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Learn about the principles of electrostatics and static electricity, including the structure of atoms, charging objects by friction, conductors vs. insulators, and the interaction between charged and neutral objects.
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Electrostatics can be explained by remembering the structure of atoms from chemistry class: Since protons are tightly bound in the center of the atom, they NEVER leave the atom. Electrons are loosely bound, so the OFTEN leave the atom.
Neutral vs. Charged Objects • A neutral atom has the same number of protons and electrons. Neutral objects also have the same number of protons and electrons. • The electrons contained within some objects are prone to move to other objects. In fact, the process of an electron leaving one object to reside (perhaps only temporarily) in another object is a common everyday occurrence.
Charged objects • Electrons are often removed from and added to an atom. • Protons and neutrons are not removable by usual everyday methods. • If something gets a negative charge, how did that happen? • If something gets a positive charge, how did that happen?
Check your understanding: True or False? • An object that is positively charged contains all protons and no electrons. • A positively charged object is an object that has an excess of positive electrons. • A positively charged object has more protons than electrons. • Protons have a positive charge.
negative -- -- positive + + + + Electric Charge When a rubber rod is rubbed against fur, electrons are removed from the fur and deposited on the rod. Electrons move from fur to the rubber rod. The rod becomes negatively charged because of an excess of electrons. The fur becomes positively charged because of a deficiency of electrons.
positive glass + + + + negative - - - - silk Glass and Silk When a glass rod is rubbed against silk, electrons are removed from the glass and deposited on the silk. Electrons move from glass to the silk cloth. The glass becomes positivelycharged because of a deficiency of electrons. The silk becomes negatively charged because of a excess of electrons.
Check your understanding: During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct explanation for why the plastic strip becomes positively charged is that ... • the plastic strip acquired extra protons from the cotton. b. the plastic strip acquired extra protons during the charging process. c. protons were created as the result of the charging process. d. the plastic strip lost electrons to the cotton during the charging process.
How do you know which becomes negative and which becomes positive?
Neg Neg Neg Pos Pos Pos The First Law of Electrostatics Like charges repel; unlike charges attract.
Conductors and Insulators: Conductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule. (metals) Insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. (plastic, rubber, glass)
Conductors and Insulators If any part of a conductor gets a charge, that charge will quickly spread out through the entire object. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging.
Check your understanding: A conductor differs from an insulator in that a conductor ______. • has an excess of protons b. has an excess of electrons c. can become charged and an insulator cannot d. has faster moving molecules e. does not have any neutrons to get in the way of electron flow f. none of these
Check your understanding: One of these charged spheres is copper and the other is rubber. The diagram below depicts the distribution of excess negative charge over the surface of two spheres. Which sphere is copper, and which sphere is rubber?
Check your understanding: Suppose that a metal sphere is charged positively by some method. The charge is initially deposited on the left side of the sphere. Yet because the metal is a conductor, the charge spreads uniformly throughout the surface of the sphere. The uniform distribution of charge is explained by the fact that ____. • the charged atoms at the location of charge move throughout the surface of the sphere b. the excess protons move from the location of positive charge to the rest of the sphere c. excess electrons from the rest of the sphere are attracted towards the excess protons
What about neutral objects – are they attracted or repelled by anything? • Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object. Positively charged objects and neutral objects attract each other; and negatively charged objects and neutral objects attract each other.
How can neutral objects be attracted to charged objects? • Recall that electrons are loosely held in some atoms (especially metal atoms.) • In conducting objects, electrons are so loosely bound that they may be induced into moving from one portion of the object to another portion of the object. • To get an electron in a conducting object to get up and go, all that must be done is to place a charged object nearby the conducting object.
Inducing charge separation in a conductor • If a negatively charged balloon is brought near a metal can, the electrons within the can will experience a repulsive force. The repulsion will be greatest for those electrons that are nearest the negatively charged balloon. • Many of these electrons will be induced into moving away from the repulsive balloon. Being present within a conducting material, the electrons are free to move from atom to atom. • This electron movement leaves atoms on the balloon's side of the can with a shortage of electrons; they become positively charged. And the atoms on the side opposite of the can have an excess of electrons; they become negatively charged. The two sides of the aluminum pop can have opposite charges.
Charge polarization Polarization is the process of separating opposite charges within an object. The positive charge becomes separated from the negative charge. By inducing the movement of electrons within an object, one side of the object is left with an excess of positive charge and the other side of the object is left with an excess of negative charge. The polarization process always involves the use of a charged object to induce electron movement or electron rearrangement.
Check your understanding: Which of the diagrams below best represents the charge distribution on a metal sphere when a positively charged plastic tube is placed nearby?
Charge polarization in insulators But what if the object being polarized is an insulator? Electrons are not free to move across the surface of an insulator. How can an insulator such as a wooden wall be polarized?
Charge polarization in insulators In a conducting object, electrons are induced into movement from one side of the object to the opposite side. In an insulator, electrons merely redistribute themselves within the atom or molecules nearest the outer surface of the object.
Charge polarization in insulators In insulators, the atoms themselves become polarized:
Example: • Remember from the computer lab, the negatively charged balloon was attracted to the wall?
One more thing… Polarization is not charging! When an object becomes polarized, there is simply a redistribution of the centers of positive and negative charges within the object. Yet, there are still equal numbers of positive charges (protons) and negative charges (electrons) within the object. While there is a separation of charge, there is NOT an imbalance of charge. When neutral objects become polarized, they are still neutral objects.
Charging by induction • We’ve already seen that objects can be charged by friction (rubbing different objects together.) • Induction charging is a method used to charge an object without actually touching the object to any other charged object.
Induction - - - - - - - - - - - - - - - - - - Uncharged Spheres Electrons Repelled + - - + + + - - + + + + + + + + Charging Spheres by Induction Separation of Charge Isolation of Spheres Charged by Induction
Induction - - - - - - - - - - - - - - - - - - + - + - - - - - + - - + + + + + + + + + Induction for a Single Sphere Uncharged Sphere Separation of Charge - - - - Electrons move to ground. Charged by Induction
The Quantity of Charge The quantity of charge(q) can be defined in terms of the number of electrons, but the Coulomb (C) is a better unit for later work. One definition is given below: The Coulomb: 1 C = 6.25 x 1018 electrons Which means that the charge on a single electron is: 1 electron: e- = -1.6 x 10-19 C
+ + + + + + + + + + + + + + ExampleIf 16 million electrons are removed from a neutral sphere, what is the charge on the sphere in coulombs? 1 electron: e- = -1.6 x 10-19 C q = -2.56 x 10-12 C Since electrons are removed, the charge remaining on the sphere will be positive. Final charge on sphere: q = +2.56 x 10-12 C
F - + q q’ r F F q q’ - - Coulomb’s Law The force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.
Practice problem: What is the electrostatic force between two metal spheres, each having 5.0 C of charge? The spheres are 0.30 m apart.
