Wireless Charging

Strike one tuning fork with a ping, then bring a second one close. Without touching it, the second fork starts to hum and tremble on its own. Wireless charging works in a similar way: without any contact, it uses an invisible magnetic field to pass energy across the gap.

Inside a charging pad is a part called a coil, made of copper wire wound into a circle. When the pad gets electricity from the outlet, waves of magnetic field quickly form around the coil, too faint to see. This field spreads gently through the air and reaches the bottom of your phone.

Inside the back of your phone, a round receiving coil just like the pad's is hiding. When the magnetic waves from the pad pass through and shake the coil inside the phone, a tingle of electric current is induced in the phone's coil and starts to flow.

This newly made electricity flows into the battery and charges it bit by bit. Even with no physical cable bridge, the magnetic field builds an invisible, virtual bridge through the air and quietly delivers the electrical energy.

In physics, this phenomenon is called the law of electromagnetic induction. Discovered by the 19th-century scientist Faraday, it says that when a wire sits where a magnet or a magnetic field is changing in strength, the electrons in the wire get pushed along and a current appears on its own.

Most smartphone wireless chargers we use every day (the Qi standard) rely on this induction method. But it has a clear limit: it only works when the pad's coil and the phone's coil face each other very closely, within a few millimeters, and line up accurately.

If the phone is a little off-center or lifted away from the pad, the magnetic waves scatter in the wrong places and charging efficiency plunges. That's why technology that attaches a round magnet to the back of a phone to snap it into the right position has recently become widespread.

Scientists are also developing "magnetic resonance charging," which can work from a bit farther away. It tunes the two coils to vibrate at the same frequency to pass energy along, so charging works even a few centimeters from the pad. Long-range wireless charging that works anywhere in a room is still at the research stage.

When you use wireless charging, you may notice the back of your phone getting much warmer than usual. Instead of racing straight down a dedicated highway like a cable, the electricity is turned into a magnetic field and then back into electricity. Along the way, some of it is lost as heat in the coil and circuits.

Because not all of the energy makes it across and some leaks out as heat in the coil and circuits, power efficiency is lower than plugging in a cable, and charging takes longer. Too much heat can also wear down battery life, so the device sometimes slows its own charging speed for safety.

That's why some fast wireless charging pads include a tiny cooling fan or a heat-spreading plate to cool things down. Manufacturers also put a lot of effort into fine-tuning the coil layout to cut heat and raise energy transfer efficiency.

It falls short of wired charging in efficiency and speed, but since the device doesn't need an exposed metal charging port, it's easier to build a sealed design that resists water. It also cuts the wear and breakdowns of charging ports caused by plugging and unplugging cables, which is why it's so popular in daily life.