Wireless Electricity Transmission
Magnetic induction is a technology that you will probably remember from your physics classes at high school.
You need two coils, a transmitter coil and a receiver coil. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery.
The presentation below shows how to build a wireless charging system using magnetic induction.
You will see that products need more than coils and alternatining currents. For energy-efficient power transfer the phone must be able to shut down the transmitter when the battery is full. The phone, therefore, needs to send control signals to the charging station.