PhysicsLAB: Electrostatics Fundamentals

A positive charge means that the object has lost electrons and is no longer electrically neutral. Each electron lost gives the particle a charge of +1.6 x 10^-19 coulombs. Positive, or vitreous, charges are classically created by rubbing a glass rod with silk. The rod becomes positive (loses electrons); the silk become negative (gains electrons). Since electric charge is conserved, the system (glass rod and silk) maintains a net charge of 0.

A negative charge means that the object has gained electrons. Each electron gained gives the particle an additional charge of -1.6 x 10^-19 coulombs. Negative, or resinous, charges are classically created by rubbing a rubber rod with fur. The rod becomes negatively charged; the fur positively charged. By definition, negatively charged objects have more mass than an identical neutral object since each extra electron has a mass of 9.11 x 10^-31 kg.

Three modes of electrifying an object: friction, conduction and induction.

Electrification by friction occurs when two surfaces are rubbed together. Examples of this were discussed above when a positive charge was created by rubbing glass with silk and a negative charge was created by rubbing rubber with fur. The following list details a larger portion of the triboelectric sequence. When any two substances shown in this list are rubbed together, the top one will become positively charged while the lower one will become negatively charged. The further apart the two substances are in the list, the greater the electrification.

+ -	Asbestos Fur (rabbit) Glass Mica Wool Quartz Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Copper, Brass Rubber Sulfur Celluloid India rubber

Charging by conduction means that the charging rod actually touches the electroscope’s knob. Since there is contact, electrons from the knob would flow onto a positive rod or off of a negative rod. Charging by conduction leaves the electroscope with a residual charge IDENTICAL to that of the charging rod.

Charging by induction means that the charging rod is brought close to the electroscope’s knob but NEVER touches it. If the electroscope is not grounded, it will remain neutral but be temporarily polarized while the charging rod is in the immediate vicinity. That is, a positive rod will induce the electrons in the scope to migrate to the knob. This redistribution of charge will result in the leaves of the scope being positively charged

If the electroscope is grounded during induction, electrons will flow from the knob to the ground if the charging rod is NEGATIVE and electrons will flow onto the knob if the charging rod is POSITIVE. The net effect once the grounding wire is removed is that the electroscope will be left with a residual charge that is OPPOSITE to that of the charging rod.

Suppose the positive rod is brought near to an insulator, for example, a piece of paper or a section of a wall. Since electrons are not free to move within an insulator, another process takes place which still results in the paper or wall becoming polarized. The particles in the insulator realign themselves - presenting an oppositely charged layer towards the charged rod. This process is illustrated below.

	positively charged rod
	top surface "-"
	polarized molecules within the insulator
	bottom surface "+"

Coulomb's Law: calculating electrostatic forces between point charges

Coulomb’s Law of Electrostatics states that

where:

F is the force measured in Newtons
k is Coulomb's constant which equals 9 x 10⁹ Nm²/C²
Q is the magnitude of each charge measured in coulombs
r is the distance between the centers of the two charges

This formula may only be used for point charges. That is, for isolated points of electric charge. It is an example of an inverse square law: if you double the distance between two charges, the force between them is reduced to 1/4^th its original size.

If F is negative, that means that the charges carry opposite "signs" -- that is, one is positive and the other is negative. The negative answer means that the point charges are attracting each other -- it does NOT mean that F is acting in a negative direction.
If F is positive, that means that the charges carry the same "sign" -- that is, either both are positive or both are negative. A positive answer means that the point charges are repelling each other -- it does NOT mean that F is acting in a positive direction.

Refer to the following information for the next four questions.

Two identical conducting spheres are initially charged and separated at a distance of 1 meter, as shown below.

	What was the initial electrostatic force between them?

	After being brought together, touched, and then re-separated, what is the new charge on each sphere?

	What is the magnitude of the new force between?

	If one sphere had originally had a charge of +46 µC and the other had a charge of -46 µC, how would your answers have changed?

Summary: Take a moment to compare the properties of gravitational forces with those of electrostatic forces.

Gravitational Forces	Electrostatic Forces
G = 6.67 x 10^-11 Nm²/kg² is VERY small gravity is a weak force inverse square force attractive only	k = 9 x 10⁹ Nm²/C² is VERY large electrostatic forces are strong inverse square force attractive and repulsive