Lecture 1 agenda: Electric Charge. Just a reminder of some things
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Lecture 1 agenda: Electric Charge. Just a reminder of some things you learned back in grade school. Coulomb’s Law (electrical force between charged particles). You must be able to calculate the electrical forces between one or more charged particles. The electric field. You must be able to calculate the force on a charged particle in an electric field. Electric field due to point charges. You must be able to calculate electric field of one or more point charges. Motion of a charged particle in a uniform electric field. You must be able to solve for the trajectory of a charged particle in a uniform electric field.
Electric Charge Read about electric charge in sections 21.1 and 21.2 in your text. You should have learned this material in your prior academic career. If you haven’t, there is important information you need to learn now! There are two kinds of charge. - like charges repel unlike charges attract charges can move but charge is conserved Law of conservation of charge: the net amount of electric charge produced in any process is zero. (Not on your starting equation sheet, but a fact that you can use any time.)
Although there are two kinds of charged particles in an atom, electrons are the charges that usually move around. - Protons are roughly 2000 times more massive than electrons and are typically bound inside nuclei. Charges are quantized (come in units of e 1.6x10-19 C). The charge of an electron is –e –1.6x10-19 coulombs. The charge of a proton is e 1.6x10-19 coulombs. That’s all the lecture time I’ll devote to sections 21.1 and 21.2.
Lecture 1 agenda: Electric Charge. Just a reminder of some things you learned back in grade school. Coulomb’s Law (electrical force between charged particles). You must be able to calculate the electrical forces between one or more charged particles. The electric field. You must be able to calculate the force on a charged particle in an electric field. Electric field due to point charges. You must be able to calculate electric field of one or more point charges. Motion of a charged particle in a uniform electric field. You must be able to solve for the trajectory of a charged particle in a uniform electric field.
Coulomb’s Law Coulomb’s law quantifies the magnitude of the electrostatic* force. Coulomb’s law gives the force (in newtons) between charges q1 and q2 (in units of coulombs), where r12 is the distance in meters between the charges, and k 9x109 N·m2/C2. q1q 2 F k 2 12 r12 *Moving charged particles also exert the Coulomb force on each other.
a note on starting equations q1q 2 F k 2 12 r12 is on your starting equation sheet. In general, you need to begin* solutions with starting equations. You may begin with any correct variant of a starting equation. QAQB For example, F k E D2 is “legal” and may be used. Don’t get hung up about starting a problem with an equation which is an exact copy of one from the OSE sheet. *“Begin” does not mean that a starting equation has to be the first thing that appears on your paper. It might be several lines before you use a starting equation.
Force is a vector quantity. Your starting equation gives the magnitude of the force. Use your diagram for the problem to figure out the direction. If the charges are opposite in sign, the force is attractive; if the charges are the same in sign, the force is repulsive. q1q 2 F k 2 12 r12 This equation just gives the magnitude of the force. If a problem asks you to calculate a force, assume that means both magnitude and direction (or else all components). 1 Also, k 4 0 where 0 8.85 10 12 C2 . 2 N m Remember, a vector has a magnitude and a direction.
Coulomb’s Law is valid for point charges. If the charged objects are spherical and the charge is uniformly distributed, r12 is the distance between the centers of the spheres. r12 - I just told you it’s OK to use Coulomb’s Law for spherically-symmetric charge distributions. If more than two charges are involved, the net force is the vector sum of all forces (superposition). For objects with complex shapes, you must add up all the forces acting on each separate charge (calculus!!). - - -
Example: a positive charge Q1 Q is located a distance d along the y-axis from the origin. A second positive charge Q2 Q is located at the origin and a negative charge Q3 -2Q is located on the x-axis a distance 2d away from Q1. Calculate the net electrostatic force on Q1 due to the other two charges. To be worked at the blackboard. You should apply the expert techniques you learned in Physics 1135 when you work Physics 2135 problems.
