One Spark is not like the other.
Rhett Allain writes: Late at night, I tend to flip through the channels just to see what’s up. If there’s a good movie on, I might watch part of it—and recently, I stumbled on Iron Man 3. I know what you’re gonna say—that’s a terrible superhero movie. But I disagree. Fantastic Four, now that’s a terrible superhero movie. Iron Man 3 wasn’t so bad. Especially not that part where Tony Stark has to go to the store and MacGyver his way into a temporary suit.
However, I did notice something annoying in a scene near the end. Iron Man needs to recharge his suit, and he improvises by connecting two cables (one red and one black) from a car battery to his suit. When he is mostly charged up, he pulls off the cables—one at a time. First he pulls the red cable off and it creates a slight sparking effect. Right after that he pulls off the black cable and it also makes a spark. See the mistake? One of the cables could easily make a spark, but not both.
But why? Of course that is the question. And now for the answer.
What Causes a Spark?
You can get a spark when air changes from an electrical insulator into an electrical conductor. This happens not at a certain electric potential difference (voltage) but at a certain electric field strength. Let me explain the difference with an example.
Suppose you have two wires connected to a 9 volt battery with the free ends of the wires held just 1 centimeter apart. The electric potential difference between these two ends is 9 volts. That’s probably not a huge surprise. If I move the wires closer together, they still make a 9 volt potential. However, the electric field depends on both the potential and the distance. As I move the wires closer together, the electric field gets stronger between the two wires. The electric field is the gradient of the electric potential and measured in units of volts/meter.
I know you didn’t like that example, so how about an analogy? Instead of electric stuff, I have a hill. The height of the hill is like the change in electric potential. The slope of the hill at some point would be the electric field. So now I have a 9 meter tall hill (instead of 9 volts). As the bottom and top of the hill move closer together (horizontally) the slope gets steeper. That’s just like the difference between electric potential difference and the electric field. Bonus: Note the importance of calling the electric potential a “difference” or “change in”—just like a hill, the change in height is the key, not just the top of the hill.
Now back to sparks. A spark is created by a high electric field, not a high voltage. At about 3 x 106 Volts/meter, you get a spark in air. With a 9 volt battery (and assuming a constant electric field for simplicity), you would need two wires to get 3 micrometers apart to cause a spark. That’s one super tiny spark.
A Simple Electric Circuit
But there are many cases where a spark is created even with low voltage. In order to understand this, let’s first look at a simple circuit—the simplest circuit you could ever imagine. It consists of a battery and a wire. That’s it. Here is what that might actually look like.
Yes, that is just a battery and a wire. OK, technically there are two rare earth magnets in there too—they just are an easy way to connect the wire to the battery. And although this is a simple circuit, it’s not very useful. In fact, it would be considered a short circuit since there isn’t really anything in the path. Without a resistor or anything, the current will get higher than it normally would. This would make the wire get hot—not too crazy hot with just a D-cell battery, but still hot.
What happens when I disconnect one of the wires? Let me show you … (read more)