Introduction to physics 212 Dr. Muhammad Ali Yousuf maliyousuf@towson.edu
Plan for Today
Course material, while I don’t have access to Blackboard: https://tinyurl.com/tuphys
Let’s start with a quick review Q: Can an object carry a charge of 2.4 × 1019 C? a) Yes, if the object is a conductor. b) Yes, if the object has electrons or protons. c) Yes, if the object is an insulator. d) No, because objects do not have charge. e) No, because charge is quantized.
Q: The smallest charge on a single particle has been measured to be 1 Q: The smallest charge on a single particle has been measured to be 1.60 × 1019 C. What can we conclude from the fact that no smaller charges have been measured? a) Charge is quantized. b) Electrons are conserved. c) Charge is conserved. d) Electrons have the smallest unit of charge. e) Charge is the same as mass.
Conductors and Insulators Not only can electric charge exist on an object, but it can also move through and object. Substances that readily conduct electric charge are called electrical conductors. Materials that conduct electric charge poorly are called electrical insulators.
You have heard of electrons and protons but as there anything inside them? Based on our current knowledge and understanding, there is nothing inside an electron – it is a true ‘point’ particle. BUT … a proton Three Quarks
Charge is conserved, but …
When materials are rubbed together, charges can be separated, particularly if one material has a greater affinity for electrons than another. (a) Both the amber and cloth are originally neutral, with equal positive and negative charges. Only a tiny fraction of the charges are involved, and only a few of them are shown here. (b) When rubbed together, some negative charge is transferred to the amber, leaving the cloth with a net positive charge. (c) When separated, the amber and cloth now have net charges, but the absolute value of the net positive and negative charges will be equal.
Q: Complete the following statement: When a glass rod is rubbed with silk cloth, the rod becomes positively charged as a) negative charges are transferred from the rod to the silk. b) negative charges are transferred from the silk to the rod. c) positive charges are created on the surface of the rod. d) positive charges are transferred from the silk to the rod. e) positive charges are transferred from the rod to the silk.
Q: Glass is a very good electrical insulator Q: Glass is a very good electrical insulator. How is then possible that a glass rod can be charged by rubbing it with cloth? a) When the rod is rubbed, it becomes an electrical conductor. b) Although the rod is an insulator, any excess charge will slowly be conducted away. c) When the rod is rubbed, the part of the rod that is in contact with the cloth becomes electrically conductive. d) Because the rod is an insulator, the charge that is transferred to the surface of the rod has difficulty moving. e) None of the above answers are correct.
Charging by induction:
Q: An initially electrically neutral conducting sphere is placed on an insulating stand. A negatively-charged glass rod is brought near, but does not touch the sphere. Without moving the rod, a wire is then attached to the sphere that connects it to earth ground. The rod and wire are then removed simultaneously. What is the final charge on the sphere? a) negative b) positive c) neutral d) It has a fifty percent chance of having a positive charge and a fifty percent chance of having a negative charge.
Q: Three identical conducting spheres on individual insulating stands are initially electrically neutral. The three spheres are arranged so that they are in a line and touching as shown. A negatively-charged conducting rod is brought into contact with sphere A. Subsequently, someone takes sphere C away. Then, someone takes sphere B away. Finally, the rod is taken away. What is the sign of the final charge, if any, of the three spheres? A B C a) + + b) + + c) + 0 d) + 0 e)
Q: Consider the conducting spheres labeled A, B, and C shown in the drawing. The spheres are initially charged as shown on the left, then wires are connected and disconnected in a sequence shown moving toward the right. What is the final charge on sphere C at the end of the sequence? a) +Q b) + Q/2 c) + Q/3 d) +2Q e) +3Q
Conservation Laws Only a limited number of physical quantities are universally conserved. Charge is one—energy, momentum, and angular momentum are others. Because they are conserved, these physical quantities are used to explain more phenomena and form more connections than other, less basic quantities. We find that conserved quantities give us great insight into the rules followed by nature and hints to the organization of nature. Discoveries of conservation laws have led to further discoveries, such as the weak nuclear force and the quark substructure of protons and other particles.
18.3 Coulomb’s Law
18.3 Coulomb’s Law COULOMB’S LAW The magnitude of the electrostatic force exerted by one point charge on another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance between them.
18.3 Coulomb’s Law Example 3 A Model of the Hydrogen Atom In the Bohr model of the hydrogen atom, the electron is in orbit about the nuclear proton at a radius of 5.29x10-11m. Determine the speed of the electron, assuming the orbit to be circular.
18.3 Coulomb’s Law
Class work This slide is NOT in your version of PPT 21. (a) Find the ratio of the electrostatic to gravitational force between two electrons. (b) What is this ratio for two protons? (c) Why is the ratio different for electrons and protons? 22. At what distance is the electrostatic force between two protons equal to the weight of one proton?
18.3 Coulomb’s Law Example 4 Three Charges on a Line Determine the magnitude and direction of the net force on q1.
18.3 Coulomb’s Law
18.3 Coulomb’s Law