An electroscope1[a device for detecting electrical charge] is a simple instrument used to detect electrostatic charge2[electric charge at rest on a body]. It is based on the principle that like charges repel each other. In this experiment, an electroscope is built using a jam jar, a copper wire3[conductive metal filament], and aluminum foil. Frictional electricity4[generation of charge by rubbing two materials together] is used to generate an electric charge, which is transferred to the copper wire. The charge distributes itself over both aluminum strips, causing them to repel due to their like charges. This experiment allows observation and investigation of electrostatic phenomena5[phenomena related to stationary electric charges], including charge separation6[separation of positive and negative charges], electrostatic induction7[generation of charge without direct contact], and Coulomb’s law8[law for calculating the force between two charges].
Null Hypothesis: A metallic conductor inside the jar cannot retain an electrostatic charge permanently, and no visible movement of the aluminum foil occurs.
1. Introduction
Electrostatic charge arises when positive and negative charges are separated. This can occur through friction, contact, or induction. An electroscope is a device that makes electric charge visible by utilizing the repulsion of like-charged bodies. It consists of a metal rod [conductive rod made of metal] that absorbs charge and transfers it to two movable metal strips. These strips repel each other when charged similarly and move apart.
The goal of this experiment is to qualitatively detect electrostatic charges. Various materials are tested for their ability to generate charge, and the transfer of charge onto the electroscope via contact or proximity to a charged object is examined.
2. Material and Methods
Material
- Empty jam jar with a screw lid
- Copper wire (~10 cm, Ø approx. 1 mm)
- Aluminum foil (~2 × 3 cm per strip)
- Nail or drill (to make a hole in the lid)
- Plastic rod (e.g., ruler, comb, PVC pipe)
- Wool cloth or silk cloth
- Optional: Glass rod for generating positive charge
Method
A hole is drilled into the lid of the jam jar, through which a copper wire is inserted. The wire is bent to form a small loop or ball at the top, serving as the contact surface for charges. Inside the jar, the wire is shaped into a hook, onto which two bent aluminum strips are placed. These strips are free to move so that they can either repel or attract each other in response to changes in charge.
To generate charge, a plastic rod is rubbed with a wool cloth, making it negatively charged. Alternatively, a glass rod is rubbed with a silk cloth to generate a positive charge. The charged rod is then brought near or touched to the copper loop of the electroscope, transferring charge to it.
Observations are made immediately after the charge is transferred. The two aluminum strips should visibly move apart as they repel each other. The degree of movement depends on the amount of charge transferred. If the wire is touched with a hand or a grounded metal object, the electroscope discharges, and the aluminum strips return to their original position.
3. Results
Charge transfer leads to a visible movement of the aluminum strips. Depending on the strength of the applied charge, the strips move more or less apart. This effect persists until the charge is dissipated through contact with a grounded object or gradually lost to the air.
Different materials produce varying effects. A plastic rod rubbed with wool generates a negative charge, while a glass rod rubbed with silk transfers a positive charge. In both cases, movement of the aluminum strips is observed.
Additionally, it is evident that charge can be transferred without direct contact. When a strongly charged body is brought near the electroscope without touching it, the aluminum strips also move apart. This occurs due to electrostatic induction, in which the electric field9[the region around a charge where forces act on other charges] of the charged body causes charge separation within the electroscope.
4. Discussion
The movement of the aluminum strips in the electroscope is a direct result of the electrostatic charge that is transferred to the system through friction, contact or induction. To understand this, it is first necessary to explain how charge is generated and why it is transferred to the metal parts of the electroscope.
Generation of charge through friction
Charge can be generated by various mechanisms. In this experiment, triboelectric charging10[charge separation by friction between two materials] is used to charge the electroscope. The following happens when a plastic rod is rubbed against a woolen cloth:
- Every material consists of atoms [the smallest building blocks of matter] that contain electrons11[negatively charged elementary particles].
- Some materials have a stronger affinity for electrons than others, i.e. they absorb or release electrons more easily.
- When two materials come into contact and are separated again, electrons can be transferred from one material to the other.
- In our case, the wool cloth emits electrons and the plastic rod absorbs them. As a result, the plastic rod becomes negatively charged.
This effect is described by the triboelectric series, which indicates which materials are more likely to release electrons and which are more likely to accept them. Materials such as glass emit electrons and become positively charged, while PVC or acrylic absorb electrons and become negatively charged.
Transfer of charge to the electroscope
As soon as the plastic rod, which now carries a negative charge, touches the copper loop of the electroscope or comes close to it, a charge separation occurs:
- Direct charge transfer through contact
- If the plastic rod touches the copper loop directly, some of the excess electrons jump over to the copper wire.
- As copper is a very good conductor of electricity, this charge is immediately distributed over the entire wire and also to the aluminum strips.
- As similar charges repel each other, the aluminum strips move apart.
- Charge transfer through induction
- If the plastic rod is only held close to the copper loop, electrons can redistribute within the wire without direct contact taking place.
- As negative charges are repelled by the plastic rod, which is also negatively charged, some electrons migrate from the upper part of the wire into the aluminum strips.
- As a result, the strips become negatively charged and repel each other.
- If the electroscope is touched by hand during this process, excess electrons can flow into the body (earthing) and a positive charge remains on the electroscope.
These two mechanisms show that charge transfer can take place both through direct contact and through the electric field of a charged body.
Why do the aluminum foil strips move apart?
The repulsion of the aluminum strips can be explained by Coulomb’s law, which describes the electrostatic force between two charges:
These are:
- F is the force between two electrical charges,
- k = 8.99 x 109 Nm2/C2 the Coulomb constant12[natural constant for the strength of electrostatic interaction],
- q1 and q2 dthe amount of charge in coulombs13[unit of measurement for electrical charge],
- r is the distance between the two charges.
As both aluminum strips have the same type of charge (both positive or both negative), the force F is positive, which means that the two charges repel each other.
If typical values are used, e.g. a charge of q = 10-9 C per strip and a distance of r = 2 cm, the result is:
This small but measurable force is sufficient to move the aluminum strips apart. The greater the charge transferred, the stronger the repulsion.
Influence of external factors on charge retention
The environment influences the stability of the charge on the electroscope. In particular, the humidity14[proportion of water vapor in the air] plays a major role:
- Water in the air can act as a conductor for electrons, so that the charge flows away quickly and the electroscope loses its effect.
- In dry air, the charge lasts much longer as there are fewer free water molecules to balance the charge.
- A sealed electroscope with silica gel15[hygroscopic material for absorbing moisture] could slow down the discharge.
Possible extensions to the experiment
Dieses Experiment kann durch verschiedene Variationen erweitert werden:
- Investigation of the triboelectric series:
- Which materials generate the strongest charge?
- How does the charge differ if a glass rod is used instead of a plastic rod?
- Investigate the influence of the environment:
- How long does the charge last in a dry vs. humid environment?
- Which materials discharge faster?
- Quantitative measurement of the charge:
- A scale could be attached to the electroscope to measure the angle of the diverging aluminum strips.
- A laser pointer could be aimed at the strips so that the deflection of the reflected light can be seen on a distant wall.
- Comparison with a professional electroscope:
- How do the results of a home-made electroscope differ from those of a laboratory model?