University of Virginia
Physics Department

## Neon Bulbs and Motion of Charge

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9
• PS. 11

Objectives

Students will

• understand how neon bulbs light up due to an electrical current produced by batteries and also by static electricity;
• use neon bulbs to investigate which object in a pair is giving up and which is receiving electrons.

Motivation for Learning

Demonstration: The Neon Bulb

Materials

• Neon bulb: This is available from Radio Shack for \$1.19 for two (Part # 272-1100B; if unavailable, use part # 272-1102 which is a similar bulb without the resistor. If you use #272-1102, you must include a 22kW resistor in series)
• Wire with alligator clips at the end.
• At least eleven 9-V batteries

Actual neon bulbs (Part #272-1100B)

Procedure

1. Light the neon bulb from a stack of eleven new 9-volt transistor batteries snapped together in series. If the batteries are not

new it may require more batteries to achieve the desired voltage. Continue to add batteries in series one at a time until the bulb lights up. (NOTE: different bulbs will require a different amount of batteries to charge it. Check to see what required voltage the neon bulbs that you have require. There are varying intensities of these small neon bulbs, but the experiment works much better if it takes less voltage to power it. The resistor is placed in series with the batteries and the lamp so that the lamp doesn't burn out from a high current.)
2. Show that the neon bulb lights when the circuit is completed.
3. Explain to the students how a neon bulb operates as described below in the background information.
4. Explain that they will be discovering the transfer of electrons with their very own Neon Charge Meters. We can use the neon bulb to determine whether objects are positively or negatively charged, as long as we have a sufficient potential difference to excite the neon gas in the bulb.

Background Information

Neon bulbs are used throughout the world as indicating lights. Other gases are available to produce bulbs with varying and dazzling colors. These bulbs indicate whether water is flowing from the water purification plant to our house or whether the water is pouring out all over Main St. They indicate whether the hand brake is on in our car and whether our gas tank is nearing empty. We rely on these kinds of bulbs for a variety of things, including denoting whether the power is on for this monitor you are looking at.

The neon bulb can be used as an alternative electroscope to determine electric charge. When one electrode of the bulb is brought into contact with an object that is charged, a flash occurs. The neon bulb contains two electrodes in a space filled with neon gas. The light inside the neon bulb occurs nearest the negatively charged electrode (or cathode). When the bulb is lit with the 9-V transistor batteries note that the side of the bulb that is lit is always the side connected to the negative voltage of the batteries. If you switch the battery leads, the side of the bulb that lights will also switch. The light will always occur near the negatively charged electrode whether the source of charge is from a battery or from static charge.

The voltage difference causes an electric field to form which accelerates the electrons released by the cathode (negative electrode). These fast electrons then collide with the neon gas in the bulb causing the neon atom to lose an electron (ionization) or to shift an electron to a higher energy state (excited state). When the excited electron decays to a lower energy state or when the positive neon atom collects a free electron, energy will be released by the neon atom in the form of a photon or light flash. This causes the the red-orange light characteristic of neon. The flash occurs nearest the negative electrode because that is where the neon atom is most likely excited.

This information can now be used to determine the charge of objects touched with the electrode of the bulb as will be done in the Student Activity. When one electrode is held in the student's hand and the other one touched to a charged object, one of the electrodes will have an instantaneous glow around it. The electrode nearest the flash is being charged negatively and is gaining electrons from the object. This means that the object that is touched is losing electrons. The electrons are traveling from the object to the cathode and into the neon gas. The students can then determine if the object is positively or negatively charged depending on which electrode lights up. If the electrode nearest the object that is touched is the one that flashes, then the object was negatively charged. If the other cathode lights up, then the object was positively charged, and the electrons flowed in the opposite direction.

### Student Activity

CAUTION: Do not plug the neon bulb into a wall socket. It may explode!

Materials (for each group of three)

• Small neon light bulb
• Several drinking straws
• Scissors
• Glass or plastic rod
• Wool, silk or fur
• Styrofoam cup

Procedure to make light shield:

Make a light shield for the neon bulb. It will be easier to see what is happening inside the bulb with a white backing made out of straw. Note: The bulb can also be taped to a white or a black piece of paper.

1. Cut about a four cm length of straw. Cut away half of it for about one cm.
2. Cut slits about a half cm up the straw.
3. Bend the leads of the neon bulb out at right angles and slide the neon bulb into the cut away part of the straw, leads first, so that you can see both electrodes of the bulb and the leads stick out from the slits in the straw.

Procedure:

Investigating charge transfer with the neon bulb.

To use the neon bulb, hold one lead in your hand and touch the other lead to a charged object while watching the electrodes. Try the following tests: (NEVER STICK A WIRE INTO AN 110-V ELECTRICAL SOCKET!):

• Charge a Styrofoam cup by charging the glass rod and then discharging it on the cup. The rod should be charged by rubbing with fur or silk and then discharged by touching the cup. Don't let anything else touch the cup besides the table it rests on. Touching something will discharge the stored charge on the cup. Repeat this process four or five times. Then touch one lead to the cup while holding the other lead. Note which electrode flashes and write it down.
• Rub part of a balloon with wool. Touch one lead to the charged portion of the balloon while holding the other lead. Note: This will only work on extremely dry days when enough charge can be built up on the balloon. Note which electrode flashes and write it down.
• Try rubbing your shoes across carpet and touching one lead of the bulb on a neutral object while holding the other lead. Note: Again this will only work on extremely dry days allowing sufficient charge to build up. Note which electrode flashes and write it down.

Data Sheet

Questions:

1. Why does the bulb light in each of those tests?

2. For each test, tell which object, your hand or the charged object, was losing electrons. You should be able to determine this by observing which electrode flashed.

3. When you rub your feet on the floor what is happening?

1. There is a potential difference created between the two electrodes because one is touching a charged object. This is called static electricity. The potential difference causes the bulb to light up.
2. The object that is touching the electrode that lights is the one that is losing electrons. Here is a short list of combinations of materials to determine the charge of the object touched. If a swatch of fur, wool or silk is used to charge a plastic (lucite, plexiglas, teflon, or bakelite) rod, the rod will be negatively charged and the swatch will be positively charged. If a glass rod is charged with silk, the silk will be negatively charged and the rod will be positively charged.
3. When you rub your shoes across the carpet, you are wiping electrons off your shoes onto the carpet, and giving yourself a positive charge. Objects that you touch will give you electrons, which will create a spark in the neon bulb near the electrode touching the object.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Questions to be answered during the activity.