Amagnetic compass is placed near an insulated copper wire. when the wire is connected to a battery and a current is created, the compass needle moves and changes its position. which is the best explanation for the production of a force that causes the needle to move?
(1)the copper wire magnetizes the compass needle and exerts the force on the compass needle.
(2)the compass needle magnetizes the copper wire and exerts the force on the compass needle.
(3)the insulation on the wire becomes charged, which exerts the force on the compass needle.
(4)the current in the wire produces a magnetic field that exerts the force on the compass needle.

most voltage drop calculators want the distance between the source and destination. then internally, they double it so both the source and return wires are calculated. but you need to know which distance the calculator uses so you're not half of what you should be or double what you should be.

most circuit breakers will not take a #6 directly, so the electrician will have to splice on a smaller pigtail. you also have to upsize the grounding wire when you upsized the ungrounded (hot) wire (see note in 250.122). this will preclude using most cable assemblies, as a cable with #6 copper will most likely only have a #10 grounding conductor. you need a #6 grounding conductor if the circuit is 20a. a #8 cable is no better, as it will have a #10 grounding conductor too. but most 20a breakers will take a #8 directly.

in your related post, you didn't say what type of load this is. if it is a light or heater, voltage drop won't be a big issue. if this is a motor, the current needs are huge at startup and the voltage drop can pose a problem. if this is a motor, knowing if the motor is under much load during startup will answer the voltage drop effect (a saw is easier to start than a compressor).