Magnetism and Electromagnets
From compass needles to MRI machines
From compass needles to MRI machines
Magnetism is a force that you have probably known about since elementary school. You played with magnets and discovered that sometimes they snap together and sometimes they push apart. You might have hung a magnet on a refrigerator or used one to pick up paper clips. But what is actually happening when magnets attract or repel each other? The answer lies in the behavior of tiny particles called electrons that swirl around atoms.
Every magnet is surrounded by an invisible field called a magnetic field. This field is not something you can see with your eyes, but its effects are very real. It exerts a force on other magnets and on certain materials like iron, nickel, and cobalt. The magnetic field is strongest at the poles of the magnet and gets weaker as you move farther away. Scientists represent magnetic fields visually using field lines — curved lines that show the direction and strength of the force at different points around the magnet.
Every magnet has two poles: a north pole and a south pole. These are the points where the magnetic force is strongest. The naming convention comes from how magnets behave in the Earth's magnetic field — a magnet's north pole naturally points toward the Earth's geographic north (which is actually a magnetic south pole, but that is a confusing detail scientists have agreed to live with). The rule for magnetic poles is simple: like poles repel, and opposite poles attract. Two north poles pushed together will push apart. A north pole and a south pole will pull toward each other.
One of the most important things to understand about magnetic poles is that they cannot exist alone. If you take a bar magnet and cut it in half, you do not get an isolated north pole and an isolated south pole. You get two smaller magnets, each with its own north and south pole. No matter how many times you cut a magnet, you always get smaller magnets with two poles. This is fundamentally different from electric charges, where positive and negative charges can exist independently.
Scientists use magnetic field lines to visualize and understand magnetic fields. These lines appear to flow from the north pole to the south pole outside the magnet, and from south to north inside the magnet. The closer the lines are to each other, the stronger the magnetic field is in that region. At the poles, where the field is strongest, the lines are packed closely together. Far from the magnet, the lines spread out and the field is weak.
You can see field lines yourself with a simple experiment. Place a bar magnet on a table and cover it with a piece of paper. Sprinkle iron filings (tiny shavings of iron available at hardware stores) lightly over the paper and gently tap it. The filings will align along the invisible field lines, creating a visible pattern that shows exactly how the field curves around the magnet. You will see the lines emerge from each pole and curve through the air to connect with the other pole, forming a beautiful symmetrical pattern.
The Earth itself is a giant magnet. Deep inside our planet, liquid iron moving in the outer core generates a magnetic field that extends thousands of kilometers into space. This field is what makes a compass work. The compass needle is a small magnet that is free to rotate. It aligns itself with Earth's magnetic field, with its north pole pointing toward the magnetic south pole near the Earth's geographic north. Sailors, hikers, and explorers have used compasses for navigation for over a thousand years, and they rely entirely on Earth's magnetic field to work.
Earth's magnetic field does more than just make compasses work. It acts as a shield that protects the planet from charged particles coming from the sun and outer space. These particles, called the solar wind, would strip away the atmosphere and expose the surface to harmful radiation without the magnetic field's protection. The auroras — the northern and southern lights — happen when charged particles from the solar wind are funneled by the magnetic field toward the poles and interact with gases in the atmosphere, creating spectacular displays of light.
Not all materials respond to magnets. Iron, nickel, cobalt, and some alloys of these metals are called ferromagnetic materials because they are strongly attracted to magnets and can themselves be magnetized. These materials have tiny magnetic regions called domains inside them. In an unmagnetized piece of iron, the domains point in random directions, so their magnetic effects cancel out. When you bring a magnet close, the domains align with the magnet's field, causing the iron to be attracted.
If you stroke a piece of iron with a magnet in the same direction repeatedly, you can align the domains and turn the iron itself into a magnet, at least temporarily. This is how you can magnetize a needle to use as a compass needle. However, the magnetization is not permanent — heating, dropping, or hammering the material can jumble the domains and destroy the magnetization. Steel, which contains iron, can hold a magnetization much better than pure iron, which is why steel is often used to make permanent magnets.
Here is something remarkable: electricity and magnetism are not separate phenomena. They are two aspects of the same force, called electromagnetism. When an electric current flows through a wire, it creates a magnetic field around the wire. This discovery, made by Hans Christian Oersted in 1820, was revolutionary because it meant that you could create a magnet using electricity. These are called electromagnets.
The simplest electromagnet is a coil of wire wrapped around a nail and connected to a battery. When current flows through the coil, the nail becomes magnetized. When the current is turned off, the nail loses its magnetization. This is what makes electromagnets so useful — they can be turned on and off at will. Increase the current, and the magnet gets stronger. Wrap more turns of wire around the core, and the magnet also gets stronger. Electromagnets can be made far more powerful than permanent magnets, and their strength can be adjusted instantly by changing the current.
A solenoid is essentially a coil of wire, usually shaped in the form of a cylinder. When an electric current flows through the coil, it creates a magnetic field inside the coil. The magnetic field is strongest at the center of the coil. If you put a metal rod inside the coil, the rod will be pulled in or pushed out depending on the direction of the current. Solenoids are the mechanism behind many everyday devices that convert electrical signals into physical motion.
Think about an automatic door lock on a building. When you present a key card, the system sends a small electrical current to a solenoid, which pulls a bolt back and unlocks the door. The same principle is used in the starter motor of a car — when you turn the ignition, a solenoid pushes the starter gear into engagement with the engine's flywheel. In a junkyard, a large electromagnet attached to a crane lifts scrap metal, and when the operator turns off the current, the magnet releases its grip and drops the metal. All of these are practical applications of the solenoid principle.
Electromagnets are everywhere in the modern world, often in places you would not think to look. Your phone or computer contains speakers that work using electromagnets. Inside a speaker, a cone is attached to a small electromagnet. When music signals pass through the coil, the electromagnet pushes and pulls against a permanent magnet, making the cone vibrate and produce sound. Louder sounds come from larger vibrations, which come from stronger magnetic forces.
Magnetic Resonance Imaging (MRI) machines, used in hospitals to create detailed images of the inside of the human body, use incredibly powerful electromagnets — so powerful that the machines weigh several tons and require enormous coils of superconducting wire cooled to near absolute zero with liquid helium. The MRI's magnetic field aligns the hydrogen atoms in your body, and then the machine detects the signals these atoms emit as they return to their normal state, building a detailed image from that data.
Other uses of electromagnets include: the motors in electric cars and appliances, which use magnetic forces to produce rotational motion; railway systems like maglev trains that use magnetic levitation to float above the tracks, eliminating friction; metal detectors at airports and beaches that use electromagnets to detect conductive metals; and recycling facilities that use large electromagnets to separate iron and steel from other materials. Without electromagnets, the modern world as we know it would simply not exist.
While permanent magnets are generally harmless, strong electromagnetic fields can damage electronic devices. Credit cards have magnetic strips that can be erased by strong magnets, which is why you should keep them away from speaker magnets. Headphones can pick up hum from nearby electromagnetic interference. Pacemakers and other medical devices can be affected by strong magnetic fields, which is why patients are screened before entering areas with MRI machines.
The interplay between electricity and magnetism has given humanity one of the most powerful forces for progress in history. From the compass needle that guided ancient sailors to the MRI machines that save lives today, magnetism — both natural and electromagnetic — has shaped human civilization in ways that are both profound and practical.