Law of Charges. Two charged rubber rods (negative) will repel each other; two charged glass rods (positive) will also repel each other. Therefore, like charges repel. In contrast a charged rubber rod will attract a charged glass rod. Therefore, unlike charges attract.
The Electron Theory of Matter. To understand the production of static (nonmoving) charges and interpret their effect, we must consider the structure of matter at the atomic level. The electron theory tells us that all the elements are composed of atoms. Each element possesses a unique atomic structure, and this determines the distinctive properties of the element.
a. The Nucleus. According to a simplified version of electron theory, the atom consists of a very dense core called the nucleus, comprised of neutrons and protons, and collectively referred to as nucleons. The proton carries a single unit positive charge. The neutron, with about the same rest mass as the proton, carries no electrical charge.
b. Electrons. The third basic particle in the atom is the electron. Electrons are simply imagined as revolving in shells or orbits around the nucleus, at various fixed distances. These electrons are sometimes referred to as orbital electrons. An electron possesses a rest mass 1/1840th the mass of a proton, and it has a negative charge of the same magnitude as that possessed by the proton. The distances between the nucleus and the electron shells, and between the shells themselves, are relatively great compared to the diameter of the nucleus. Thus, the atom is comprised mainly of empty space.
c. Neutrality of the Atom. Normally, the number of revolving electrons is the same as the number of protons in the nucleus. Hence, the total unit negative charges of the electrons are equal to the total unit positive charges of the nucleus. These charges neutralize each other, and the atom is said to be neutral in terms of charge. This is the normal state to which a charged atom tends to return.
d. Loss and Gain of Electrons. Since both the proton and the neutron are about 1840 times more massive than the electron, it is evident that nearly all of the mass of the atom is contained in the nucleus. The protons and the neutrons are closely bound together and cannot be disrupted except by enormous forces. Electrons, on the other hand, move with relative ease from one atom to another. Setting up a charge on a body, therefore, is always explained in terms of either the loss or gain of electrons - not of protons.
Conductors and Insulators. Some materials conduct electrons better than others. Substances having a large number of free electrons are good conductors. This is true of metals because their free electrons move easily between neighboring atoms. Silver, copper, and aluminum, in that order are the best of the common conductors. Substances whose atoms hold on to their electrons strongly are poor conductors or good insulators. They do not readily transmit electrons. Glass, mica, rubber, sulfur, and dry air are examples of good insulators. When an insulator is charged, the charges do not travel; they stay where they are placed. This is what is meant by the term electrostatics - the charges stay where they are put. When a conductor is charged, the charges spread out all over its outer surface.
Charging by Contact. An object may receive an electric charge by being brought into contact with a charged body. This is charging by conduction. In this case, electrons actually move from one body to another. The charge received is always of the same sign as that of the charging body.
The Electroscope. This is a simple yet sensitive instrument employed to detect the presence, kind, and amount of electric charge. In its simplest form, it consists of a very thin, low-mass carbon-fiber tube with a straight pin pierced through its center of mass. It is in turn mounted to and balanced on a vertical S-shaped metal bracket to which a flat metal disc is attached at the top exterior of the body of the electroscope. The bracket is supported by an insulator and enclosed in a cylindrical metal housing with a dial graduated in 10-degree increments on the surface of a rigid back-plate. When a positive or negative charge is placed on the knob, it spreads over the bracket and the rotating arm. Since the bracket and arm become charged alike, they repel each other. The amount of rotation of the arm, hence the extent of the repulsion, indicates the strength of the charge. The arm will remain in this position as long as the charge remains constant.
Electron and Nucleon Facts:
Rest Mass of Electron (me) = 9.1 x 10exp-31 kg
Rest Mass of Proton (mp) = 1.7 x 10exp-27 kg
Rest Mass of Neutron (mn) = 1.7 x 10exp-27 kg
Charge of a Single Electron = 1.60 x 10exp-19 Coulomb (C)
1 coulomb (C) = 6.25 x 10exp25 electrons (elementary charges)
Transfer of Charge
Charging by Contact (Conduction). An exchange of electrons, or transfer of charge, causes the phenomenon associated with the rubber and glass rods. If the rubber rod is rubbed against fur and then brought near a low-mass non-conducting ball (such as a pith ball) suspended from an insulating thread the negative charge on the rod repels the electrons in the pith ball, causing them to move to the far side of the ball. As a whole, the ball remains neutral in sum charge, but the redistribution of electrons causes the ball to become electrically polarized. The side of the ball closest to the rod becomes positively charged and the side of the ball farthest from the rod becomes negatively charged. The rod's excess electrons attract the protons on the near side of the ball and repel the electrons on the far side. The force of attraction is stronger (owing to the shorter distance between the rod and the ball's positively charged near side), and so the ball moves toward the rod. When the ball touches the rod, some of the rod's excess electrons transfer to the pith ball. The ball gains electrons and becomes negatively charged. This method of transfer is called charging by contact (or conduction). The ball and the rubber rod are now both negatively charged and they repel each other. The ball moves (and remains) as far away from the rod as possible.
When a glass rod is rubbed against a swatch of silk and placed near a pith ball the charged glass rod also polarizes the ball but in the opposite fashion. The ball's electrons are attracted to the positively charged glass rod and move to the side of the ball closest to it. When the ball touches the rod, some of the ball's electrons transfer to the rod, leaving the ball with more protons than electrons. (The rod remains positively charged because it gains only a few electrons.) At this point both the ball and the glass rod are equally positively charged and the force of repulsion keeps them apart.
Two pith balls that have made contact with a charged rubber rod will repel each other because they have both gained electrons and acquire a negative charge. Two pith balls that have contacted a charged glass rod will repel each other because they have lost their electrons and become positively charged. Finally, two pith balls that have touched different rods attract one another because they have become oppositely charged.
Charging by Induction. A charged object may induce an opposite charge in a neutral object without touching it. First, the neutral object must be grounded - that is, it must be connected to an object so large that it can accept or give up a significant number of electrons without becoming noticeably charged. (The Earth is often used as a ground.) Once the neutral object is grounded, a charged rod is is brought near it without touching it. If the rod is negatively charged, it repels the electrons of the neutral object, forcing them to transfer into the ground. If the object is then disconnected from the ground while the negatively charged rod is nearby, the object will then be left with a net positive charge. Vice-versa with a positively charged rod. In both cases, the charge acquired by the previously neutral object is opposite of that of the rod used to charge it.
Conservation of Charge. In all cases of charge transfer, the law of conservation of charge applies: Charge is never created or destroyed; a gain of charge in one place must correspond to a loss of that type of charge in another place.
Detection of Charge. The presence of excess charge on an object can be detected by bringing the object near an electroscope. If either a positively or negatively charged object is brought near the rod of an electroscope, the electrons within the electroscope are forced to rearrange themselves, and the electroscope becomes polarized. The bracket and the rotating arm acquires the same type of charge as the charged object, and they separate. A neutral electroscope cannot be used to distinguish between positively and negatively charged objects. However, an electroscope can be charged (either by contact or induction) and then used to detect the presence and type of charge brought near it. The arm and bracket of a charged electroscope are separated due to the charge on them. If an object brought near the knob of the charged electroscope has the same charge as the electroscope, the arm will diverge even farther from the bracket. If the object brought near the charged electroscope is oppositely charged, the arm will return to a vertical position.
Materials
solid rubber rod
glass rod
swatch of fur
swatch of silk
pith ball
confetti
10-cm length of thread
insulated stand
electroscope (with rotating arm)
Procedure and Data
A. Types of Charges. Vigorously rub the solid rubber rod with the fur swatch. Bring the charged rod close to a pile of confetti.
1. Result: ______________________________________________________________
Charge the solid rubber rod again by rubbing it against the fur. Slowly bring the charged rod toward the pith ball suspended from the insulated stand. Allow the pith ball to touch the solid rubber rod.
2. At first the solid rubber rod ___________________ (attracts, repels) the pith ball and then _____________ the pith ball.
Charge the glass rod by rubbing the rod vigorously with the silk swatch. Bring the charged rod close to the suspended pith ball.
3. The charged glass rod ____________________ (attracts, repels) the pith ball.
B. Charging the Electroscope by Contact. Charge the rubber rod again by rubbing it against the fur. Now bring the charged rod near the knob of the electroscope.
1. Result: ____________________________________________________________________________
Remove the rubber rod.
2. Result: ____________________________________________________________________________
Touch the knob of the electroscope with the charged rubber rod.
3. Result: _____________________________________________________________________________
Bring the charged rubber rod near the knob of the electroscope.
4. The arm of the electroscope _____________________ (becomes more vertical, becomes more horizontal).
Charging the Electroscope by Induction. Charge the rubber rod again. Bring the charged rod near the knob of the electroscope.
5. The arm of the electroscope _____________________ .
Ground the electroscope by touching the knob with your finger, still holding the charged rod near the electroscope.
6. The arm of the electroscope _____________________ .
First take your finger away from the knob and then remove the rubber rod.
7. The arm of the electroscope _____________________ .
Again bring the charged rubber rod near the knob of the charged electroscope.
8. The arm of the electroscope _____________________ .
Questions
1. Briefly define and explain charging by induction. How does induction allow us to gather charge? Explain why work is done when rubbing the rubber and glass rods with fur and silk respectively. What role does friction play in this situation?
2. a. Rubber rods and fur define negative charges. Explain this statement
b. Glass rods and silk define positive charges. Explain this statement.
c. What is the charged particle that is mobile in each of the above cases?
d. Why is this particle so mobile?
3. Describe what occurs during the process of grounding.
4. What phenomenon occurs when objects possessing like charges are placed near one another? What phenomenon occurs when objects possessing unlike charges are are placed near one another?
5. With a charged rod in your grasp, test a fine stream or individual drops of water from the lab faucet. Describe your observations. What does this tell you about the electrical nature of water molecules? Briefly describe how a charged object can attract matter.
6. When an object charges another by contact, what is the sign of the charge that is acquired? When an object charges another object by induction, what is the sign of the charge that is acquired?
