Lightning is the electronic discharge between particles in the air andclouds and the ground. Electricity is carried through current, or the flow of electrons, and lightning is caused by very large currents, which is why it can be so deadly when it strikes. Large currents that causelightning also cause high voltages. Voltage is the “potential difference” between two places, meaning it describes the ability and likelihood of electric charge to flow from one place to another. If current is relatively low, voltage can be very high and still very safe. The Van de Graaf generator in this experiment operates at high voltages but low currents, similar to the static electricity you experience after rubbing your shoes on carpet and touching a doorknob on a dry day.
You may have seen a Van de Graaf generator in a science museum before. It is an electrostatic generator, and creates static electricity by building up very large voltages on its surface by moving a belt over a terminal and the electric charge accumulates on the surface of a hollow metal sphere.These spheres can hold high enough potential differences to produce a visible spark when objects are brought close. A small, table-top generator can get up to 100,000 V (volts)! One in a museum can get up to 5 megavolts—that’s 5,000,000 V! This is much higher than a typical battery, which is about 1.5 V. This apparatus was invited in 1929 by Robert Van de Graaf, an American scientist.
What will happen if you touch the generator when it is on?
- 2 Van de Graaf generators
- Rubber-soled shoes
- Metal fork
- Costume wig
- Wear rubber-soled shoes! This will help insulate you from the ground.
- Set up the Van de Graaf generator on a table top.
- Touch the generatorwhile it is off to discharge any static electricity currently on the surface.
- Turn on the generator.
- Touch it! What happens? How does it feel?
- Keeping your hand on the generator that is on, bring your free hand closer to the metal fork. What happens?
- Turn off the generator.
- Place the costume wig over the generator. Turn it on and observe what happens.
- Turn off the generator and remove the wig.
- Turn on the generator.
- Take the metal object (like a fork) and rub it on an insulated object, like the carpet or clothing. Why do you have to do this?
- Bring it close to the sphere and watch what happens.
Touching the generator while it’s on will cause your hair to stand up! Similarly, the hair on the wig will stand up when the generator is on. Bringing your free hand close to a metal object will discharge electricity from your body and you will hear an audible pop! Bringing a positively charged charged metallic object close to the negatively charged generator will produce a visible spark, like tiny lightning!
Rubber shoes help to insulate you from the ground, allowing charge to build up on your body rather than flow straight through you into the ground, the area with the lowest electric potential. A plastic stepping stool or other object made of insulated material would also work well. Carpet is also a decent insulator, which is why you can often scoot around the carpet with shoes on and shock your friends with the static electricity that you build up.
When the generator is on, it is negatively charged as the electrons amass on the sphere’s surface. When you touch the sphere, the electrons flow onto your body, which is neutrally charged. As the electrons flow over the surface, they reach the hairs on your arms and on your head. Opposite charges repel each other, and because the hairs are so light the charges repel enough to move the hair away from the rest of your body. This is the same reason the strands of the wig stand up.
The sound produced when static electricity is discharged is caused by the electrons jumping across the air barrier all at once! Lightning makes a sound as well, the electric discharge that happens in the air creates the audible boom we know as thunder. We see the lightning first before we hear the thunder because light travels faster than sounds.
Rubbing a metal object like a fork on the carpet or on fabric will cause many of the electrons to move onto whatever you are rubbing the object on. This will create a positively charged object. Bringing this near the negatively charged Van de Graaf generator can create a potential difference large enough to produce a visible spark. If you don’t get a spark, it is likely that your object is not positively charged enough.