Hey all, I just had a quick question about powering my Arduino through an outlet. I was digging around my room and found a 5V 4A power supply laying around. Does this mean that if I connected the 5V to the 5V of the Arduino, 4A would flow through it? I’m thinking about using it later for a project (so that it can be powered through an outlet), so would it be safe?
No, 4A will not flow into the mystery Arduino. The load (Arduino) will pull only the current that it needs.
Before using a power supply, make sure that it outputs a well regulated 5V. Voltage greater than 5.5V can damage the Arduino.
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Ohh, okay, I seem. So if it makes sense, (suppose we have a 5V, 2A supply, such as a phone charger,) it will give me 5V and 2A max. I can connect any load to it as long as it uses 5V and doesn’t draw over 2A (assuming a perfect world scenario).
Thank you!
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That's how things work.
Or not. I don't know about a "word scenario" but if it is a reputable brand, it presumably will safely deliver the rated current. If something off eBay, well ... 
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I see that you've "got it" but sometimes it helps to put things in an extreme perspective ( it does for me).
Consider an automotive battery, capable of 600+ amps.
You connect your phone charger to the battery and to the phone. Its clear that 600+ amps will not be flowing to your phone.
Then you look on the printing on your phone charge. It says input current 1Amp. So you have a source capable of 600 amps but you are drawing 1 Amp.
Two other important bits of information:
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Is you connected you 5V 4A to your Arduino, and you wanted to control a 500ma motor (using a relay or some driver) you cannot connect the 500ma motor to any of the 5V pins on the board, it must be connected directly to the 5V 4A supply.
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This is one of my pet peeve's. Real cheap ac adapters from China rarely meet the stated output requirements. The sellers count on the fact that most users buying supplies on ebay will never draw more that 50% of the rated current. You can google ac adapter teardown and find many sites showing the poor build quality of these adapters.
And the worst issue with cheap AC adapters. If they are not UL approved** the isolation between mains voltage and the DC output is woefully inadequate. This means that if the adapter is dropped one too many times or allowed to get too hot too many times, one of the input pins could become connected to the output. Not a good situation.
** and a recognized brand.
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Not so extreme, but I generally refer to my power point (Wall outlet for US speakers) rated 10 Amps at 240 V, when I plug in my IKEA 3 W LED lamp which will draw about 0.015 Amps.
(OK, that is actually just as extreme! 
)
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I've found over the years the "trick" of taking a situation you think you understand but not 100% sure, mentally push the conditions to an extreme condition. Things often become clear 
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That’s a wonderful way of putting things into perspective for me, thank you so much! I’m actually not sure what I might do with it just it. Might use it to automatically control bird feeder/cage when my family and I are out
That’s a wonderful idea, definitely will help me in the future. Thank you!
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