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.

Resistors are used to limit voltage and/or current in a circuit and can have a fixed or variable resistance.  Variable resistors (often called potentiometers or rheostats) are used when dynamic control over the voltage/current in a circuit is needed, for example, in a light dimmer or volume control.

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

Solved for current, I = \( \frac{V}{R} \) = \( \frac{100}{40} \) = 2.5 A

Current is the rate of flow of electrons per unit time and is measured in amperes (A). An ammeter is used to measure the electric current in a circuit.

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 DC is a battery. In contrast to the constant one-way flow of direct current, alternating current changes direction many times each second. Electricity is delivered from power stations to customers as AC because it provides a more efficient way to transport electricity over long distances.