Insulators have valence shells that are more than half full of electrons and, as such, are tightly bound to the nucleus and difficult to move from one atom to another.

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.

Capacitors store electricity and are used in circuits as temporary batteries. Capacitors are charged by DC current (AC current passes through a capacitor) and that stored charge can later be dissipated into the circuit as needed. Capacitors are often used to maintain power within a system when it is disconnected from its primary power source or to smooth out or filter voltage within a circuit.

The transistor is the foundation of modern electronic devices. It is made entirely from semiconductor material (making it a solid state device) and can serve many different functions in a circuit including acting as a switch, amplifier, or current regulator. A transistor works by allowing a small amount of current applied at the base to control general current flow from collector to emitter through the transistor.

Connecting the 6 batteries in series multiplies their voltage while keeping their current the same yielding a 72V 15A configuration. Connecting the 6 batteries in parallel multiplies their current while keeping their voltage the same yielding a 12V 90A configuration. Using a series-parallel connection, 3 batteries can be connected in series and 3 can be connected in parallel resulting in a 36V 45A configuration.