Until the early nineteenth century, electricity and magnetism were thought to be completely unrelated phenomena. That changed in 1820 when the Danish physicist Hans Christian Ørsted discovered that an electric current flowing through a wire deflected a nearby compass needle. This simple experiment provided the first clear evidence that moving electric charges produce magnetic fields, establishing the connection between electricity and magnetism.

An electromagnet is a magnet whose magnetic field is produced by an electric current. Unlike a permanent magnet, an electromagnet can be switched on and off simply by controlling the current, and its magnetic strength can be adjusted by changing the amount of current flowing through the wire. This controllability makes electromagnets indispensable in modern technology.

Electromagnets are used in countless applications, including electric motors, generators, relays, loudspeakers, magnetic locks, particle accelerators, and medical devices. One of the most familiar biomedical applications is magnetic resonance imaging (MRI), where extremely strong superconducting electromagnets produce the magnetic fields needed to generate detailed images of soft tissues without using ionizing radiation.

One of the first clues that electromagnets and permanent magnets produce the same type of magnetic field comes from the behavior of iron filings. As shown in Figure 35.4, the pattern formed around a current-carrying coil closely resembles that around a permanent bar magnet. In both cases, the filings align with the magnetic field, revealing its shape.

This similarity shows that electromagnets and permanent magnets share the same basic properties: both have north and south poles, like poles repel, unlike poles attract, and magnetic poles always occur in pairs.

Electromagnets become much stronger when a ferromagnetic core, such as iron, is placed inside the coil. The magnetic field generated by the current aligns many of the magnetic domains within the core, greatly increasing the overall magnetic field. This combination of an electric current and a ferromagnetic material produces magnets that are far stronger than either component alone.

Large electromagnets with iron cores are used in scrapyard cranes, industrial machinery, research laboratories, and particle accelerators. When extremely strong magnetic fields are required, superconducting electromagnets are often used. Because superconductors have essentially zero electrical resistance, they can carry enormous currents without overheating, enabling magnetic fields far stronger than those produced by ordinary electromagnets.

The combination of electromagnets and ferromagnetic materials is also essential for storing digital information. Tiny magnetic regions within specially designed materials can be magnetized in different directions to represent binary data (0s and 1s). Although modern solid-state drives are increasingly common, magnetic hard disk drives remain widely used because they provide inexpensive, high-capacity data storage.

Current: The Source of All Magnetism

In the previous section, we learned that an electromagnet produces a magnetic field whenever an electric current flows through a wire. This naturally raises an important question: if permanent magnets produce magnetic fields without an obvious electric current, where does their magnetism come from?

The answer is that magnetism ultimately originates from the motion and intrinsic properties of electric charges. Inside every atom, electrons possess magnetic properties because they move around the nucleus and because they have an intrinsic property called spin. Each electron therefore behaves like a tiny magnetic dipole. In most materials, these tiny magnetic dipoles point in random directions, so their magnetic effects cancel. In ferromagnetic materials, however, many electron magnetic moments align within magnetic domains, producing a strong overall magnetic field.

Although the complete explanation requires quantum mechanics, the important idea for this course is simple: all magnetic fields ultimately arise from electric charges and their motion. This unifies electricity and magnetism into a single branch of physics known as electromagnetism.

One striking difference between electricity and magnetism is that electric charges can exist individually as positive or negative charges, whereas isolated north or south magnetic poles have never been observed. Every magnet that has ever been studied has both a north pole and a south pole. Even if a magnet is cut into smaller pieces, each piece becomes a new magnet with its own pair of poles.

Scientists have long searched for hypothetical particles called magnetic monopoles, which would behave as isolated north or south magnetic poles. Their discovery would profoundly change our understanding of electromagnetism. Although some modern theories predict that magnetic monopoles could exist, no experimental evidence for them has yet been found.

Interactive Exploration: Magnets and Electromagnets

Magnetic fields can be produced either by permanent magnets or by moving electric charges. In this simulation, you will explore both types of magnetism by observing how a compass responds to a bar magnet and by building an electromagnet using a battery and a coil of wire.

As you investigate, notice how the direction of the magnetic field changes around a magnet and how the strength of an electromagnet depends on the electric current flowing through the coil. These observations will help you understand the close relationship between electricity and magnetism introduced in this chapter.