Why does polarity matter for MIG welding with gas vs flux-core wire?

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GENIERE
I have time on my hands these days so I bought a Mig/flux welder. I have no idea what I’ll do with it but I always wanted one. The machine allows welding with wire that has a flux core or with a gas “shield. Presumably the gas or flux prevents oxidation of the metal when heated.

The question I have is that when gas is used with solid (no flux core) welding wire you are directed to connect the wire to the positive terminal and the material to be welded is connected to the negative terminal. When using flux core (no gas) wire you must reverse the polarity. I haven’t used gas but incorrect polarity with the flux wire makes it much harder to get a good bead started. I can’t think of a good reason why the polarity would matter. Anyone have a thought?
 
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Changing polarity has several effects.

From Wikipedia:
In welding, the positively charged anodewill have a greater heat concentration, and as a result, changing the polarity of the electrode affects weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds.[30] Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.