Electrostatic Electricity, Electric Charge (CH 15) EXPERIMENT NO. 1 ( Electrostatics)

1. Electrostatic Electricity, Electric Charge (CH 15) EXPERIMENT NO. 1 (Electrostatics) • Benjamin Franklin’s Experiment (1706----1790) • About Electrostatic Electricity • Electric charge and Electric Field • Conductors and Insulators • Static Electricity” - Fundamentals • Experiments and demonstrations • Electric Charge Detection and Measurement • Electrostatic Charging • Charging by Polarization • What is a Capacitor? • Coulomb’s Law, Electric Field Force and Exercises • What is an electric field? Exercises • New Terminology • Quiz No. 1 • Homework No. 1

2. Lightning Bolt Las Vegas, Nevada 2013 On Earth, the lightning frequency is approximately 40–50 times a second. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA)

3. Lightning strike Barstow airport 2015 Aircraft operating in U.S. airspace have been required to be equipped with static discharge wicks. A plane is designed to conduct the excess electricity through its skin and structure to the wicks to be safely discharged back into the atmosphere.

4. Benjamin Franklin’s Experiment (1706----1790) KITE After failing to exploit the energy of lightning Benjamin Franklin devised the lightning rod, an iron rod attached to the highest point of a structure and connected to wires leading to the ground.

5. About Electrostatic Electricity • Most electric charge is carried by the electrons and protons within an atom. Electrons carry negative charge and protons carry positive charge. They attract each other. Two protons repel each other, as so do two electrons. • Protons and electrons create electric fields which exert a force called Coulomb force, which radiates outward in all directions. The electric field radiates outward from a charged particle and it decreases in strength as the square of the distance from the source (1/r2). • Because protons are generally confined to the nuclei imbedded inside atoms, they are not nearly as free to move as are electrons. Therefore, we nearly always mean a surplus or deficit of electrons. When an imbalance exists, electrons are able to flow, an electric current is created, In its broadest sense electricity describes phenomena associated with the interaction between electrically charged objects. The simplest one is the electrostatics where the charged objects (particles) are at rest.

6. Electric Charge and Electric Field The electric charge q, is a fundamental property of matter. It is associated with particles that make up the atom: electron and proton. Electron charge -e = -1.60 x 10-19 C. Proton +e = 1.60 x 10-19 C. Neutron has 0 C. The SI unit of an electric charge is the Coulomb C; [C = As}. It is equal to the charge of approximately 6.241×1018electrons. Proton’s mass is 104 times larger than the mass of an electron. q = ne– [C or As] An electric field E, is a space radiated by electric charges q, and acted upon by an electric force F. E = F/q [V/m] F = Eq [(V/m) As = Ws/m = N]; (Note: Nm = Ws)

7. Conductors and Insulators What distinguishes broad group of substances is whether they can transmit or conduct electric charge. Metals are good conductors. Rubber, glass, most plastics are insulators. In conductors, the electrons in the outermost orbit are loosely bound. They can easily leave. In insulators, most of the electrons are tightly bound. The charge does not move easily.

9. “Static Electricity” - Fundamentals What is “Static Electricity”? Static electricity is the imbalance of positive and negative charges. Positive and negative charges attract. In an atom, the nucleus is made up of Protons ( Positive ) and orbiting Electrons (negative). The electrostatic forces between the protons and electrons are what hold the electrons in orbit. They are about 36 orders of magnitude stronger than the gravitational forces between them. How can we produce “Static Electricity”? By placing two different materials (insulators) in contact with each other, charges are built up at the surface of the materials. This comes from two effects: 1) The material with more electrons in the outer orbit “pushes” electrons in the other material away from the surface (like charges repel); and 2) some of the electrons will move between the materials. When the materials are separated, the charges remain until they bleed off. Why do the materials need to be insulators? In insulators the outer electrons are tightly held, so the electrons making up the charge cannot move easily through the material and dissipate, they remain “static” at the surface of the material.

10. Electric Charge Characteristics of Materials Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the Triboelectric series. TRIBOELECTRIC SERIES your hand glass nylon wool fur MORE POSITIVE silk paper cotton hard rubber polyester polyvinylchloride plastic If these materials are rubbed together, the one higher on the list gives up electrons. Thus becomes positively charged.

11. Electric Charge Detection and Measurement How can we detect and measure Static Charges? There are three types of Electroscopes: The Pith Ball Electroscope; the Foil Electroscope and; the swinging needle Electroscope. Two types will be demonstrated in class: The Pith Ball Electroscope: This instrument, invented in 1754 by a British apprentice John Canton, consists of two balls of lightweight nonconductive material, originally pith, suspended by a silk thread from an insulated stand. Pith Ball Electroscope showing charge effects Typical Pith Ball Electroscope

12. Electric Charge Detection and Measurement The Gold Foil Electroscope: This instrument, invented in 1787 by a British physicist Abraham Bennet, consists of a vertical metal rod from witch hang two parallel strips of thin flexible conductive material (originally gold). When the metal rod is touched with a charged object, the strips spread apart in a “V”. This is because some charge on the object is conducted through the rod to the strips. Since they receive the same signed charge, they repel each other. Original gold foil Electroscope Modern Electroscope

13. What causes the rods to attract or repel? • Like electric charges repel each other, and unlike electric charges attract each other. The repulsive and attractive forces are equal and opposite. • Example: • Glass Rod (positive charge) • Rubber Rod (negative charge) How can an object become electrically charged?

14. Electrostatic Charging • Charging by Friction • If a hard rubber rod is rubbed by fur it acquires a negative charge; • Rubbing a glass rod with silk will give it a positive charge; • Charging by Conduction (Contact) • “Conduction” refers to the flow of a charge during the short period of time the electrons are transferred. • Charging by induction • A negatively charged rubber rod (charged by friction) can affect the charge of another object without being in contact of that object. • Charging by Polarization • Charging by contact and induction involves removal of charge from an object and creating an electric field around an object to be polarized.

15. Examples of Electrostatic Applications • Charged by Friction • Balloon and Wall Comb and Paper • Electrostatic Air Cleaners • Electrostatic Copier • Capacitors used in electronics, camera flash, defibrillator

16. Charging by Polarization Some materials are more susceptible to become polarized than others. A dielectric is an electrical insulator that can be polarized by the action of an externally applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as in a conductor, but only slightly shift from their average equilibrium positions causing the dielectric polarization inside. The positive charges are displaced along the direction of the field while the negative charges shift in the opposite direction. This creates another electric field. A polarized electric object stores energy that can be released on demand. When designed for that purpose it is called capacitor which is commonly used in electronic circuits.

17. What is a Capacitor? Capacitance C = k ε0 x A/d ε0 = 8.85 x 10-12 [C2/(Nm)2] C = q /V [As)/V] Energy stored U = ½ qV = ½ CV2 [VAs] To maximize the energy stored a capacitor should have: (1) large plates, (2) a close distance, and (3) high polarizability material between them.

18. Coulomb’s Law and Electric Force Following Benjamin Franklin’s work, electrical research advanced by leaps. Quantitative measurements were carried out in 1785 by the French physicist Charles-Augustin de Coulomb (1786----1806). He showed that attraction (or repulsion) between given (electric) charges, q varied inversely as the square of their distance, r from each other. It became known as the Coulomb’s Law where Electric force, F between two electric charges between two points is: F = k(q1 x q2)/r2 ; k = 9.00 x109 Nm2/C2 F = [(Nm2/C2)x C2/m2] = [N] W= Fxm [Nm] Example: Two point charges q1 = -1 nC and q2 = +2 nC; distance, r between them is 0.30 m. What is the electric force, F on each particle? A: F = 9.00 x 109 Nm2/C2 x (1x10-9 C x 2x10-9 C)/0.09 m2 = 18.0 x 10-9 N/0.09 F12 0.30m F21 = 18.0 x 10-9 N/9x 10-2 = 0.20 x 10-6 N; F = 0.20 μN

19. Coulomb’s Law and Electric Force Exercises Exercise 21, Ch. 15 Compared to electric force the gravitational force between two protons is a, about the same, b, somewhat larger, c, very much larger, d, very much smaller. A.: 21 d

20. What is an electric field? • The electric force, like the gravitational force, is an “action at a distance” force. Its range is infinite. The electric field is a domain where the presence of electric charges exert an electric force. Charges in one field can interact with charges in another one. The electric field is a vector field that enables us to determine the force exerted on a charge in a particular location. • An electric field E [V/m],is the region of space surrounding electrically charged particles, q[As] where electric charges are acted upon by an electric force F E = Fq/qo • E = k(qox q)/qor2 ; k = 9.00 x109 [Nm2/C2] • E = kq/r2 [Nm2/C2/m2] = [N/C] • Exercise: • The electric field due to positive charge (a) varies as 1/r; (b) points toward the charge, (c ) points away from the charge or (d) has a finite charge. 41c

21. What is an electric field? Exercise No. 55 Ch. 15 • What would be the magnitude and the • direction of a vertical electric field that would just support the weight of a proton on the surface of the Earth? • (b) of an electron? • A: 55 1.0 x 10-7 N/C; upward; 5.6 x 10-11 N/C downward

22. What is an electric field? Exercises 2. An electric force acts vertically downwards on an electron. The direction of the electric field at that point is: (a) up, (b) down, (c ) zero, (d) undetermined.A: 43b 3. A positive charge is inside an isolated metal sphere. Describe the situation of electric field and the charge on the sphere. Also, how would it change if the charge were negative. A: 47Yes, by induction the inner surface of the sphere would be negatively charged while the outer surface would become positive and would continue radially outward as if emanating from the center point of charge. If the charge were negative the field lines would reverse direction. 4. Could the electric field due to two charges ever be zero at some location nearby? If, yes, describe and sketch the situation. A: 49 Yes! When the electric fields are equal in magnitude and opposite in direction at a location. + q1 q2 F2 F1

23. The electric force and work can be expressed as a function of an electric field F = Eq [N] or [(N/C)C] = Eq [(V/m) As] = [VAs/m] = [Ws/m] = [N] W= FxL [Nm = Ws] The electric force and work can be expressed as a function of electric field: F = Eq [N] or [(N/C)C] F = Eq [(V/m) As] = [VAs/m] = [Ws/m] = [N] W= FxL [Nm = Ws] • Exercise: • The magnitude of electric force between two point charges is given by a) the charge-force law, b) conservation of charge, c) Coulomb’s Law or d) both a) and b) c

24. New Terminology Electricity is the set of physical phenomena associated with the presence or flow of an electric charge. Electrostatics:---The electrostatics is an electricity phenomenon associated with the interaction between electrically charged objects while they remain at rest. Electric Field, E:Itis the region of space surrounding electrically charged particles q, acted upon by an electric force F. E = F/q [V/m] Electric charge:-- The electric charge q, is a fundamental property of matter associated with particles that make up the atom: electron and proton. Electron, proton, neutron: Electron charge e-- = -1.60 x 10-19 [C]. Proton +e = 1.60 x 10-19 [C]. Neutron has 0 C. (C=As) Electric current---is a flow of electrically charged particles. I = q/t, [A] Conductors:--------In conductors, the electrons in the outermost orbit are loosely bound. They can easily leave. Insulators:In insulators, most of the electrons are tightly bound. The charge does not move easily. Capacitor:----------An electronic component that can store and release energy. A capacitor passes AC, but will not pass DC once charged. Coulomb’s Law:--It states that attraction (or repulsion) between given (electric) charges, q varied inversely as the square of their distance, r from each other. Electric force:-----It is the force, F exerted by the charge in an electric field. (F=Eq [N])

25. Quiz No. 1 Name:_____________________________ • What causes the rubber rods to attract or repel? Explain. ____________________________________________________________________________________________________________________________________ • Being an “insulator” how come an object made of rubber can be electrically charged?__________________________________________________________ • What is the main feature of an object that has been charged by polarization?__________________________________________________________________

26. Electrostatic Electricity, Electric Charge (CH 15 Homework No. 1 (Electrostatics) Due date: October 4 or 5 October 2016 Name: _____________________ • If you rub a glass rod with a silk cloth: a) what charge polarity will the glass rod acquire?); (b) What method of charge transfer is being used? __________________________________________________ • If you charge a glass rod and balloon each with a silk cloth, will the rod and balloon attract or repel each other? ______________________________________________________________________________ • No. 23 In calculating planetary orbits around the sun, why can astronomers safely ignore the electric force? ______________________________________________________________________________ • No. 26 Coulomb’s Law is also called an inverse square law. What does it tell you about the relationship between force and distance? ____________________________________________________________ • An electron and a proton are separated by 2.0 nm. Using the Coulomb’s Law we can calculate the magnitude of the force on the electron, F = k(q1 x q2)/r2.= 5.8x 10-11 N; What is the net force on the system? _____________________________________________________________________________ • The electric field due to negative charge (a) varies as 1/r; (b) points toward the charge, (c ) points away from the charge or (d) has a finite charge. __________________________________________________ • Could the electric field due to two charges ever be zero at some location nearby? If yes,describe and sketch the situation. ____________________________________________________________________

27. Positive and negative lightning On Earth, the lightning frequency is approximately 40–50 times a second or nearly 1.4 billion flashes per year and the average duration is 30 microseconds. CG lightning can occur with both positive and negative polarity. The polarity is that of the charge in the region that originated the lightning leaders. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs. Unlike the far more common "negative" lightning, positive lightning originates from the positively charged top of the clouds (generally anvil clouds) rather than the lower portion of the storm. Leaders form in the anvil of the cumulonimbus and may travel horizontally for several miles before veering towards the ground. A positive lightning bolt can strike anywhere within several miles of the anvil of the thunderstorm, often in areas experiencing clear or only slightly cloudy skies; they are also known as "bolts from the blue" for this reason. Positive lightning typically makes up less than 5% of all lightning strikes. Because of the much greater distance to ground, the positively charged region can develop considerably larger levels of charge and voltages than the negative charge regions in the lower part of the cloud. Positive lightning bolts are considerably hotter and longer than negative lightning. They can develop six to ten times the amount of charge and voltage of a negative bolt and the discharge current may last ten times longer. A bolt of positive lightning may carry an electric current of 300 kA and the potential at the top of the cloud may exceed a billion volts — about 10 times that of negative lightning. During a positive lightning strike, huge quantities of extremely low frequency (ELF) and very low frequency (VLF) radio waves are generated. As a result of their greater power, as well as lack of warning, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999. The standard in force at the time of the crash, Advisory Circular AC 20-53A, was replaced by Advisory Circular AC 20-53B in 2006, however it is unclear whether adequate protection against positive lightning was incorporated. Aircraft operating in U.S. airspace have been required to be equipped with static discharge wicks. Although their primary function is to mitigate radio interference due to static buildup through friction with the air, in the event of a lightning strike, a plane is designed to conduct the excess electricity through its skin and structure to the wicks to be safely discharged back into the atmosphere. These measures, however, may be insufficient for positive lightning.