What I’m talking about is, for example, a keurig coffee machine having two differently shaped prongs where one prong is bigger and wider than the other, so you have to insert the wider prong into the wider port.
However, something like a phone adapter doesn’t have this, and the two prongs are the same size and shape.
Why is this?
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One of the wires in the outlet is “hot” and one is “neutral.”
For something where someone might come in contact with the metal inside the device (or water which is in contact with the metal), you want to make sure that anything they touch is connected to neutral in the outlet, which means you want to make sure the plug only goes in one way, to ensure that the right wire goes to neutral. For something where there’s little/no chance anyone will come in contact with the insides, you don’t need to worry so much.
You will often see both prongs the same on things that are DC powered where things are hitting a transformer first. But for AC powered things it’s often beneficial to know where hot vs neutral is. The smaller prong on the right is the hot.
The different sizes of the blades is to enforce polarity. One of the blades is “hot” and one is “neutral”. This is both a safety thing and an electrical design thing.
For devices without a dedicated ground pin, the chassis of the appliance may be grounded to the neutral in which case it is important to ensure that the neutral blade of the plug is in the neutral slot of the socket (otherwise, touching it would be dangerous). A device may also simply require the correct polarity to function correctly.
For some devices, the polarity simply does not matter.
Within the outlet are 2 different wires, Hot and Neutral. When nothing is plugged in, they’re not connected. When you plug something in, it connects the 2 (through the device) completing the circuit.
For some devices, like your Keurig, it’s important to plug it in so one specific side is connected to the Hot, while the other connects to Neutral. The electricity flows in one direction, and if you plug it in backward, it will flow through the device the wrong way and potentially break the internal circuitry. So the prongs are different sizes to prevent you from plugging in the device the wrong way.
Other devices, like charging blocks, it doesn’t matter which way you plug it in. The internal circuitry allows the bi-directional flow of electricity. For charging blocks, they just need to get some of the electricity and convert it to DC that is then used to charge your phone. The first step in this process is like an off-ramp on a highway: both sides of the highway can exit the highway and end up going the same way.
For security reason, if your machine has a security mechanism to block current leaks (current coming from the hot higher than the current going in the neutral, so some current is leaking somewhere else, potentially someone’s body), it’s important to block the current as close as possible from the hot, so it’s beneficial that the hot be known, which requires different plugs.
It’s physically impossible for any electrical machine to have a circuit that care which way is the hot when there’s no current leak because the current alternate from the hot to the hot, so there’s actually no current direction through the circuit.
However, if there’s a current leak, there’s a difference, because whatever receive the leaked current acts as a neutral not as a hot and is in circuit with the hot.
Not all machines have this security system since it’s not mandatory. Also, there are other security systems inside your plug or breaker to reduce the risk.
As others have said, the two slots in the outlet are a “hot” and a “neutral”. If you stuck a fork in the hot side, you’d zap yourself because you’ve completed a circuit with the floor (“ground”). However the neutral side is effectively connected to the floor so you’d be fine (please don’t stick a knife into either side of the socket!).
Your device has a plug that runs from the hot blade, through its circuits, and to the neutral blade. If you were to touch some part of the circuit, such as the inside of a lamp socket, you could zap yourself.
Since your device already has a switch, we want to put that on the hot wire, before the circuit. So we differentiate the prongs to force the plug to sit the right way around. Now touching the circuit when the switch is off is harmless. Some older devices didn’t have this and you could zap yourself on the chassis if you plugged it in the opposite way.
Your phone charger and similar devices convert the outlet power to a lower voltage before it goes anywhere, so it’s not (as) dangerous if you accidentally touch it. It’s not necessary to differentiate the prongs because it doesn’t matter which way it gets plugged in.
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One prong has 110v on it, the other prong has 0v on it and is the “return path” for the electricity.
You can make a device safer by putting the power switch on the 110v line so there is no live electricity inside the device when it’s switched off. If you had a symmetrical plug on those devices it would be 50/50 whether you plugged it in the right way to have the power switch on the 110v line or not.
But for some devices, especially ones without a power switch, it’s not a big deal so they have a symmetrical plug.
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