English: Generic block diagram of a wireless power system that works by (nonresonant) inductive coupling (electromagnetic induction). It works like a transformer. It consists of a "transmitter" unit consisting of an oscillator that produces an alternating current of the correct frequency in a "primary" coil. The alternating current produces an alternating magnetic field (B, green) which extends to the "secondary" coil in the receiver unit and induces a voltage in it by Faraday's law of induction. The alternating current in the secondary is rectified to DC (if DC is needed) and powers a load. This is a simplified version of how wireless charging units for portable devices like the Qi and PowerPad systems work. Wireless charging stands for tools such as electric toothbrushes operate at mains frequency (50/60 Hz) and so do not need the oscillator, but must use an iron core inside the coils in order to transfer power at low frequencies.