Electricity is delivered from power stations to customers as AC because it provides a more efficient way to transport electricity over long distances. Most households use electricity with a frequency of 60Hz.

An amplifier is a device that takes a small input signal and creates a larger output signal. This makes it most similar to a step-up transformer which takes a smaller input voltage and creates a larger output voltage.

All electricity is the movement of electrons which are subatomic particles that orbit the nucleus of an atom. Electrons occupy various energy levels called shells and how well an element enables the flow of electrons depends on how many electrons occupy its outer (valence) electron shell.

Ohm's law specifies the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = IR.

Solved for resistance, R = \( \frac{V}{I} \) = \( \frac{200}{4} \) = 50 Ω

Direct current flows in only one direction in a circuit, from the negative terminal of the voltage source to the positive. A common source of direct current (DC) is a battery.