The Principle of Wireless Charging: Revealed Through a Simple Experiment—It’s Not Magic

Brands like Huawei, Xiaomi, and Apple, as well as modern tech products like Tesla cars, widely use wireless charging technology. This technology has greatly simplified our lives, but behind its seemingly complex façade lies a simple physical principle. You might be surprised to learn that these principles were covered in middle school physics. Let’s dive into how wireless charging works and uncover the secrets behind it.

Direct Current vs. Alternating Current

First, we need to understand the basic concepts of Direct Current (DC) and Alternating Current (AC). Direct Current refers to an electric current that flows in a single direction. For example, when you connect a battery to a light bulb, the current flows from the positive terminal to the negative terminal of the battery, lighting up the bulb. This is direct current.

Alternating Current, on the other hand, is different. When you connect a light bulb to an outlet, the bulb lights up because of the AC flowing through it, but the direction of the current alternates continuously. In the United States, this alternating current changes direction 60 times per second, meaning the current’s direction reverses 60 times per second.

The Principle of Electromagnetic Induction

The key to wireless charging lies in electromagnetic induction. Here’s how you can understand this principle through a simple experiment:

1. Experimental Observation: Insert a magnet into a coil, and you’ll see the needle of an ammeter move to the right and then return to its original position, indicating that current is generated. When you remove the magnet, the needle moves to the left and then returns to its original position, showing that current is generated in the opposite direction. Inserting or removing the magnet quickly will make the ammeter needle move more, because the current generated is stronger.
2. Changing Magnetic Fields: When the magnet is inserted into the coil, the free electrons inside the coil start to flow, creating a magnetic field that resists the magnet’s entry. When the magnet is removed, the coil generates a force to resist the magnet’s departure. This current generated by the changing magnetic field is called induced current. The strength of the induced current depends on the rate of change of the magnetic field and the number of turns in the coil.

Application of Wireless Charging Technology

So, how does the principle of electromagnetic induction apply to wireless charging technology? The working principle of wireless charging is quite similar to the experiment described above.

1. Charging Coil: When the coil in the charger is energized, it generates a changing magnetic field. This changing magnetic field is caused by the alternating current. Since the direction of the alternating current continuously changes, the magnetic field also changes constantly.
2. Receiving Coil: When the receiving coil in a smartphone comes close to the charging coil, the changing magnetic field induces a current in the receiving coil. Just like the moving magnet in the experiment, the alternating current causes the magnetic field in the coil to continuously change, so current can flow continuously.

The clever aspect of wireless charging technology is that it doesn’t require you to move your phone around like the moving magnet. The alternating current in the charging coil provides a continuously changing magnetic field, allowing the phone to charge as long as it is near the charger.