A magnetic field is an invisible force field that surrounds any magnet or moving electric charge. It’s the reason a compass needle swings north, why magnets stick to your fridge, and how an MRI machine can see inside your body. You can’t see a magnetic field directly, but its effects are everywhere, from the Earth’s core to the tiny speaker in your phone.
How Magnetic Fields Are Created
Every magnetic field traces back to the same source: moving electric charges. When electrons flow through a wire, they generate a magnetic field around that wire. When electrons orbit and spin inside an atom, they create tiny magnetic fields too. In most materials, these atomic-level fields cancel each other out. In magnetic materials like iron, they line up and reinforce each other, producing a field strong enough to pull on a paperclip from a distance.
This is why electricity and magnetism are so closely linked. Run a current through a coil of wire and you get an electromagnet. Spin a magnet inside a coil of wire and you generate electricity. The two forces are really two sides of the same coin, which is why physicists group them together as “electromagnetism.”
What Field Lines Tell You
Scientists visualize magnetic fields using field lines, the curved paths you’ve probably seen in textbook diagrams of bar magnets. These lines aren’t physical objects, but they encode real information. They flow from the north pole of a magnet to the south pole. A compass needle placed anywhere along a line will point in the direction the line travels.
The spacing of the lines tells you how strong the field is at that point. Where lines are packed closely together (near the poles), the field is strong. Where they spread apart (farther from the magnet), the field is weak. The strength is exactly proportional to the number of lines passing through a given area.
How It’s Measured
The standard unit for magnetic field strength is the tesla (T), named after inventor Nikola Tesla. One tesla is a very strong field. For weaker fields, scientists use the gauss, where 1 tesla equals 10,000 gauss. Earth’s magnetic field at the surface measures roughly 30 microteslas, or about 0.3 gauss. That’s weak enough that you need a sensitive compass to detect it, but strong enough to guide migrating birds across continents.
For comparison, a typical fridge magnet produces about 5 millitesla at its surface. A clinical MRI scanner operates at 1.5 or 3.0 tesla, meaning it generates a field roughly 100,000 times stronger than Earth’s. Research MRI machines reach 7 tesla or higher.
How Materials Respond to Magnetic Fields
Not all materials react to magnets the same way. Materials fall into three broad categories based on their magnetic behavior.
- Ferromagnetic materials like iron, nickel, and cobalt respond powerfully. They can produce magnetizations many orders of magnitude stronger than the field applied to them, which is why iron sticks firmly to a magnet. These materials can also retain magnetism after the external field is removed, which is how permanent magnets work. Interestingly, ferromagnetic materials physically change shape very slightly when magnetized, a phenomenon called magnetostriction.
- Paramagnetic materials like aluminum and platinum are weakly attracted to magnets. Their atoms have small magnetic moments that don’t fully cancel out, so they align with an applied field, but the effect is too faint to feel with your hands.
- Diamagnetic materials like copper, water, and bismuth actually repel magnetic fields, though extremely weakly. All materials are diamagnetic at a basic level, but this effect is so subtle it’s only noticeable when the other two types of magnetism are absent. Superconductors are the extreme case: they perfectly repel magnetic fields, which is how magnetic levitation demonstrations work.
What Magnetic Fields Do to Moving Charges
A magnetic field exerts a force on any electric charge that moves through it. The force pushes the charge sideways, perpendicular to both its direction of travel and the direction of the field. This is fundamentally different from gravity or electric fields, which pull objects along a straight line toward or away from their source.
This sideways push is why charged particles spiral in magnetic fields rather than flying straight. It’s the principle behind electric motors (where current-carrying wires are pushed by magnets to spin a shaft) and particle accelerators (where magnetic fields bend the paths of subatomic particles into circles). The force depends on the charge’s speed, so a stationary charge sitting in a magnetic field feels no magnetic force at all.
Earth’s Magnetic Field
Earth generates its own magnetic field deep in its core, where convection currents in molten iron create a natural dynamo. This field extends tens of thousands of kilometers into space, forming a protective bubble called the magnetosphere. It shields the planet from the solar wind, a stream of charged particles from the Sun that would otherwise strip away the atmosphere over time.
Researchers at the University of Oxford found evidence that this field has existed for at least 3.7 billion years. Ancient rocks from that era captured a field strength of at least 15 microtesla, roughly half the modern value, suggesting Earth has had magnetic protection for most of its history.
The field isn’t perfectly stable. The magnetic poles wander slowly over time, and the north and south poles have completely flipped hundreds of times over Earth’s history. The last reversal happened about 780,000 years ago.
Electromagnets and How to Make Them Stronger
An electromagnet is simply a coil of wire carrying electric current. Unlike a permanent magnet, you can turn it on and off, and you can control its strength. The field inside the coil depends on two things you can adjust: the amount of current flowing through the wire and the number of loops packed into a given length of coil. Double the current and you double the field. Triple the loops per meter and you triple it.
This tunability makes electromagnets extraordinarily useful. Small ones trigger your doorbell chime. Massive ones power MRI scanners, steer particle beams at research facilities, and lift cars at scrap yards. Wrapping the coil around an iron core dramatically boosts the field, because the iron’s ferromagnetic properties amplify the effect many times over.
Magnetic Fields in Everyday Life
You interact with magnetic fields dozens of times a day without thinking about it. Your phone’s speaker works by vibrating a coil inside a permanent magnet’s field to produce sound waves. The electric motor in a washing machine, a blender, or an electric car converts magnetic force into spinning motion. Computer hard drives store data by magnetizing tiny spots on a spinning disk, with each spot representing a 1 or a 0.
Around the house, magnetic strips seal your refrigerator door to save energy, hold knives on wall-mounted racks, and keep cabinet doors latched. Credit card stripes encode account information magnetically. Even a simple doorbell uses an electromagnet to strike a chime when you press the button.
Safety and Exposure Limits
At everyday levels, magnetic fields pose no known health risk. Earth’s field surrounds you constantly, and the fields from household magnets are far too weak to affect your body in any measurable way. The International Commission on Non-Ionizing Radiation Protection sets the general public exposure limit for static magnetic fields at 400 millitesla for any part of the body. For occupational settings, workers can be exposed to fields up to 2 tesla around the head and trunk.
The main safety concerns involve very strong fields, like those in MRI rooms. At those strengths, magnetic fields can pull on metal objects with dangerous force, turning a forgotten wrench into a projectile. People with certain medical implants, particularly pacemakers or devices containing ferromagnetic components, face additional risks because strong fields can interfere with electronic circuits or physically move implanted metal. This is why MRI facilities screen patients carefully before scans.