Understanding electromagnetism is the dividing line between simply wiring components together and actually engineering a reliable system. Below, we break down the core mechanics, look at real component specifications, and explore how this force dictates your daily work on the bench and in the field.

The Core Mechanics of Electromagnetism

Electromagnetism operates on two primary principles: Ampere’s Law and Faraday’s Law of Induction. When current flows through a wire, it generates a concentric magnetic field around the conductor. Conversely, if you move a magnet past a wire—or change the magnetic field strength around a stationary wire—you induce an electromotive force (voltage) across that wire. This is the exact mechanism that allows alternators to generate grid power and electric motors to convert electrical energy into mechanical torque.

Worked Numeric Example: Calculating Relay Pull Force

Let’s look at a real-world component: the ubiquitous Omron G5V-2 12V DC signal relay. When you apply 12V to the coil, how much magnetic field strength are you actually generating to pull the mechanical contacts closed?

• Coil Resistance (R): 288 Ω (measured at 20°C)
• Applied Voltage (V): 12V DC
• Current (I): Using Ohm's Law, I = V / R = 12V / 288Ω = 0.0416 A (41.6 mA)
• Number of Turns (N): Approximately 800 turns of fine enameled copper wire

The Magnetomotive Force (MMF) is calculated as N × I:

MMF = 800 turns × 0.0416 A = 33.3 Ampere-turns.

If the magnetic core length (l) of the relay is roughly 0.04 meters, the magnetic field strength (H) is:

H = MMF / l = 33.3 / 0.04 = 832.5 Amperes per meter (A/m).

This specific field strength is precisely engineered to overcome the spring tension of the relay's armature, closing the contacts reliably without overheating the 288-ohm coil. If you were to apply 24V to this 12V coil, the current would double to 83.2 mA, the field strength would jump to 1665 A/m, and the coil would rapidly overheat and burn out due to excessive I²R power dissipation.

Electromagnetic Parameters in Real Components

Every inductive component you install relies on the electricity-magnetism relationship. However, the way they utilize magnetic fields varies wildly based on their core material, winding geometry, and intended frequency. The table below compares the electromagnetic specifications of four common components you will encounter in control panels and power supplies.

Notice the drastic difference in inductance. The toroidal transformer has a massive 4.5 H inductance because it is designed to store and transfer energy via a continuously changing magnetic field at 60 Hz. The stepper motor, conversely, has only 2.8 mH of inductance because it must switch current on and off thousands of times per second; high inductance would limit the rate of current rise (di/dt) and stall the motor at high speeds.

Where You Meet This in Practice

The electricity-magnetism relationship forces specific design choices in real installations. Ignoring these physical realities leads to destroyed components, noisy data lines, and failed inspections.

1. Inductive Kickback and Flyback Diodes

When current flows through an inductor (like a solenoid valve or relay coil), energy is stored in the magnetic field. If you suddenly open a switch or transistor to cut the power, the magnetic field collapses instantly. According to Faraday’s Law of Induction, this rapid change in flux induces a massive voltage spike to keep the current flowing. The formula is V = -L(di/dt). Because the time (dt) is near zero, the voltage (V) can spike to hundreds or thousands of volts, instantly destroying the controlling MOSFET or transistor.

The Fix: Always wire a flyback diode (like a 1N4007) in reverse parallel across DC inductive loads. When the switch opens, the diode provides a safe recirculation path for the collapsing magnetic energy to dissipate as heat.

2. Transformer Action and Ground Loops

Transformers rely entirely on a changing magnetic field linking a primary and secondary coil. This provides galvanic isolation, which is critical for safety. However, parasitic capacitance and stray magnetic flux can couple high-frequency noise between windings. In sensitive audio or instrumentation installations, this is why we use electrostatically shielded transformers and ensure inductors are mounted at 90-degree angles to one another to minimize mutual inductance and crosstalk.

3. Skin Effect in High-Frequency Wiring

In DC circuits, current flows evenly across the entire cross-section of a wire. In AC circuits, the changing magnetic field generated by the current induces eddy currents within the wire itself, pushing the primary electron flow to the outer edge (the 'skin') of the conductor. At 60 Hz mains frequency, this effect is negligible for standard AWG wire. But in high-frequency applications like induction heaters, solar inverters, or RF transmitters operating at 20 kHz or higher, the skin effect drastically increases effective resistance. This is why high-frequency installations require Litz wire (many individually insulated thin strands) or hollow copper tubing.

Common Confusions and Field FAQs

When discussing the relationship between electricity and magnetism, terminology often gets tangled. Here is what people commonly confuse it with, followed by answers to frequent bench questions.

Permanent Magnetism vs. Electromagnetism

Beginners often confuse the static magnetic field of a permanent magnet (like a neodymium fridge magnet) with electromagnetism. Permanent magnetism arises from the quantum spin alignment of electrons within the material's atomic structure. It requires no external power source and cannot be dynamically scaled. Electromagnetism requires active current flow; the moment you remove the electrical power, the magnetic field collapses (save for minor residual hysteresis in the core material).

Magnetic Flux (Webers) vs. Flux Density (Teslas)

These two are constantly mixed up on spec sheets. Magnetic flux (Φ), measured in Webers (Wb), is the total amount of magnetic field passing through a given area. Think of it as the total volume of water flowing through a pipe. Magnetic flux density (B), measured in Teslas (T), is the concentration of that flux per square meter. It is the 'pressure' or intensity of the magnetic field at a specific point. A transformer core might have a high total flux, but if the cross-sectional area is large, the flux density (and the risk of core saturation) remains low.