Static Electricity - Chapter Outline Lesson 1: Basic Terminology and Concepts The Structure of MatterNeutral vs. Charged Objects Charge InteractionsConductors and Insulators Polarization Lesson 2: Methods of Charging Charging by FrictionCharging by Induction Charging by ConductionGrounding Lesson 3: Electric Force Charge Interactions RevisitedCoulomb's Law Inverse Square Law Newton's Laws and the Electrical Force Lesson 4: Electric Fields Action-at-a-DistanceElectric Field Intensity Electric Field LinesElectric Fields and Conductors Lightning Lesson 5: Electric Potential Electric Field and the Movement of Charge Electric PotentialElectric Potential Difference
objectives • Know: • Charge is quantized. • Charge on an electron and proton. • Unit of charge. • Understand: • Relationship between fundamental charge and charge in coulombs. • Law of Conservation of Charge. • Be able to: • Convert from fundamental charges to coulombs • Determine the charge on two or more objects after they come in contact with one another.
The Structure of Matter • ATOMS- All material objects are composed of atoms. • Atoms contain a dense center called the nucleus and a larger surrounding of mostly empty space that contains the electrons.
Electrons • Electrons are present in the region of space outside the nucleus. They are negatively charged and weakly bound to the atom. Electrons are often removed from and added to an atom by normal everyday occurrences. These occurrences are the focus of this Static Electricity unit.
Protons and Neutrons • The nucleus of the atom contains positively charged protons and neutral neutrons. These protons and neutrons are not removable by usual everyday methods. It would require some form of high-energy nuclear occurrence to disturb the nucleus. Protons and neutrons will remain within the nucleus of the atom. • Electrostatic phenomenon can never be explained by the movement of protons.
Check Your Understanding • ____ are the charged parts of an atom. • Only electrons • Only protons c. Neutrons only d. Electrons and neutrons e. Electrons and protons f. Protons and neutrons
Charged versus Uncharged Objects • Electrically charged objects are formed when neutral objects lose or gain electrons. • Note: protons and neutrons can not be removed, only electrons can be removed or Added! Charged objects contain unequal numbers of protons and electrons
Charged Objects as an Imbalance of Protons and Electrons Positively charged Negatively charged
example • Which part of an atom is most likely to be transferred as a body acquires a static electric charge? • proton • neutron • electron • positron
Charge as a Quantity Like mass, the charge of an object is a measurable quantity. The charge possessed by an object is often expressed using the scientific unit known as the Coulomb(C). One Coulomb of charge is an abnormally large quantity of charge. An object with -1 C of charge would need an excess of6.25 x 1018 electrons, an object with a shortage of 6.25 x 1018 electrons would have a total charge of +1 C. The units of micro-Coulombs (1 µC = 10-6 C) or nano-Coulombs (nC = 10-9 C) are more commonly used as the unit of measurement of charge.
+1.6 x 10 -19 Coulomb is called an elementary charge. The charge of one electron = -1e = -1.6 x 10 -19 C. The charge on a singleproton = + 1e = +1.6 x 10 -19C. If an object is charged, it possesses more or less ____________________________of electrons. It can not possess a fraction of an electron. An object can only have a charge that is multiple of the elementary charge – ______________of 1.6 x 10-19 C. whole numbers multiple
An object can not have a charge of 3.2 × 10-19 C 4.5 × 10-19 C 8.0 × 10-19 C 9.6 × 10-19 C Example
example • What is the smallest electric charge that can be put on an object? • 9.11 × 10-31 C • 1.60 × 10-19 C • 9.00 × 109 C • 6.25 × 1018 C
Determine the total charge of a charged object • Determine the difference between the number of electrons and the number of protons to find the excess charge. • if there are lesselectrons, the total charge = the number of excess charge x (+1.6x10-19 C) • if there are more electrons, the total charge = the number of excess charge x (-1.6x10-19 C) • Similarly, if the net charge is given, one can divide the net charge by the elementary charge (1.6x10-19 C) to determine the excess number of electrons or protons.
example • An object possessing an excess of 6.0 × 106 electrons has a net charge of • 2.7 × 10-26 C • 5.5 × 10-24 C • 3.8 × 10-13 C • 9.6 × 10-13 C
example Which quantity of excess electric charge could be found on an object? 0.25 elementary charges 5.25 × 10-19 C 6.40 × 10-19 C 1.60 elementary charges
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. 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. example
objectives • Know: • Definition of insulator, conductor • Charge is transferred in solids by electron movement only. • Understand: • How charges interact with each other • How to detect charges • How charges flow during polarization events. • Be able to: • Explain how charged object attract neutral objects
Charge Interactions The electric force is a non-contact force. Any charged object can exert this force upon other objects - both charged and uncharged objects. The nature of the electric force: Opposites attract. likes repel.
The Electric Force and Newton's Third Law This electric force exerted between two charged objects is a force in the same sense that friction, tension, gravity and air resistance are forces. And being a force, the same laws and principles that describe any force describe the electrical force. One of those laws was Newton's law of action-reaction. (balloons) Force of D upon C is the same in magnitude as Force of C upon D. they are action and reaction forces. Force of B upon A is the same in magnitude as Force of A upon B. they are action and reaction forces.
Interaction Between Charged and Neutral Objects 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; Negatively charged objects and neutral objects attract each other. Any charged object - plastic, rubber, or aluminum - will exert an attractive force upon a neutral object. And in accordance with Newton's law of action-reaction, the neutral object attracts the charged object.
Charge detection If two objects repel each other… one can conclude that both objects are charged and charged with the same type of charge. One could not conclude that the balloons are both positively charged or both negatively charged. If two objects attract each other… one can conclude that at least one of the objects is charged. The other object is either neutral or charged with the opposite type of charge. You cannot draw a conclusion about which one of the objects is charged or what type of charge (positive or negative) the charged object possesses.
example A lightweight sphere hangs by an insulating thread. A student wishes to determine if the sphere is neutral or electro statically charged. She has a negatively charged hard rubber rod and a positively charged glass rod. She does not touch the sphere with the rods, but runs tests by bringing them near the sphere one at a time. The student notes that the sphere is attracted to both rods. This test result shows that the charge on the sphere is positive negative neutral
example A negatively charged plastic comb is brought close to, but does not touch, a small piece of paper. If the comb and the paper are attracted to each other, the charge on the paper may be negative or neutral may be positive or neutral must be negative must be positive
Conductors and Insulators • The behavior of an object that has been charged is dependent upon whether the object is made of a conductive or a nonconductive material. • Conductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule. • In contrast to conductors, insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule.
Examples of conductors and insulators • Examples of conductors include • metals, • aqueous solutions of salts • graphite, • water • human body. • Examples of insulators • plastics, • Styrofoam, • paper, • rubber, • glass • dry air.
The division of materials into the categories of conductors and insulators is a somewhat artificial division. It is more appropriate to think of materials as being placed somewhere along a continuum.
Human body is a conductor • Along the continuum of conductors and insulators, one might find the human body somewhere towards the conducting side of the middle. When the body acquires a static charge it has a tendency to distribute that charge throughout the surface of the body. • phet
Water is a conductor • Water, being a conductor, has a tendency to gradually remove excess charge from objects. Since humidity levels tend to vary from day to day and season to season, it is expected that electrical affects (and even the success of electrostatic demonstrations) can vary from day to day.
insulators vs. conductors charge on a conductor is quickly distributed across the entire surface of the object. Why do think this happens? • Charge on an insulator will remain at the initial location of charging. The insulating cups are use to prevent charge from escaping to the surroundings as well as to provide for a convenient handle.
Distribution of Charge via Electron Movement • Predicting the direction that electrons would move within a conducting material is a simple application of the two fundamental rules of charge interaction. Opposites attract and likes repel. • The excess negative charge distributes itself throughout the surface of the conductor. This is because electrons wish to manipulate their surroundings in an effort to reduce repulsive affects.
Check your understanding • Suppose that a conducting sphere is charged positively by some method. The charge is initially deposited on the left side of the sphere. Yet because the object is conductive, the charge spreads uniformly throughout the surface of the sphere. The uniform distribution of charge is explained by the fact that ____. a. the charged atoms at the location of charge move throughout the surface of the sphere b. the excess protons move from the location of charge to the rest of the sphere c. excess electrons from the rest of the sphere are attracted towards the excess protons
A conductor differs from an insulator in that a conductor ________. • has an excess of protons • has an excess of electrons • can become charged and an insulator cannot • has faster moving molecules • does not have any neutrons to get in the way of electron flow • none of these
objectives • Know: • Definitions polarization and electroscope. • Charge is transferred in solids by electron movement only. • Understand: • How charges flow during polarization events. • Be able to: • Draw and interpret pith ball and electroscope diagrams.
Polarization - Why a charged object attract neutral object • In an atom, the protons are tightly bound in a nucleus and incapable of movement. 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. • By placing a charged object near a neutral conducting object you can create electron movement.
No electrons have been added to or subtracted from the can yet there is a charge at either end of the can; overall the can is electrically neutral. This arrangement of charge is called polarization.
Polarization is the process of separating opposite charges within an object. • The polarization process always involves the use of a charged object to induce electron movement or electron rearrangement. • 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. Charge becomes separated into opposites. • Polarization is not charging – the total charge in a polarized object is still zero just like before.
The Electroscope • An electroscope is a device which is capable of detecting the presence of a charged object through polarization.
How Can an Insulator be Polarized? • In an insulator, electrons merely redistribute themselves within the atom or molecules nearest the outer surface of the object.
Polarization is Not Charging • When an object becomes polarized, there is simply a __________________ of the centers of positive and negative charges 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 ______________ objects. redistribution neutral
example • An inflated balloon which has been rubbed against a person's hair is touched to a neutral wall and remains attracted to it. Which diagram best represents the charge distribution on the balloon and the wall? a b c d
example • The diagram below shows three neutral metal spheres, x, y, and z, in contact and on insulating stands. Which diagram best represents the charge distribution on the spheres when a positively charged rod is brought near sphere x, but does not touch it? C D A B
Lesson 2: Methods of Charging • Charging by Friction • Charging by Induction • Charging by Conduction • Grounding - the Removal of a Charge
objectives • Know: • Definitions of conduction, induction, and grounding. • Charge is transferred in solids by electron movement only. • Understand: • How charge is transferred by friction, conduction, and induction. • Be able to: - Draw and interpret pith ball and electroscope diagrams.
Charging by Friction • When two objects are rubbed together electrons may be transferred from one object to another. One object gains electrons and the other object loses electrons, so both objects have a charge.
When wool is rubbed against a PVC pipe, the PVCsteals electrons from the wool because it has higher electron affinity compared to wool. The PVC strip ends up with a negative charge while the wool ends up with a positive charge When wool is rubbed against a Nylon strip, the wool will steal electrons from the Nylon because wool has higher electron affinity than Nylon. As a result, the Nylon ends up positively charged and the wool ends up negative.
How do we know which object will gain electrons and which will lose electrons? • electron affinity determines which object will gain electrons. • The property of electron affinity refers to the relative amount oflove that a material has for electrons. High affinity means the material has more pull to electrons. • The more love of electrons a material has the more likely it is to steal electrons from the other object during charging by friction