Electricity & Magnetism is a fun topic, especially for teachers who enjoy watching teenagers act like themselves.
I have several demonstrations that can give students mild electric shocks. The van de Graaf generator and Wimshurst machine are traditional physics classroom gadgets. With the Wimshurst machine, I came up with a diabolical plan. I suggested that if one student held each electrode, and they placed their fingertips about an inch apart, when enough static charge had built up, a spark would jump from one to the other. Two girls jumped up to try it, giggling and shrieking predictably. When they were finished, I wrote on the board: “Girls 2. Boys 0.” That was enough to get half the class to try it.
After the Wimshurst machine, I hooked up the van de Graaf generator, which turned out to be powerful enough to make their hair move a little, but not enough to make any of it stick straight up.
Next was a plasma globe, which was fun to look at. I pointed out that it “finds” their fingers because even though glass is an insulator, some charge does get through. I proved this by putting an aluminum foil tent over the plasma ball, which gave them noticeable shocks. Some students clearly were conflicted—they wanted to experience the shocks, but were also afraid of them. Of course I let them decide for themselves, dissuading their classmates from applying peer pressure. In the end most of them decided to try the demos, but what I found fascinating was that a significant fraction of the most fearful, once they had experienced the shock, were the most eager to do something to increase the effect!
The other interesting revelation with the plasma globe came when a student accidentally leaned on a metal object when she had her hand on the globe (sans aluminum foil). She discovered that a lot more charge was passing through her body than she had realized! What I found more interesting was that after she shrieked and pulled her hand away, several other students wanted to try it.
I also have an induction coil, which I used to make a Jacob’s Ladder. (No, I didn’t let them give themselves shocks with the induction coil!) The arcing was not hot enough to ignite paper, or even to light a match. However, on a whim I decided to see what a lit match would do. It turned out to be interesting—the arc moved toward the flame. This makes sense; a flame is a redox reaction between molecular oxygen and some sort of fuel. Because the reaction involves electron transfer, it served as a source/sink for electrons jumping from one electrode of the induction coil to the other. It made a nice connection between physics and chemistry.
After I was finished with the demos, I told my students that I was the envy of the entire faculty—I was sure that most of their teachers would love an opportunity to apply electric shocks to just one of them, and I got to do it to half the class!
Jeff, have you run into any safety / perceived safety issues with electrical demos? In my college Intro Physics class, I’m happy to do things that startle my students badly (I short a high-voltage capacitor, which makes a bang like a gunshot) but I try to avoid significant physical pain, because I worry that someone might not think it’s fun. And god forbid some student has a serious heart condition. I guess a Wimhurst machine might strike the right balance between fun and pain: getting hit by our Van de Graaf hurts a *lot*.
Also, a technical point: it’s not true that “some charge gets through” the glass in a plasma globe. In high school physics terms, the charges inside the plasma globe and the charges inside your finger attract each other through Coulomb’s law, but never flow across the glass. Charge surges in and out of the globe thousands of times a second, causing the charge in your finger to do the same. In electrical engineer’s terms, your finger and the globe act as a capacitor: at the radio frequencies the plasma globe is driven at, the impedance is small even though the resistance is essentially infinite.
The van de Graaf I have in my classroom is pretty tame. I wasn’t able to get a spark longer than about 1-2 cm from it. (This may be in part because the belt is probably quite old.) The shock from the plasma globe with the aluminum foil tent was probably the most dangerous–the sparks actually left tiny burn marks on some students’ fingers.
If the plasma globe and aluminum foil tent are acting as a capacitor, then I don’t understand where the charge in the capacitor is coming from.
“If the plasma globe and aluminum foil tent are acting as a capacitor, then I don’t understand where the charge in the capacitor is coming from.”
The ground wire on the plasma globe supplies charges to the inside; the atoms in whatever material is touching the globe supply charges to the outside. The globe contains a transformer, which acts as an oscillating charge pump. One end of the transformer is connected to the central electrode, the other end is tied to an electrical ground. Every few microseconds, it pumps charge from the ground into the globe. When you touch the globe, opposite charges in your body are attracted toward the globe and build up on your fingers. A few microseconds later, the pump reverses direction, reverses the charge on the globe, and the charge flows through your body the opposite way. But no charge flows across the glass.
Cool. I hadn’t realized that it oscillates. Thanks for the explanation.
I’m a lay person, but based on what you two just discussed here, could you please explain how a fluorescent bulb “lights up” when in contact to the plasma globe if there’s no transfer of electrons? Thanks.
I would guess that if the globe is oscillating, it will induce oscillating charges in the fluorescent bulb, much like AC current.
Jason, if you’re still following this thread, can you provide a better explanation?