Direct current is a flow of electric charge in only one direction and has a steady state of constant voltage in the circuit. Rectifiers, batteries, commutator-equipped generators, and fuel cells are some examples of devices that generate direct current. Nowadays, most applications use a time-varying voltage source. Alternating current is a flow of electric charge that periodically reverses direction. An alternating current is produced by an alternating emf that is generated in a power plant. If the alternating current source varies periodically, particularly sinusoidally, the circuit is known as an alternating current circuit. Some electrical devices, such as fans, bulbs, air conditioners, and motors, run on alternating current. Different alternating current voltages and frequencies are used in homes, offices, and other places around the world. The potential difference between two electrical outlets in a typical home alternates sinusoidally with a frequency of 60 Hz or 50 Hz and an amplitude of 156 V or 311 V depending on whether you live in the United States or Europe, respectively. The electrical outlets in the US and Europe have a potential difference of 120 V or 220 V, respectively. However, these voltages are the typical voltages in the outlets, not the peak values. When a resistor is connected to an alternating current voltage source, the voltage and current vary in time across the resistor. The voltage fluctuates sinusoidally with time at a fixed frequency on either the battery terminals or the resistor.