Abstract

Objective

The goal of this project is to measure how many volts are produced by sparking barbecue fire starters. You can do this by measuring the distance the spark can travel. Two metal balls connected to the sparking element are used as electrodes for this experiment. How is the distance affected by objects in the spark gap?

Introduction

Air normally acts as a good insulator (or dielectric). However, when the voltage across an air gap becomes sufficiently high, electrons are stripped from the air molecules, ionizing the air and allowing current to flow. This is what is happening when lightning strikes during a thunderstorm. The process is called dielectric breakdown, and the voltage at which it occurs is the dielectric breakdown voltage.

The dielectric breakdown voltage for air is approximately 3000 V/mm (= 3 kV/mm), but also depends on other factors such as the geometry of the gap and the air pressure.

Figure 1. Photo of a piezoelectric barbecue fire starter. When the button is pressed, a high voltage is generated between the positive electrode (red) and the ground electrode (six "fingers" surrounding the positive electrode), and a spark jumps across the gap.

When the red button is pressed, a spring-loaded hammer inside the plastic handle hits a piezoelectric (quartz) crystal. Enough voltage is generated to produce a spark in air. The spark ignites the volatile lighter fluid and the resulting flames heat and ignite the charcoal.

In this project you'll learn how you can use two metal spheres (e.g., Baoding exercise balls, pinballs, or large ball bearings) with the barbecue fire starter in order to get a good estimate of the voltage produced by the piezoelectric mechanism. You'll connect the electrodes up to the metal spheres, and measure the distance that the spark can jump between them (see Figure 2, below). From the distance, you can calculate the approximate voltage between the two spheres. You can also investigate what happens when sheets of material (either conducting or insulating) are placed between the two spheres.

Figure 2. Measure the distance the spark can jump, both with and without sheets of material between the conducting spheres.

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• piezoelectricity,
• piezoelectric crystal,
• dielectric,
• dielectric breakdown voltage.

Questions

• What are some other applications of piezoelectricity?
• What are the typical voltages created in other applications?

Bibliography

• The following Wikipedia links will be useful:
  
  • Wikipedia contributors, 2006. "Piezoelectricity," Wikipedia, The Free Encyclopedia [accessed May 26, 2006] http://en.wikipedia.org/w/index.php?title=Piezoelectricity&oldid=55283535.
  • Wikipedia contributors, 2006. "High Voltage," Wikipedia, The Free Encyclopedia [accessed May 26, 2006] http://en.wikipedia.org/w/index.php?title=High_voltage&oldid=55292514.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• piezoelectric barbecue fire starter (available at your local hardware store; the one in the picture above is made by Char-Broil (model 4638, PN 3496404),
• metal spheres (e.g., Baoding exercise balls, pinballs, or ball bearings with diameter at least 25 mm),
• modeling clay,
• electrical tape,
• large piece of stiff cardboard,
• ruler,
• screwdriver,
• sheets of conducting and insulating materials, e.g.:
  • paper,
  • plastic,
  • aluminum foil.

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Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms and concepts above.
2. The apparatus for this experiment is quite simple. You need a non-conducting surface to hold the two spheres and the barbecue starter electrodes. You also need a ruler for measuring the distance between them. Figure 3 shows our setup, made with a piece of cardboard folded into a V-shaped groove. We held the electrodes in place with electrical tape. The ground electrode remained fixed in place. We moved the positive electrode when changing the gap distance between the conducting spheres. The straight edge of the protractor was used as a ruler because it was a handy size for the experiment.

   Figure 3. Barbecue sparker experimental apparatus. Fold the cardboard into a V-shaped groove, and tape the electrodes in place.

3. Place the conducting spheres on the cardboard V-groove, with an air gap between them. Each sphere should be in firm contact with one of the electrodes. Use a bit of modeling clay to hold the spheres in place, if needed.
4. Measure the distance of the air gap. Hint: you can use a sheet of paper held taut and just touching each ball to get an accurate reading on the ruler.
5. Press the button of the sparker and see if a spark is created across the gap. (Remember not to touch the conducting spheres!) You may want to dim the lights to see the spark better.
6. Repeat at least three times to be sure that your result is consistent. If there was no spark, first discharge any remaining potential between the two spheres by shorting them with a screwdriver (hold it by the insulated handle and touch the metal shaft to both spheres).
7. Adjust the gap and repeat steps 4–6. If there was no spark, make the gap smaller. If there was a spark, make the gap larger. Keep going until you have determined the largest gap that reliably produces a spark. Use this distance to calculate the voltage produced (remember from the Introduction that the dielectric breakdown voltage of air at standard temperature and pressure is approximately 3000 V/mm).
8. How does the maximal gap distance change if you insert a sheet of material between the conducting spheres (see Figure 4, below)? For example, you could use paper, plastic, or aluminum foil (fold a larger piece in half 5 times and press together). Can you explain the results? What happens if the sheet of insulating material is smaller than diameter of the conducting spheres?

   Figure 4. Closeup of barbecue sparker experimental apparatus with a sheet of plastic between the conducting spheres.

Variations

• Does the shape of the electrodes matter? Can the distance be increased with a different electrode shape?
• Advanced. The dielectric breakdown voltage of air changes with pressure. If you are good at building things, you could extend this project to investigate the effect of pressure. You'd need a clear plastic chamber that could be sealed tight. The electrodes would go inside the chamber, and you would need to be able to change and measure the distance between them. You would also need an access port for pumping air (or another non-flammable gas), in or out of the chamber, and a way to measure the pressure. How does the maximum distance between the electrodes change with pressure? With different gases (e.g., nitrogen, carbon dioxide, or helium)?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

Written by Richard Blish, Ph.D.  

Edited by Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-06-09 21:30:00