In this blog post, we’ll examine how products commonly found around us—including anti-theft devices at grocery stores—utilize the principle of electromagnetic induction, and explore how this fundamental principle is applied in real-world devices.
What is electromagnetic induction, and how does it occur?
When we go to the checkout counter after shopping at a supermarket, we often see two large pillars standing on either side. These are devices installed to prevent theft; if you try to pass through the exit with an unpaid item, a beeping alarm sounds. Therefore, if you attempt to leave the store with an unpaid item, the alarm will go off, and you’ll be stopped by a store employee. So, what exactly is the principle behind these anti-theft devices?
Copper Cylinders and Magnets
Before explaining the principle of anti-theft devices, I’d like to explain the underlying theory of electromagnetic induction to aid your understanding. The term “electromagnetic induction” may sound unfamiliar, but it is a phenomenon that occurs quite frequently around us. Before explaining this, let’s note that electricity and magnetism are very closely related. This can be seen in the following phenomenon: If you run an electric current through a wire in a laboratory and place a compass next to it, what happens? The compass needle begins to move. Why does this happen? It is because a magnetic field is generated around the wire when an electric current flows through it. As such, electricity and magnetism are inextricably linked. Based on this relationship between electricity and magnetism, the phenomenon of electromagnetic induction occurs; electromagnetic induction refers to the generation of an electric current due to changes in a magnetic field. More precisely, when the magnetic flux passing through a wire or coil changes, an induced electromotive force (EMF) is generated, and if the circuit is closed, an induced current flows. The dictionary definition may seem somewhat difficult, but it becomes easier to understand when examined through the following example. Suppose there is a copper cylinder about 1 meter long and a bar magnet in a laboratory. After securing the copper cylinder vertically, drop the magnet onto it. How long will this take? Will it take the same amount of time as dropping it in free air? No, it takes much longer. This is precisely due to the phenomenon of electromagnetic induction. The principle is as follows. First, a magnet has its own magnetic field. Therefore, the moment the magnet approaches the copper cylinder, the magnetic flux passing through the cylinder changes. In other words, a change in the magnetic field occurs within the copper cylinder. This induces an electric current in the copper cylinder, and the magnetic field generated by this current is oriented in a direction that opposes the change in magnetic flux caused by the magnet’s motion. This is Lenz’s Law. As a result, a force arises that opposes the magnet’s fall, causing it to fall much more slowly than it would in air. Of course, since the force of gravity continues to act downward on the magnet, it eventually falls downward. However, as long as the magnet is moving, the magnetic flux passing through the copper cylinder continues to change, inducing an electric current; the magnetic field generated by this current opposes the change in the magnet’s motion. Consequently, the magnet’s rate of descent slows down. In other words, this is the very principle of electromagnetic induction.
Electromagnetic Induction and Anti-Theft Devices
Let’s apply this to an anti-theft device. The posts of an anti-theft system contain coils and electronic circuits for transmitting and receiving signals, which generate a specific electromagnetic field and detect the response from anti-theft tags attached to objects. Many items have anti-theft tags attached along with barcodes. These tags do not have the same structure as simple magnets; depending on the method used, there are various types, such as radio frequency (RF) systems that utilize circuits and resonant structures to respond to electromagnetic fields, or acoustic-magnetic (AM) systems that utilize the properties of magnetic materials.
For example, some acoustic-magnetic tags use magnetic materials that respond to alternating magnetic fields of a specific frequency to send a signal back to the detection system. Therefore, when a tag passes through the detection zone, an electromagnetic interaction occurs between the device at the entrance/exit and the tag, and the device can detect this and sound an alarm. In this regard, anti-theft devices can be considered practical applications of electromagnetic induction, electromagnetic resonance, and detection technology in everyday life.
But why can we walk out without setting off an alarm, even though we don’t necessarily remove the anti-theft tag attached to the item after paying? It’s because the tag is deactivated or rendered unresponsive to detection during the checkout process. Although the methods vary depending on the type of anti-theft system, the tags on paid items are processed so that they no longer trigger a normal alarm signal from the detection devices at the exit. Therefore, even if you carry a paid item into the detection zone at the exit, the system does not interpret it as a theft. In other words, rather than simply explaining it as “removing the tag’s magnetic field,” it is more accurate to understand that, depending on the anti-theft technology used, the tag’s electromagnetic response is deactivated or its detectable state is altered.
Electromagnetic Induction in Everyday Life
As we’ve seen, the anti-theft devices we’ve casually walked past all along are based on the scientific principles of electromagnetic induction and electromagnetic detection technology. These technologies are used not only in supermarkets but also in various other locations, such as libraries, to detect when items are being removed. Furthermore, metal detectors used in security screenings at airports and other locations operate by detecting magnetic changes induced in metal objects using an electromagnetic field that varies over time. This is how they check whether we are carrying any metal objects when we pass through a metal detector.
There are other products that utilize the principle of electromagnetic induction as well. Microphones, commonly found in karaoke rooms, and generators that produce electricity are also prime examples of devices that utilize the interaction between electricity and magnetism. In particular, generators are representative applications of electromagnetic induction, generating electricity by changing the magnetic flux through motion. Many products and devices we encounter in our daily lives utilize this relationship between electricity and magnetism in this way, and electromagnetic induction is not merely a phenomenon confined to the laboratory but serves as the foundation for various technologies that make our lives more convenient.
