When the card is placed within range of the reader, the antenna coil and capacitor, which form a tuned circuit, absorb and store energy from the field, resonating at the frequency emitted by the reader. This energy is rectified to direct current which powers the integrated circuit. The chip sends its ID number or other data to the antenna coil, which transmits it by radio frequency signals back to the reader unit. The reader checks whether the ID number from the card is correct, and then performs whatever function it has been programmed to do for that ID number. All the energy to power the card comes from the reader unit, so passive cards must be close to a reader to transmit their data.