LIGHTNING  How is lightning formed?  Brian becomes well-grounded in the physics of lightning.
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Getting Started

Begin the lesson by explaining that lightning is an extreme example of the same static electricity that shocks your fingers when you touch a metal door knob after rubbing your feet on a thick carpet.

The powerful bolts from the sky are proof of the old saying, "Opposites attract." To demonstrate, blow up a balloon, draw a face on it with a permanent marker, and then hang it on a string about head high for the students in your class. Have volunteers rub the face vigorously with a piece of wool. Does the face turn toward that person? Why? Ask the volunteer to step away from the balloon, then move toward it. Have other students try this same exercise. Ask: How close to the balloon do you have to be before it reacts to you? What do you think is happening?


Overview

Scientists know that lightning results from a complicated interplay of positive and negative electrical charges occurring in the 2,000 or so thunderstorms taking place on Earth at any given moment.

To learn about what causes lightning, scientists had to learn about the interaction of electrons and positive ions. Electrons, tiny particles orbiting the outside of atoms, carry a negative charge. Positive ions are atoms or molecules that have lost an electron. Atoms and molecules normally have equal positive and negative charges, making them neutral.

When different materials come in contact, electrons are transferred and one of the materials gains an excess of electrons and becomes negatively charged. When an object with a lot of positive or negative charges gets close to an object carrying the opposite charge, a spark jumps across the space between them to neutralize the charges.

In a thunderstorm, that spark is a lightning bolt. It's only a couple of inches wide, but it leaps between the clouds and the earth at a remarkable 90,000 miles per second. The power in the stroke is three million megawatts, comparable to all the power generated in the United States at any one instant.

The separation of positive and negative charges necessary for lightning begins during a thunderstorm, when rising water droplets collide with falling hailstones in the middle of the cloud. The hail strips electrons from the droplets and the top of the cloud becomes positively charged, while the bottom becomes negatively charged. What we see as lightning happens in a two-step process. Static electricity builds up between the earth and the cloud and a spark in the form of an invisible lightning bolt comes down from the cloud.

Just before this bolt reaches the ground, it is met with an upward moving, positively charged spark. When the two collide, an explosion occurs as the return stroke travels up the bolt-the result: a visible flash called lightning.


Connections
1 Why do most people hit by lightning survive? How do airliners, which are each hit by lightning about once a year, manage to keep flying?

2. Florida has some of the strongest lightning activity in the country, while there is very little in Oregon and Washington state. Why?

3. While the initial charge of most bolts of lightning travels from the cloud down to the ground, the flash we see actually travels from the ground up to the cloud. Why does the flash look like it is traveling down?



STATIC ATTACK
LIGHTNING:
Student Activity
Charge up a balloon and watch the reactions.

MAIN ACTIVITY:

Only scientists with sophisticated equipment can actually study lightning, but the same forces of static electricity that drive lightning can be studied on a much smaller and safer scale.

In this activity you'll play with electrons, creating negative and positive charges like those that form in thunderstorms. Instead of creating bolts of lightning that can fry trees, you'll use the charges to attract and repel objects.

Materials

  • balloons
  • Styrofoam packing pellets or puffed rice cereal
  • strips of wool cloth
  • salt and pepper (The little packets from fast food restaurants work well.)
  • 1. Inflate a balloon and rub it with a wool cloth.
    2. Bring your balloon close to a handful of the Styrofoam pellets and watch what happens.
    3. Many of the pellets will cling to the balloon. Wait for several minutes and observe what happens to the pellets.
    4. Try to explain what forces were involved in the pellets being attracted to the balloon, then explain the odd behavior of the pellets that followed.
    5 Mix together a small pile of salt and pepper, recharge the balloon with the cloth, then hold the balloon very close to the salt and pepper. What happens? Is it what you expected?
    6. Go back through the experiments and chart, in writing, the positive and negative forces involved in each step and how they caused the motions you observe.

    Questions

    1. Based on what you've seen, can you explain why rubbing your feet on a carpet and then touching another person or something metal causes a shock? Why does static electricity seem to build up more in dry air than in moist air?

    2. Some materials, such as wool and human hair, give up electrons very easily and produce a negative charge in an otherwise neutral object. Other materials, suchas rubber, don't. Why?

    3. After you've explored the charges on the balloon, experiment with other objects available in your school, such as a rubber rod, a glass rod, or a piece of PVC plumbing pipe. Rub these objects with silk, fur, or wool, and see what happens when you bring them near your balloon. Do the objects attract or repel the balloon? Why?


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    Resources

    Books and articles

    Burroughs, W. (1996)
    The Nature Company guides to weather.
    New York: Time-Life Books.

    Day, J. (1991)
    Peterson first guides, clouds and weather: A simplified field guide to the atmosphere.
    Boston: Houghton Mifflin Company.

    Newcott, W. R. (1993, July)
    Lightning: Nature's high-voltage spectacle.
    National Geographic, p. 80.

    Web sites

    Automated Weather Source Online
    aws.com/index.html

    Boston Museum of Science
    www.mos.org
    (Click on the electricity exhibit)

    Lightning Retardant Cable
    207.158.219.249/
    links.htm#11

    National Weather Service, Newport, NC, Home Page
    www.eastnc.coastalnet.com/
    weather/nwsmhx/lightng.htm

    NOVA Online: Lightning!
    www.pbs.org/wgbh/
    pages/nova/
    teachersguide/lightning/

    The first of a series of linked pages that explore lightning and include several activities for the classroom.


    Try This:

    Ask a meteorologist from a local TV station, your regional National Weather Service office, or a local college or university to come to your class and talk about lightning. How do weather forecasters predict lightning? Are there different types of lightning?

    Try This:

    Research the dangers of lightning and develop a safety quiz for your school. Find the safest place to be in a lightning storm. What do you do if you're caught outdoors? Is it really safe to be in a car? Is it dangerous to take a shower or talk on the phone in a lightning storm? Why?

    Try This:

    Cut out small squares of paper and put them on a table. Put the top of a clear plastic, take-out container over the pieces of paper and rub the top vigorously with a wool cloth for 2 or 3 minutes. What happens if you stop rubbing and wait 15 minutes? What happens if you rub the plastic again?





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    Twin Cities Public Television





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