Talk It Over

Have you ever noticed that socks cling to your clothes and to each other when they come out of the dryer? If so, you have noticed static electricity. It is an electrical charge that builds up in an object when electrons move to or from its surface. In the case of socks in the dryer, opposite charges (+ and –) build up and attract, causing the pieces to cling.

You can see the effects of static electricity and measure how long a static charge lasts.

Get

• Thermometer with a humidity dial
• 2 round balloons
• String
• Ruler
• Table
• Tape
• Rubber ball
• Silk or polyester scarf or fabric piece
• Rubber band
• Stopwatch

Go

1. Note and record the time, date, temperature, and humidity.
2. Blow up 2 round balloons to approximately the same size. Tie them off. Attach a piece of string about 30 cm (12 in.) long to each balloon.
3. Tape the strings to the tabletop, so the balloons hang over the edge. One should be in line with the table leg. Make them hang about 10 cm (4 in.) apart.
4. Tape the balloon near the table leg to it. Tape it securely, so it cannot move.
5. Put the rubber ball inside the scarf and secure it with the rubber band, like this:
6. Run the covered ball over the moveable balloon exactly 20 times. For each rub, move the ball from the top to the bottom of the balloon. Turn the balloon as you go so you are sure you rub the entire surface of the balloon.
7. Let go and notice what happens. When the moveable balloon sticks to the stationary balloon, start the stopwatch.
8. Stop the stopwatch when the moveable balloon falls away. (Be patient. This could take a while.)
9. Repeat this experiment at other days and times, when the temperature and humidity vary.

Stay Safe

Rub a balloon on your hair and you may hear a crackle or feel your hair stand on end. The static charge won't hurt you, but the sound will give you a good idea of what static electricity is all about.

Go Easy

Instead of investigating temperature and humidity, you can compare different materials used to rub the balloon to answer the following question: Does one material produce a longer lasting static charge than another? Try comparing wool, silk, polyester, plastic wrap, and any other rubbing materials you can think of.

Go Far

Conduct the experiment indoors and outdoors under as wide a range of temperatures and humidities as possible.

Show Your Results

Put times in a data table like this for "Go Easy":

Make a bar graph that compares the materials you tried (on the horizontal axis) by how long the charges they induced lasted (on the vertical axis).

For "Go," use a data table like this. Complete a column for every trial of the experiment.

Make a line graph showing how long the charge lasted (on the vertical axis) by the temperature (on the horizontal). Make a separate graph of the same type for humidity. See whether the slopes of the lines reveal anything about the effects of either temperature or humidity on how long a static charge lasts.

For "Go Far," make a scatterplot of your data to look for possible correlations between temperature, humidity, and how long a static charge lasts. (Correlate means vary together in some predictable way.) If, for example, static charges last longer on warm days, then the correlation between those two variables would be positive. If they last longer on cold days, then the correlation would be negative. If the numbers are random in relation to each other, then there is no correlation.

The first step toward finding a correlation is a scatterplot graph. Use your data to make the graph. Put temperature or humidity along the horizontal axis. Put the time the charge lasted on the vertical. Then, for each trial, find the point where temperature (or humidity) and charge-time intersect. Put a dot at that point. For example, if the humidity was 40 percent and your charge lasted 50 minutes, your first dot would be placed like this:

After you have made a dot for every data set, study where the dots lie. If they appear to lie (more or less) along a straight line, then draw it. If your line slants upward, you may have a positive correlation. If it slants down, you may have a negative correlation. If the points are so spread out that no line seems to match them, you may have found that temperature (or humidity) and charge-time are not correlated.

Tips and Tricks

• If you can't find scarves or fabric pieces to rub on your balloons, try jackets, shirts, socks, or hats. Check the label to determine fiber content.
• If you don't have a thermometer with a humidity dial, check your local weather report online at http://weather.noaa.gov.
• Don't touch your balloons while you are timing. You'll take away some of the charge. Even walking by or fanning the balloons with air can make a difference, so set up your experiment out of drafts and high-traffic areas.

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