Britta Briggs

Britta Briggs

This week’s question was asked by Britta Briggs, fifth grade at LaGrange Elementary School.

Teacher: Chelsey Juliot.

QUESTION: Why do wires spark when you plug something in?

ANSWER: Perfectly normal and quite harmless. Let’s use a table lamp as an example. A perfectly normal two-prong plug or a three-prong plug will sometimes spark when plugged into a wall socket if the lamp switch is closed or “on.”

Why is that? The closed circuit in the switch means that the last open connection in the circuit will be at the point where the plug meets the wall socket. The plug and socket are the final links in the chain that will let the current flow. This open circuit, when closed, may cause an arc to jump across the air gap in the last fraction of an inch. Generally, the spark caused by this occurrence is quite harmless.

If the lamp is already plugged in and the switch is turned on, there must be a spark inside the switch. That’s what switches are designed to handle. Many switches have mechanical springs that quickly snap the switch open or closed even if you move it slowly.

If you want to avoid that spark, you should turn off whatever lamp or appliance you are using, before you put the plug into the wall socket.

The spark or electric arc will be larger if there is a bigger load. For example, it will be a bigger spark if the lamp is 100 watts rather than 25 watts. A high-wattage halogen lamp is more likely to create a spark compared to an incandescent lamp. An electric heater is often 1,000 watts or more. An electric heater will experience a spark, whereas a radio will not. Also, if the load or appliance has an inductive motor, such as a vacuum cleaner, expect an arc. Again, a spark may occur if the vacuum cleaner switch has already been turned on before plugging it in.

There are a few small electronic devices that will spark when plugged in even when turned off. These devices contain capacitors that store electricity and charge up when first plugged in. That brief burst of power may cause a momentary spark. Devices such as computer power supplies or flat screen televisions contain capacitors.

The same spark phenomena may happen when any lamp or appliance is unplugged from a wall socket. Again, it will occur only when the appliance or lamp is “on” or operating.

What is this electric spark that jumps from metal plug to metal socket? A spark is the moving of an electric charge, or current, through air. But air is supposed to be an insulator and does not conduct electricity. Indeed, it is, but if the electric field exceeds a certain value, the air gets ionized and becomes highly conductive, enabling the charge to move. An electric field occurs around any charge, just as a magnetic field surrounds a magnet, and a gravitational field surrounds a mass.

Where does the light of the spark come from? So much current can flow through that very tiny gap between plug and socket, the temperature of the air can rise to several thousand degrees. The air turns to plasma, the metal on the tip of the plug turns to vapor. With those high temperatures comes the emission of light. It’s a tiny lightning bolt. You may hear a pop sound. That’s the thunder.

It is amazing how much science is involved in the plugging and unplugging of lights and appliances.

Oddly enough, a spark across a gap of conductors is the basis of the carbon arc lamp. It was the only electric light available to light large areas for 100 years from 1800 to 1901. It works by hooking two carbon rods to a source of electricity. With the other ends of the rods spaced at the right distance, electrical current will flow through an “arc” of vaporizing carbon, creating an intense white light capable of lighting a large length of street or a large factory interior. It was cheaper to light streets with the arc lamp than gas or oil lamps. Carbon arc was utilized as the light for movie projectors in theaters for many years.

Here is some general advice from electricians. Any socket that shows burned or blackened marks should be replaced. Water can cause an outlet to spark. Outlets known as ground fault circuit interrupters will cause a circuit to shut down rather than flowing along an unintended path, such as water or a person. GFCI can be tricky to install properly, so it’s prudent to let a professional handle it. A plug, outlet or cord should never become hot or emit smoke or odors.

Consultant: Jeff Gray of Gray Electric.

Send questions and comments to:

Larry Scheckel is a retired Tomah High School physics teacher.

If your question appears in this column, you will receive a free Value Meal from McDonald’s and a coupon from Pizza Hut.


Tomah Journal editor

Steve Rundio is editor of the Tomah Journal. Contact him at 608-374-7785.

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