Welcome Back! Physics 1308 Coulomb s Law Charles-Augustin de Coulomb 1736-1806
Announcements Assignments for Tuesday, August 28th: - Reading: Chapter 22.1-22.2 - Watch Video: https://youtu.be/tzq79csdttw Lecture 3 - The Electric Field Homework 1 Assigned - due before class on Tuesday, August 28th. - Numerical answers to WileyPLUS problems and answers to all WileyPLUS questions should be entered into WileyPLUS - You must also generate a set of written solutions to ALL your assigned questions and solutions to ALL your assigned problems for each assignment. One of those problems or questions will be randomly selected and graded each week for methodology. - Written solutions will need to follow the homework policy and will be graded according to that policy. We expect that you will produce good-quality homework solutions! Points will be lost for not adhering to the homework guidelines. Dr. Cooley will be out of the office the week of August 27th. Her office hours are cancelled for that week. Please email her if you have any immediate concerns or needs.
Dr. Cooley s office hours will be in FOSC 151 on Wednesdays from 3-4 pm and Fridays from 1-2 pm or by appointment. Ms. Jasmine Liu s office hours will be in a location FOSC 153 on Fridays from 10-11 am and Mondays from 2-3 pm.
Please join us for the PHYSICS Department Kick-Off Event Drop in from 6:30 7:30 pm Tuesday Aug 28 th Heroy Hall 153 Pizza & Ice cream Meet the Physics faculty and undergraduate Physics majors Learn about opportunities for undergraduate research Get free advice about majoring or minoring in Physics Find out about the Society of Physics Students (SPS)
Quiz Time
Review Question 1 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 50/50 chance of having either a positive or negative charge
Matter and Size An atom is about a tenth of a nanometer in size (10-10 m, called 1 Angstrom ) We in the macroscopic (human-sized) world are about 1 meter in size The macroscopic world is 10 billion times bigger than the atoms that compose it. To us, they are like idealized geometric points whose individual size can be neglected, but whose sum explains us. There are approximately 10 27 atoms in our body... more atoms in our body than there are stars in the visible Universe (estimated to be at least 100 billion trillion, or 10 23 ).
Charges and Point Charges Charge distributions can be complex...... but as far as we know, all charge distributions are made from point-like charges. Coulomb's Law describes the electric force between two point charges It will be our archetype for solving problems involving charge distributions.
Key Equations and Constants Coulomb s Law: Newton s Equation: F = 1 q 1 q 2 1 m F ~F = G m 1m 2 4 0 r 2 r 2 ˆr F = 1 q 1 q 2 4 0 r 2 Electrostatic Constant: k = 1 4 0 =8.99 10 9 N m 2 /C 2 Permittivity Constant: 0 =8.99 10 12 N m 2 /C 2
Problem 1 A current of 0.300 A through your chest can send your heart into fibrillation, ruining the normal rhythm of heartbeat and distributing the flow of blood (and thus oxygen) to your brain. If that current persists for 1.34 minutes, how many conduction electrons pass through your chest? Hint: 1 A = 1 C/s Solution: Find the total charge in 1.34 minutes. q =(0.300 C/s)(1.34 min)(60 s/min) = 24.12 C The number of charges is given by the formula q = ne n = q e = 24.12 C 1.602 10 19 C =1.51 1020 n = 1.51 x 10 20
Problem 2: Ions Near a Cell Membrane Membrane potential is the difference in electric potential between the interior and exterior of a biological cell (typically from -40 mv to - 80 mv). Barrier serves as an insulator and diffusion barrier to movement of ions. Ion transporters or ion pump proteins actively push ions across the membranes. Ion pump proteins and ion channels are electrically equivalent to a set of batteries and resistors inserted into the membrane (more on that later). Do the ions feel an electrostatic force? By Synaptidude, CC BY 3.0, https://commons.wikimedia.org/w/index.php? curid=21460910
Problem 2: Ions Near a Cell Membrane Let s make a simple model. What is the electrostatic force that the potassium ion (charge = +1e, m = 6.5 x 10-26 kg) exerts on the sodium ion (charge +1e, m = 3.8 x 10-26 kg)? Solution: The electrostatic force is given by Coulomb s Law: ~F = 1 q 1 q 2 4 0 r 2 We need to take note of two things. First, we need to calculate the numerical value of the charge for both ˆr ions. Second, notice that this is a two dimensional L = 2.8 x 10-9 m problem. We will have to take account of both the x- and y- directions.
Recall that the charge of a particle is given by: q = ne ~F = 1 q 1 q 2 4 0 r 2 ˆr We are also need r and ˆr = ~r ~r ~r = p (2L) 2 +(2L) 2 = p 8L 2 = p 8L (1) 2L r ~r has x- and y- components ~r = xî + yĵ Plug in their values: 2L ~r = 2Lî +2Lĵ =2L( î + ĵ) (2)
Now we can get ˆr ˆr = ~r ~r = 2L( î + ĵ) p 8L ~r = p 8L ~r =2L( î + ĵ) Now, let s put it all together. ~F = 1 q 1 q 2 4 0 r 2 ˆr = k (1e)(1e) 8L 2 [ 2 p 8 ( î + ĵ)] =(8.99 10 9 N m 2 /C 2 ) (1.602 10 19 C)(1.602 10 19 C) 8(2 10 9 m) 2 [ 2 p 8 ( î + ĵ)] ~F =(2.6 10 12 N)( î + ĵ)
The End (for Today!) https://steemit.com/steemstem/@kowalskii/electrostatic-coulomb-s-law