eli5: Why is there a tiny spark sometimes when plugging into an outlet?
In: Physics
If you live in a dry climate, you might see a blue or blue-white spark when you plug a plug into an outlet. That is static electricity that built up on you and your device as you moved around. Electric charge that built up on you (from you rubbing the carpet or whatever) jumps to the grounded (or close-to-grounded) wires in the outlet. As the charge jumps through the tiny air gap into the wire, it heats the air and makes it glow blue.
If you slowly plug in a high powered device (like a vacuum cleaner), and the device is switched on, you might see an orange or orange-white spark when you plug it in. That is little bits of the metal of the plug vaporizing from the high current draw through your device. In the moment you plug in the plug, electric current starts to flow. If you plug it in slowly, then for a tiny fraction of a second the electric current goes through a tiny contact patch at the front of the plug. If your device is drawing enough current, it heats or even vaporizes that little tiny bit of metal, making the orange spark.
That an electrical arc. That will only happen if there is something inside the unit that comes on instantly. Like a little light or if it’s meant to come on just by plugging it in. You can also leave some thing that uses a lot of electricity on and then just unplug it and see a large arc. The size of the arc is depending on how much electricity is being used
The main reason are capacitors inside the devices that load immediately after plugging in. Think of them as tiny rechargeable batteries. They load extremely fast and consume a lot of power in a very short time. Think of it as a very very short short-circuit. The short moment before a stable connection is made means that almost-short-circuit can ionize (charge) air and create what’s called a spark gap. Plugs and outlets are built to withstand this spark.
Electrons *really* hate being near other electrons, and repel each other with great force. Electrons also really struggle to move through air (despite what the existence of lightning would make you think). When you bring a plug near a high voltage outlet (think high water pressure), as you bring the plug closer, you’re reducing the distance through the air electrons would have to jump to escape to somewhere else. When that distance (and thus the resistance) is small enough, the electrons jump and you get a spark.
Technically, that happens every time we plug things in, but especially if its something that takes a lot of power and is plugged in slowly when it is visible to the naked eye..
Voltage is a difference in electrical potential between two entities. The two sides of an (AC) outlet are always in a state of difference in voltage, meaning there is always a difference in potential between them. Electric potential always “wants” to be the same. So when presented with a conductive path, that electric potential will equalize. When you add resistance to the path, you get current flow. That is what the windings in a motor, or the filament in a lightbulb do. In the case of the light, the resistance provided by the filament in the light bulb, causes it to heat up and emit light. When you approach a socket with a plug of a device that is off, the resistance within the device is too great for any current to flow, and you get no spark. When you approach the socket with the plug of a device that is on, there is a path within the device for current to flow, and it will flow as soon as the path is made.
Now the path, and the spark… The air around our light sockets and what we breathe is not nothing. It is a mix of gasses, and airborne solids. Electric current will flow through this air when the resistance is low enough, and/or the electrical potential is great enough. As you approach a socket with the plug, the resistance to current flow between the two becomes less and less as the air gap becomes smaller (and thus the resistance to current flow). Right before you have contact, the difference in potential between the plug and the energized outlet overcomes the resistance in the air gap. Just like a light bulb, when you pass enough current through the resistance in the air, it will heat up the electrons of the gasses and solids in the air and emit light, or “spark”. It really is the same as lightning in principal. The main difference is that the difference in potential is far far greater in a storm, as the charged energetic mass of a storm cloud, gets higher and higher (due to turbulent friction of the molecules in the cloud) compared to the “ground” of earth. Eventually, the resistance of the atmosphere is overcome, and the potential difference between the cloud and earth is momentarily equalized via the violent arc we know as lightning. That little click you might hear when the spark is made at the outlet, is baby thunder.
Thanks for asking this, OP! It’s always scared me when that happens but I never thought to ask on here.. turns out it’s not scary after all haha, thank you!
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