Determining Charge with an Electroscope

Emmette Cox
Product Management Coordinator for Physical Science

The triboelectric series will give you a starting point for finding out what combination of materials provides a good result when trying to produce a static charge. The following sections provide a little more information on how to test for static electricity and how to determine which objects have positive or negative charges.

Review

Students already should be familiar with the following basic information about static electricity.

1. Electricity (static or current) is a result of the quantity charge. Charge is a physical property of matter, just like mass or volume. Charged objects experience a force when placed in an electric field.
   
2. There are 2 types of charge, positive and negative.
3. Like charges repel and unlike charges attract (Fig. 1).
   
   

   

   
   Charge is detected when 1 charged object exerts a force on another object. Several factors affect the amount of force between 2 charged objects, such as the amount of charge on each object and the distance between the objects.
4. Charge is a result of subatomic particles in an atom. An atom is made up of protons and neutrons (which make up the nucleus) and electrons (which orbit the nucleus). Protons have a positive charge. Neutrons have no charge. Electrons have a negative charge.

Testing for charge

Testing to see if an object is charged is fairly straightforward. In Fig. 2, a negatively charged balloon is brought near a piece of paper. The paper is normally neutral, with the positive and negative charges evenly distributed. However, when the negatively charged balloon is brought near the paper, the balloon repels the electrons in the paper to the far side of the paper, leaving the side near the balloon slightly positive. The attractive force between the negative charges on the balloon and the positive charges on the upper surface of the paper causes the paper to move toward and stick to the balloon.

In Fig. 3, a piece of glass with a positive charge is brought near a piece of paper. In this case the electrons are attracted to the glass and move to the top surface of the paper. This phenomenon is called charging by induction.

Another useful tool for detecting static charge is an electroscope (Fig. 4). When a negatively charged object is brought near the metal end of the scope (Fig. 5), the negative charges in the metal of the scope travel to the leaves, which both become negatively charged and repel each other. When a positively charged object is brought near the electroscope (Fig. 6), negative charges in the metal of the scope move toward the positive object, causing the leaves to have a positive charge, and the leaves repel each other. The stronger the charge, the greater the separation of the leaves.

Finding polarity

The triboelectric series will help you select materials from your electrostatics kit that generate a strong charge on an object. The electroscope will allow you to detect a charge.

How do you know if an object is positively or negatively charged? Follow these steps:

1. Select a pair of materials from the triboelectric series. Charge the objects, then select 1 to use in your experiment. In Fig. 7 through Fig. 11 below, the object has a negative charge.
   
2. Bring the object near the electroscope, as shown in Fig. 7. The leaves will separate, indicating that the object you are testing is charged.
   
3. While the object is near the electroscope, touch the terminal of the electroscope with your finger. This grounds the electroscope. In Fig. 8, the line from the electroscope represents a path to ground (the 3 horizontal lines at the end of the “wire” are the symbol for ground). When the negative object is brought near the electroscope, some of the electrons are repelled down to the leaves as before, but some take this alternate path through your finger and leave the electroscope. Attaching the electroscope to “ground” essentially means attaching the electroscope to the earth, which can act as an infinite source (or sink) for electrons.
   
4. While the negatively charged object is still near the electroscope, remove your finger, disconnecting the electroscope from ground. Some of the electrons have left the electroscope through the ground wire. The leaves will move toward each other.
   
5. Finally, move the negatively charged object away from the electroscope. The electroscope is now positively charged. The leaves will repel each other.

Now the electroscope will allow you to identify whether an object is positively or negatively charged. If you bring a negative object near the electroscope, the electrons in the scope will be repelled toward the leaves, balancing the charge. The leaves will move toward each other.

If you bring a positive object near the electroscope, it will attract the electrons toward the terminal. The leaves will move away from each other.

You can also use this process with a positive object to charge the electroscope. In this case, grounding the electroscope with your finger will allow extra electrons to move to the electroscope, which will become negatively charged.

Further investigation

Static electricity is an interesting phenomenon, and that's probably all many of us think about it—it's interesting but not very useful. However, we know that without current electricity to power our homes and run our computers and cell phones, our lives would be very different. The electric technologies we rely on every day couldn't have been built without some early scientist taking an interest in static electricity.

Static electricity can be beneficial or damaging. Some smoke stacks use electrostatic devices called scrubbers to help remove particulates from the smoke. Static electricity can damage the circuitry in some electronic devices. Therefore, parts for a computer might be shipped in special packaging to prevent damage from static electricity.

What other uses have scientists found for static electricity? Can you come up with an experiment to develop your own triboelectric series?

Suggested materials

These materials may help your investigation of static electricity.

• Metal Case Electroscope (item #756165)
• Pith Electroscope (item #756154)
• Flask-Type Electroscope (item #756160)
• Pith Ball Electroscope (item #756156)
• Leyden Jar (item #756237)
• Electrophorus (item #756246)
• Friction Rod Set (item #755819A)
• Electrostatic Charge Kit (item #751150)
• Electrostatics Kit (item #755811)
• Balloon, 9", Pack of 100 (item #974032)