There’s Something Wrong With This Iron Man 3 Scene
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.
That wasn’t very exciting.
A More Complicated Electric Circuit
Here is another circuit. Watch closely as I connect the final cable to the battery.
It’s dark so you can see better, but notice that there is a spark both when I connect the battery and when it is disconnected. Here is a picture of the whole circuit.
There are two significant differences with this second circuit. First, I am using two 9 volt batteries instead of one D-cell battery (at just 1.5 volts). Second, I have another device in this circuit—a coil of wire (on the left). We call this an inductor.
There is a pretty cool relationship between electric and magnetic fields and that’s the basis of the inductor. Let me start with a simpler example. Suppose I have a coil of wire and a magnet. As I move the magnet in and out of this coil, it will produce an electric current. Here is an example of that.
The device on the right is essentially a sensitive meter to detect electric currents (called a galvanometer). If you look carefully, you might be able to notice that a current is produced when the magnet is moving—not just when there is a magnetic field. There must be a changing magnetic field to produce a current. But what if I replaced this moving magnet with a coil of wire with a changing current running through it? It could produce the exact same effect as the moving magnet.
Even better—I don’t need two separate coils. With just one coil and a changing electric current, part of the coil will make a changing magnetic field that will induce an electric current in the other part of the coil. This is the essence of an inductor. Really, the best way to think of an inductor is as a device that produces a change in electric potential that is proportional to the rate of change of the electric current. When the current is constant, there is not electric potential difference across the inductor is zero volts. When the electric current is changing very rapidly (like going from some current to zero when you open a switch), the electric potential difference across the inductor can be huge.
And here is the answer to sparking wires: If the circuit contains an inductor then opening or closing a switch (or pulling off a wire) will create a very large change in current (since it goes to zero). This huge current change makes a giant change in electric potential across the inductor. The giant electric potential difference means that you can get super high electric fields around the contact points. The electric field can be large enough to be at the 3 x 106 V/m level that would produce a spark.
But wait! What if your circuit doesn’t have an inductor in it? Here’s a surprise for you: Every circuit is an inductor. Since every circuit makes a loop, it at least has some inductance in it—so every circuit can spark when you open or close a switch.
Only One Spark
Back to the Iron Man scene. Why would there only be a spark on the first cable that is removed from suit? Let’s step through this in parts. First, I assume that there is a large current running from the battery to the suit and then back to the battery. This is a complete circuit—everything is good. Second, Tony pulls off one of the cables. If there is an inductance in the suit (highly likely) then this rapid decrease in electric current would induce a large voltage to create a spark. Finally, Tony pulls off the other cable. But there’s a big difference—he’s already broken the circuit. No complete circuit means no electric current and no change in current. There shouldn’t be a second spark.
But in the end, it’s just a movie and not real physics. Technically, this would be a scientific error—but it doesn’t really have any impact on the plot. And even if it was a plot-related error, that’s OK (as I have said many times before). I still like Iron Man.