Susan KangLab Partners: February 12, 2014Lab #2: Electrostatics

Abstract

The purpose of this experiment is to observe how positive and negative electric charges interact with each other by rubbing isolator materials and learn how to measure the amount of charge using the principles of a Van de Graaff generator and an electroscope.

Introduction

Electrostatics is the branch of science that deals with the phenomena arising from what seem to be stationary electric charges. It involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge.

The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which yields static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena of contact electrification and the triboelectric effect. Rubbing two nonconductive objects generates a great amount of static electricity. This is not just the result of friction; two nonconductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some examples of these substances are rubber, plastic, glass, and pith. Conductive objects only rarely generate charge imbalance except, for example, when a metal surface is impacted by solid or liquid nonconductors. The charge that is transferred during contact electrification is stored on the surface of each object.

Versorium is a device for detecting the presence of static electricity. The versorium is a needle constructed out of metal which is allowed to pivot freely on a pedestal. The metal needle would attract charged bodies that are brought near it, turning towards the charged object. Since it is able to distinguish between charged and non-charged objects, it is an example of an electroscope. It can be noted that the versorium is of a similar construction to the magnetic compass, but is influenced by electrostatic rather than magnetic forces. Versorium only signifies the presence of charge, not whether the charge is positive or negative. The type of charge is determined by following the initial charging of the device with the application of a known charge (from amber, fur or some other familiar, well-studied material). In addition, it is useful in determining the relative amount of charge present. The versorium does not identify

positive and negative charges. Either a positive or a negative charge will attract the aluminum. It does show that a static charge is present, even if it is too weak for our senses to detect.

When you rub two different materials together, one will lose electrons and have a positive charge. The other will gain electrons and have a negative charge. In some ways, static charges are like magnetic charges. Two charges that are different will attract. Charges that are the same will push apart or repel. If you bring a rod that is negatively charged near the aluminum that is not charged, something neat happens. The negative charges in the end of the aluminum are pushed away (like charges push away), leaving a positive charge on the end of the rod. Opposite charges attract, so the positive charge on the end of the aluminum is pulled towards the negative charge on the rod.

An electroscope is a common demonstration apparatus used to illustrate electrostatic principles of charging and charge interactions. The electroscope is most commonly used as a charge-detecting device. The electroscope consists of a plate (near the top), a support stand (which connects to the plate and extends through the center of the scope), and a needle which rests upon the support stand and is free to rotate about its pivot. The plate, support stand, and needle are all made of a conducting material which allows for both the free flow of electrons and the distribution of any excess charge throughout the electroscope. By observing any deflection of the needle, the presence of charge in either the electroscope or a nearby object can be determined.

One common demonstration performed with the electroscope involves the induction process of charging. In the induction process of charging, a charged object is brought near to but not touching the electroscope. The presence of the charged object above the plate of the electroscope induces electrons within the electroscope to move accordingly. With the charged object still held above the plate, the electroscope is touched. At this point electrons will flow between the electroscope and the ground, giving the electroscope an overall charge. When the charged object is pulled away, the needle of the electroscope deflects, thus indicating an overall charge on the electroscope.

A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe. It is an electrostatic generator that is capable of producing large static electric potentials. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. On a Van de Graaff generator, if you put a load, the current (amperage) remains the same. What varies is the voltage, which varies with the load. The large sphere acts as a capacitor to store the charge transferred up its supporting column. The small sphere (connected to the ground potential) will draw an arc from the larger when the air gap breakdown voltage is exceeded.

Methods

Part A: Van De Graaf demonstration-Record observations about how different objects interact with the Van De Graaf when turned on-Discharge Van De Graaf after every use.

The objects include: Electric Whirl, Animal Fur, A gas exchange bulb, A conductor, Fluorescent Tube, Tissue Paper.

Part B: Versorium-Set up versorium by placing the strip foil on the needle. Place it, so it is stable and can spin freely-Make sure the versorium is discharged by touching it-Take one of the rods and one of the fabrics. Rub the fabric vigorously against the side of the rod -Place the rod about one centimeter away from the foil of the versorium and record any observations of behavior. Record if the interaction is attractive or repulsive, strong or weak.-Discharge the rod and the versorium-Rub the same material against the same rod to charge it and touch the charged rod to the versorium. Once again record observations about the resultant behavior. -Repeat steps above for the rest of the materials and the same rod-Repeat steps for the different rods (glass, nylon, and plastic) and a combination of sample materials (cotton, silk, wool, and animal fur) for both induction and conduction

Part C: Electroscope-Discharge electroscope by placing the small sphere against the plate located on top of the electroscope-Take one of the rods and a sample material. Rub the sample material against the side of the rod-Place the rod near the electroscope, around one centimeter away, for induction Observe the degree of deflection from the center of the arm. Record the interaction if it was strong or weak-Discharge the rod and electroscope-Using the same material and rod, charge the rod by rubbing the material against it.-Touch the tip of the rod to the small plate at the top of the electroscope for conduction Observe how much the arm moves. Record any observations-Repeat steps for the different rods (glass, nylon, and plastic) and a combination of sample materials (cotton, silk, wool, and animal fur) for both induction and conduction

Part D: Electrostatic Attraction and Repulsion-Charge a PVC rod with cotton and observe and explain how the pith balls react-Record observations such as attractive repulsive, weak or strong.-Repeat using the different materials

Results

Table 1: Van De Graff Demo ObservationsMaterial Observations ExplanationsElectric Whirl - Originally not moving

- Turned on- whirl starts to spin- Discharge-ceased to move

The ends of each wire acquire an electrostatic charge. The tip has a strong electric field due to its sharpness, the field ionizes nearby atoms and repulsion causes the object to spin.

Animal Fur - Floats above, and then falls off- Hairs on fur stand individually- Discharge-lies flat on the sphere

The hairs of the fur acquire a charge causing individual strands to repel and separate and stand. Eventually the entirety of the fur becomes charged and experiences repulsion form the Van de Graaf.

Gas discharge tube - Lights up- crackling noise.

The gas inside the tube becomes excited by the influx of charge from the Van de Graaf, causing light to be emitted.

Conductor A long spark along with crackling was created in close proximity to dome

The electric fields of the conductor and sphere create a visible current between the two objects.

Fluorescent Tube Light emitted when brought near dome A current runs through the tube causing it to light up.

Tissue Paper Pieces stood up The tissue paper becomes charged and repels from one another.

Table 2: Versorium : Induction and Conduction for rods and materialsRod Material Induction ConductionGlass Cotton No Reaction Strong attractionGlass Wool Weak attraction Strong attractionGlass Satin Strong attraction Weak attractionGlass Fur Strong attraction Weak attractionPlastic Cotton Weak attraction Weak attractionPlastic Wool Weak attraction Weak attractionPlastic Satin Strong attraction Weak attractionPlastic Fur No Reaction Weak attractionNylon Cotton No Reaction Strong attractionNylon Wool No Reaction No ReactionNylon Satin Weak attraction Strong attractionNylon Fur Strong attraction Strong attraction

Table 3: Electroscope: Induction and Conduction for rods and materialsRod Material Induction ConductionGlass Cotton No Reaction No reactionGlass Wool No Reaction No reactionGlass Satin No Reaction No reactionGlass Fur No Reaction Moved slightly

Plastic Cotton No Reaction Moved slightlyPlastic Wool No Reaction No ReactionPlastic Satin Moved Slightly Moved slightlyPlastic Fur No Reaction Moved about 3

degreesNylon Cotton Moved about 15

degreesMoved about 50 degrees

Nylon Wool No Reaction No reactionNylon Satin No Reaction Moved around 3

degreesNylon Fur Moved about 20

degreesMoved about 30 degrees

Part D: Electrostatic Attraction and RepulsionMaterial Induction ObservationsCotton No ReactionWool No reactionSatin No ReactionFur Strong Attraction

Conclusion/Discussion

Some materials are better than others at accepting or donating electrons. To measure the ability to transfer electrons, an electroscope and a versorium were used. The amount of charge transferred is reflected in the extent of movement produced in the devices. The pith balls showed how charged objects can cause the polarity of neutral objects to shift.Experimental Uncertainties:

-Whether or not the electroscope and versorium were fully discharged before each use-How charged the rods became-Humidity

Questions

1. A conductor allows for electrons to move freely amongst atoms or molecules. Inductors impede the flow of electrons between atoms and between molecules. Glass and plastic are not conductors they are categorized as insulators. Electrons are bound more tightly to the atom in conductors, but that does not completely impede all movement of electrons. Although they’re not conductors, they can still carry charges, if enough is created.

2. Yes, a charged body can attract a neutral conductor ball hanging on an isolating wire. The charged body induces a dipole in the uncharged body. Electrons in the uncharged body move away or toward the charged body depending on the charged body’s polarity. This causes the side closest to the body to adopting the opposite charge of the charge body, resulting in the attraction of the two bodies. The number of protons and electrons

does not vary, instead the distribution across the surface of the object changes to polarize the material. To become charged, electrons must be transferred. While there is a separation of charge, there is not an imbalance of charge. When neutral objects become polarized, they are still neutral objects.

3. One can test if an object is charged, by placing that object in proximity to neutral pith balls. If the object is charged, the ball would polarize and become attracted to the object.

4. A charged body may induce a dipole in an uncharged body resulting in attraction. Uncharged bodies are able to attract each other due to induced charged but uncharged bodies will never repel each other. Repulsion only occurs between two similarly charged bodies. So, repulsion is a sure way to test for charge. Even if you know the type of material beforehand, repulsion is still the only finite way to test for charge.

5. Dry conditions are necessary for observation of charge separation because moisture would prevent accumulation of charges. Moisture makes the atmosphere more conducive, so charge is more likely to be transferred to the surroundings instead of accumulating within the charged material. Voltage does not get large in the Van de Graaf in humid conditions, because the humidity causes the charge to be transported away from the Van de Graaf. A Van de Graaf strips air molecules of their electrons; the belt transports the charges upward where they accumulate on the metal sphere. Water in the air causes the charge to leak away, preventing the build up and therefore high voltage.

6. A sharp edged lighting-rod tip would be better than a spherical tip. The surface of a sphere distributes charge equally, so the electric field strength is the same everywhere. Sharply curved surfaces result in strong electric fields. The greater concentration of charge surrounding a pointed object enhances its conductive ability by ionizing the surrounding area. This would be more efficient in making sure the lightning would go through it and nit through anything else in the house. The unequal distribution would better, for if the surface was equally charged everywhere there is more of a chance that it could be distributed to surrounding objects. The Van de Graaf is spherical and therefore charge is equally distributed. The rods were sharper edged causing the charge to stay in a single area.