generators and microphones

• alternators rotate a coil in a magnetic field. As the coil spins, a current is induced in the coil. this current changes direction every half turn. ac generators have slip rings and brushes so the contacts don't swap every half turn, this means they produce an alternating potential difference
• dynamos have a split - ring commutator, this swaps the connection every half turn to keep the current flowing in the same direction
• oscilloscopes show how the PD generated in the coil changes overtime. . for ac this is a line that goes up and down crossing the horizontal axis. for dc the line stays above the axis (pd is always positive) so it's still direct current. the height of the line is the generated potential difference at that time. increasing frequency of revolutions increases overall pd.
• microphones create current from sound waves. lastly, the changing pattern of potential difference is passed through an amplifier and then into a moving coil loudspeaker, increasing volume of sound
• microphones are loudspeakers in reverse. soundwaves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet, causing the coil of wire to move in the magnetic field, generating a current. the movement of the coil depends on the properties of the sound wave. louder sounds make diaphragm move further. microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit