Explain what rectifiers is and what are the types of rectifiers?

A rectifier is an electrical device that transforms A.C or alternating current into direct current (D.C), which flows in only one direction. The types of rectifiers are 

• Half wave rectifier: It uses one p-n junction 
• Full wave rectifier: It uses two p-n junction

Mention how NPN and PNP transistor works?

In a circuit when NPN is used, 

• No current flowing from A to D = No flow from X to Z
• Current flowing from A to D = Current allowed to flow from X to Z 

When PNP is used, 

• No current flowing from A to D = Current is allowed to flow from X to Z
• Current flowing from A to D = No current flow from X to Z 

Mention what are the types of semi-conductors?

There are two types of semi-conductors intrinsic and extrinsic.

Again in extrinsic semi-conductors you will have N-type semiconductors and P-type semiconductors.

Explain how you decide what size of electrical wire do you need?

Your installation of conductors will depend on a few factors like gauge of the wire, wire capacity, etc. For wires, smaller the wire gauge larger the capacity of the wire to handle current. 

For example, low voltage lighting and lamp cords will have 18 gauge, electric furnaces or large electric heaters are of 6 gauge.