When you plug your coffeemaker into a hard-to-reach receptacle, you may have to get down on your hands and knees and make two attempts because the plug can only be inserted one way. That isn't true for the plug on your computer or cell phone charger; it can be inserted either way. That seems much easier, so why the difference?
Polarized Plug vs. Unpolarized Plug
If you look at the coffeemaker plug, you'll see that one prong is slightly larger than the other. It's a polarized plug, and it will only go one way into a polarized outlet that has matching slots. The charger plug with two identical prongs isn't polarized. You can also plug it into a polarized outlet, but it will go just as easily into an unpolarized outlet, although these aren't common anymore.
Making the prongs on a polarized power cord different sizes is a way to ensure that the hot leg of the electrical circuit always contacts the hot terminal of the appliance you're using. This is important for appliances with switches because it guarantees that the appliance won't become energized if the switch is off. A charger doesn't have a switch, which is why it has a nonpolarized power cord.
What Is Polarization?
In a North American AC circuit, the current changes direction 60 times per second, so it's meaningless to talk about positive and negative poles. An AC circuit does have two "legs," though. One is the hot leg, which comes directly from the power source, and the other is the return leg, which completes the circuit back to the source. If the circuit is broken by a switch, the hot leg is energized while the neutral leg isn't.
When you use an unpolarized plug, the hot leg of the circuit could be on either side of the device you're using. This is fine for nonswitched devices, which is why you can use a nonpolarized outdoor extension cord for your Christmas lights. It creates a hazard for switched appliances, though. All the electrical components on the hot side of the switch could be energized when the switch is off, and you could get a shock.
This scenario can't happen if the device has a polarized power cord. The hot leg is always on the same side of the device, and manufacturers place switches as close to the hot terminal as possible. When the switch is off, the device is electrically neutral.
The NEMA 1-15 Plug and Receptacle
The standard residential outlet in North American homes is the NEMA 1-15R, which accepts both polarized and unpolarized plugs. An appliance with a polarized power cord has a NEMA 1-15P plug with prongs that match the outlet. It will only go in one way. An appliance with a nonpolarized power cord also has a NEMA 1-15P plug, but the prongs are identical, so it can go in either way.
A grounded NEMA 1-15P has three prongs. It's polarized because it can only go into the receptacle in one direction. The blades don't have to be different sizes, but they are on some three-prong plugs anyway.
Some newer appliances and light-duty power tools are double insulated. The two layers of insulation ensure that users can never come in contact with any electrical or metal parts. You can never get a shock, so they don't need polarized plugs. These come with a NEMA 1-15 nonpolarized power cord.
A Short Bit of History
Modern plugs and outlets are not much different from those patented and sold by Connecticut inventor Harvey Hubbell in 1903. The most important modifications to Hubbell's design were the addition of a ground pin and the introduction of polarized blades, which occurred in the early 1900s. These modifications didn't become standardized until midcentury updates in the electrical code mandated them.