Electricity 1 Topics in Electricity Monitoring AC Alternating current electricity is made by electrical generators. The current and voltage change over time. Circuit Theory Looking at circuit theory. Finding the relationships between potential difference, current and resistance. Internal Resistance All electrical sources can be considered a source of emf with an internal resistance. Capacitors Capacitors are such an important component in electrical devices. They are used in many timing circuits and can block ac signals. Semiconductors Semiconductors and pn junctions are found in all electronic equipment.