Physics high-school May 24, 2026

How Does Wireless Phone Charging Work?

Energy transfer through changing magnetism

A phone sitting on a wireless charging pad with magnetic field lines linking a coil in the pad to a coil inside the phone.

A wireless charger uses electricity in the pad to make a changing magnetic field. That changing field passes through a coil inside the phone and makes electricity there. The phone uses that electricity to charge its battery, with some energy lost as heat.

Big Idea. NGSS HS-PS3-5 connects wireless charging to devices that convert energy from one form to another.

Wireless charging feels like a small trick because there is no plug going into the phone. The charger is still moving energy from the wall outlet to the battery. It just uses magnetic fields instead of metal contacts. Inside the charging pad is a coil of wire. Inside the phone is another coil. When alternating current moves through the pad coil, the magnetic field around it grows, shrinks, and flips direction many times each second. That changing field reaches the phone coil and pushes charges in that wire. The phone then changes that electrical energy into a form the battery can store. This process is electromagnetic induction. It is the same physics behind many generators and transformers. The main limits are distance, alignment, coil size, and heat. A small gap can work well. A large gap wastes much more energy.

Two coils, no plug

A cutaway view of a wireless charging pad and phone showing a primary coil in the pad and a secondary coil in the phone. — Wireless charging starts with two nearby coils

A wireless charger is built around a coil in the pad and a coil in the phone. The pad coil is called the primary coil because it receives energy from the outlet. The phone coil is called the secondary coil because it receives energy from the magnetic field. The two coils do not touch. They only need to be close enough for the changing magnetic field from one coil to pass through the other. Most phone chargers use a flat spiral shape. That shape fits inside a thin pad and inside a thin phone. The phone also has circuits that control charging, protect the battery, and stop charging when needed. The pad and phone usually communicate before full power begins. This helps the charger send the right amount of power and avoid heating stray metal objects.

The phone is not powered by contact. It is powered by a linked magnetic field.

Changing current makes changing magnetism

A pad coil with alternating current arrows and magnetic field loops that expand through the phone coil above it. — Alternating current creates a changing magnetic field

A steady current makes a steady magnetic field. Wireless charging needs a changing field, so the pad uses alternating current. Alternating current reverses direction over and over. As the current changes, the magnetic field around the pad coil also changes. The field grows, collapses, and reverses direction. This changing magnetic field spreads through the small gap and through the phone coil. The phone coil does not need a battery to start feeling that field. The field is part of the space around the pad coil. The faster and stronger the change, the larger the effect in the receiving coil. Chargers are designed to use frequencies and power levels that work with their coils and safety systems. This is why a phone may charge poorly on an incompatible pad.

A changing current is what makes the magnetic field useful for charging.

Induction pushes charges

A changing magnetic field passing through a phone coil and causing charge motion around the wire. — A changing field induces voltage in the phone coil

Electromagnetic induction happens when a changing magnetic field passes through a loop of wire and produces a voltage. The voltage pushes charges around the phone coil. In physics terms, the changing magnetic flux through the coil matters. A simple version of Faraday's law is $\mathcal{E}=-N\frac{\Delta \Phi_B}{\Delta t}$. The symbol $N$ means the number of turns in the coil. More turns can increase the induced voltage. A faster change in magnetic flux can also increase it. The minus sign shows that the induced effect opposes the change that caused it. That idea is called Lenz's law. In a phone, the induced current is not sent straight into the battery. Circuits first convert and regulate it so the battery charges safely.

Induction turns a changing magnetic field back into electrical energy.

Distance and alignment matter

Two side-by-side examples comparing aligned coils with strong field linkage and misaligned coils with weak field linkage. — Small changes in position can change charging efficiency

Wireless charging works best when the coils are close and lined up. The magnetic field from the pad coil is strongest near the coil. Farther away, less of that field passes through the phone coil. A thick case, a phone that is off center, or a gap caused by a stand can reduce the energy transfer. The charger may respond by lowering power or stopping. Alignment matters because the phone coil only receives the part of the field that passes through its loop area. If the coils overlap poorly, much of the field misses the receiving coil. This is why many chargers use magnets, guides, or multiple coils. These features help place the phone where the magnetic link is stronger. Better alignment usually means faster charging and less heat.

Efficiency drops when the magnetic field misses the phone coil.

Energy does not all reach the battery

An energy flow diagram showing wall outlet energy moving to a pad coil, magnetic field, phone coil, battery, and heat losses. — Wireless charging converts energy in several steps

Wireless charging is convenient, but it is not perfectly efficient. Some energy becomes heat in the coils, electronic parts, battery, and nearby materials. Heat is a sign that energy has been transferred into random motion of particles instead of stored chemical energy in the battery. Phones and pads include sensors and control circuits to limit heating. If the phone gets too warm, charging may slow down. Efficiency depends on coil design, distance, alignment, power level, and the materials between the pad and phone. A good system keeps the coils close, sends power only when a device is detected, and adjusts power during charging. The energy path is still clear. Electrical energy from the wall becomes magnetic field energy near the pad, then electrical energy in the phone, then stored chemical energy in the battery.

Charging is an energy conversion process, not energy created from nothing.

Vocabulary

Electromagnetic induction: The process in which a changing magnetic field produces a voltage in a conductor.
Primary coil: The coil in the charging pad that uses alternating current to create a changing magnetic field.
Secondary coil: The coil in the phone that receives energy from the changing magnetic field.
Magnetic flux: A measure of how much magnetic field passes through an area, such as the area inside a coil.
Efficiency: The fraction of input energy that becomes useful output energy instead of being lost, often as heat.

In the Classroom

Coil alignment test

30 minutes | Grades 9-12

Use two small coils, an AC signal source, and a voltmeter to measure induced voltage in the receiving coil. Students move the coils closer, farther apart, and off center, then graph how voltage changes.

Energy transfer map

20 minutes | Grades 9-12

Students make a flow diagram for a wireless phone charger. They label each energy form and identify where heat losses can occur.

Faraday's law model

35 minutes | Grades 10-12

Students compare coil turns, changing magnetic field strength, and induced voltage using a simulation or teacher demonstration. They connect each observation to the terms in Faraday's law.

Key Takeaways

• Wireless charging uses two coils separated by a small gap.
• Alternating current in the pad creates a changing magnetic field.
• The changing magnetic field induces voltage in the phone coil.
• Charging works best when the coils are close together and well aligned.
• Some energy is lost as heat, so wireless charging is not perfectly efficient.

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