Of course fireworks are popular in the USA around July 4th. But there is perhaps no simpler firework than the basic sparkler. The sparkler is usually some type of stick with a thick paste of a metallic fuel. When this fuel burns, it produces tiny glowing sparks that shoot off in all directions.

Of course, there are some interesting physics here too. It's not just sparklers. There are always cool things around us that show both basic and advanced physics concepts. For now, we will just look at the sparkler. Here we go.

Temperature vs. Heat ——————–

These sparks from the sparkler are HOT. I mean super hot. It depends on the kind of sparkler, but the temperature of these sparks can be anywhere from 1800°F to 3000°F (1000°C - 1600°C). How hot is that? Here are some temperatures of some other things you might have seen.

Human body = 98°F (37°C)

Your oven can get up to 550°F, but the element (the heating part) can get up to 1500°F (815°C).

The glowing filament in an incandescent lightbulb is about 4500°F (2500°C).

Iron melts at 2800°F (1500°C).

The temperature of a spark from a sparkler is very high. Aren't these sparklers dangerous? Actually, they can be when not used safely. But if one of these sparks hits your skin, it's not going to do serious damage. Why not? This is because there is a difference between temperature and energy. The hot spark has what we call thermal energy. The thermal energy an object has depends on its temperature, its mass and the type of material it is made from. Since these sparks have such low mass, they don't have very much thermal energy. Without much energy, they don't have the capacity to do very much damage to your skin.

Here is another example. Suppose you like leftover pizza. You put the leftover (and cold) pizza on a thin piece of aluminum foil and then into the oven to warm it up (let's say at 300°F). After a few minutes, the pizza is warm. How do you get this pizza out of the oven? Just grab the aluminum foil with your finger and thumb and slide it out of the oven. But wait! Isn't that aluminum foil very hot? Yes. It's probably around 300°F. However, it is very thin with a low mass. Just like the spark from the sparkler, this aluminum foil doesn't have enough energy to do damage to your fingers.

Temperature is not the same as thermal energy.

Size Matters ————

There is another thing about sparks that make them safe: their size. It turns out that these tiny hot sparks don't stay super hot for long. Why? Because they are so small. Is that crazy? It seems crazy, but it isn't. In fact, smaller things cool off quicker than bigger things. Why?

Volume matters. Let's say I have a small hot piece of metal (the shape doesn't matter but let's say it's a cube). What would happen if it were twice as wide? If I double one side of a cube, I have to double all dimensions of a cube. This means that a cube twice the length will have 8 times the volume. With 8 times the volume, it will have 8 times the thermal energy.

Surface area matters too. How does this thermal energy get out of the metal? It has to interact with stuff outside of it through its surface area. If you double the size of a cube, you increase the surface area by a factor of 4 (each face of the cube is 4 times larger).

Now let's put this together. Doubling the size increases the thermal energy by a factor of 8 and increases the surface area by 4. It has way more energy but can only cool off a little bit faster. This isn't a big surprise, so don't act like it is. What cools off faster? Big hot muffins or small hot muffins? You know the smaller muffins get cooler quicker.

The same thing happens with these sparks.

Blackbody Radiation ——————-

If you hang around physicists, you might hear them talking about these "black bodies". What is a blackbody? A blackbody is any object that emits radiation due only to its temperature and not by reflecting light of any kind. Since a true black object doesn't reflect light anyway, we call these objects black bodies.

The cool thing about blackbodies is that the color of light they give off is related to their temperature. Actually, an object with some temperature produces a range of light colors. The peak of this color distribution is related to the temperature of the object. I know this is confusing, but I can summarize by saying that cooler objects produce longer wavelength light (in the infrared part of the spectrum) and hotter stuff can glow red. Even hotter stuff glows white-ish.

Really, you should just play with awesome flash applet from PhET. Here is a screenshot of the applet.

Screen capture from a PhET simulation.

You can see that if you change the temperature to about 1900 K (that is the temperature unit Kelvins and it is the same as about 3000°F), the color of the object would be dark red. If you zoom in the vertical scale of the graph (click the "+" button until the vertical scale goes up to 1) then you can see a curve showing the intensity of different wavelengths produced. Most of the light at this temperature is longer than visible light such that you can't see it. Only the tail end of this curve is in the visible spectrum. That's why a very hot stove element appears dark red.

What About a Green Spark? ————————-

Go back to that blackbody applet. Adjust the temperature until you find a blackbody color that is green. No, it's not green at around 5000 K. I would call that white. It seems you can't get the color green with just blackbody radiation. There has to be something else going on.

There is another way to produce light other than having an object get hot. You almost certainly have one of these objects in your house—a fluorescent lightbulb. The old style incandescent bulb produces visible light by increasing the temperature of a tiny filament inside the bulb. However, for the fluorescent bulb light is produced by exciting a gas. The fluorescent bulb is actually a little complicated so let's look at the neon light.

Image: NASA. A neon light along with its characteristic colors.

The neon light produces just certain colors of light. You can see these colors by using one of these spectral glasses (they're very cheap to get). The different colors of light correspond to different energy level transitions in the atom. Yes, an electron in the atom moves down to a lower energy level it produces light. Different atoms have different energy levels such that each atom has its own signature colors of light it produces.

It's not just electrons in atoms that can be excited to higher energy levels to produce light. Molecules can also have particular energy levels. This is how we get green light. Barium chloride just happens to have energy level transitions that correspond to the color green. But if the barium chloride has to get hot to excite the electrons, wouldn't it be a blackbody? No. There is something different about a blackbody producing light and a single atom or molecule producing light. A blackbody has to be a dense object like a metal (but not a gas). In these solid states, the light produced is that of a blackbody. If you have a gas or vapor, then the individual atoms can be excited to produce light. This is what happens with the green sparkler. That the barium chloride vapor is green—actually, the flying sparks don't look as green.

Image: Rhett Allain

See. This isn't so difficult. Just about any thing we see around us can be used as a starting point to talk about some cool physics. Sparklers just happen to be awesome.