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What’s the worst that can happen if I put a switch in the neutral instead of the phase?
I just found out that 3 light bulbs sockets in my home are connected this way. It came to my mind to fix it, but then I recalled the flow of current from positive to negative is just a convention, since the electrons are charged negatively and they actually flow from N to L, so I can't really find a solid excuse to mess with the wiring for the next 2 hours.
No, you don't want to switch the neutral.
If those light bulb sockets are a fixed installation in your house (such as a ceiling light as opposed to a table lamp), then that's not good and quite likely not to code. We can't know for sure because you haven't said where this is nor filled in your profile (remember, your profile is a courtesey for us, not really for you). The wiring in your house not being to code is not only potentially unsafe, but can open you up to various liability issues, especially if you ever sell or rent out your house.
The hot wire is the one with significant potential on it with respect to ground. Many things are tied to ground, so you can be easily connected to ground depending on what you're touching, like a radiator, water faucet, sink, computer chassis, and other things you might not realize are grounded. The hot wire is therefore dangerous.
The neutral wire is connected to ground at the breaker box, which is connected to physical ground nearby. If you switch the hot line and leave the neutral, then the whole device will be at neutral potential. That's OK. If you switch the neutral, then the whole device will be at hot potential. In theory that's OK since all of that is supposed to be insulated. However, stuff happens, and by switching neutral instead of hot you have removed one layer of safety. That is not a good idea.
NO
In this situation, electrocution can occur while changing a light blub even with the switch off. Two hours of your life in trade for everyone else's, in their entirety, should be a solid excuse.
Fix it.
One of the first things they taught us was to "Always switch the hot". On 240 volt, you switch both hots.
With line voltages, they always make the hot parts as small and inaccessible as possible. That is why the hot tab inside a lamp socket is down in the bottom, where it's hard to touch. The same goes for the smaller hot hole in an outlet vs the larger neutral hole. In furnaces, any high voltage terminals are insulated, while the low voltage doesn't need to be.
This is why I always use a non-contact voltage indicator, because I never know what traps someone else has set up for me.
Neutral is very close to ground potential due to being tied to ground at the distribution panel. Live is... live. A connection from neutral to ground will not likely kill you, but a connection from live to ground stands a decent chance of doing so.
And more of the circuit is live than neutral if you switch only the neutral, including any electrical devices on the circuit.
If the circuit is wired to switch the "hot"; which is the correct way to wire an AC circuit. The only way there should be a voltage potential at the fixture or device, is if the switch is closed. When wired this way, there's always a safe path for current to flow when the circuit is energized.
If, however, the "neutral" is switched, there would always be a voltage potential at the fixture or device. A safe path for current to flow only exists when the switch is closed. In this situation you could potentially become the path to ground, especially when the switch is off.
You mention electricity flowing from negative to positive, which suggests you're familiar with DC current. With alternating current (AC), "electricity" flows from a higher potential to a lower potential.
In addition to electrocution as others noted, you may also see LED lights with a faint glow when the switch is off. You have 120V on the HOT, and the weak capacitive coupling to GND can be enough to very weakly turn on the LED bulb.
I saw two instances of this in my house on switched NEUTRAL:
As Nick stated in his comment : "What's the worst?" you ask. "Somebody gets electrocuted".
What this means is .. The person CHANGING the light bulb has the possibility of inadvertently touching the lamp threading - the socket being HOT all the time. So if you are looking to kill someone changing a bulb this is how that is done. FIX IT RIGHT. Not only will you save someone in your family but also someone else who purchases the home from you.
As some have said about the code: In the United States it is against the National Electric Code, I believe all states adhere to this code and then adopt more stringent codes but I could be mistaken. I am not sure about how territories of the United States handle this - you will need to check if you are in any of those areas.
EDIT: I will add in this how do you know you have a neutral ? The wires are white? If so maybe you have a 3-way or 4-way switch configuration ? If a 4-way those three switches operate your single light - in which case they are not neutrals but travelers.
Fix it. But turn the power off first. I have never seen that before, but every house I have owned or built had the hot and neutral reversed on at least one outlet. Another possible hair burner.
