An amplifier unit can be used to amplify music from devices with small speakers,
such as smartphones. In the amplifier unit there is a coil connected to an amplifier
and a loudspeaker as shown.
A smartphone is placed on the amplifier unit so that the coil in the smartphone
speaker is above the coil in the amplifier unit.
Explain how the music played by the smartphone is also played through the
loudspeaker in the amplifier unit.