Electrostatics

Which spheres experience the greatest attraction?

Properties of “Charge” Reflects relative number of electrons in a substance Conserved Units of Coulombs (C) An electron has a charge of 1.6 x C of charge

Significant Charge Amounts When we rub balloons on rabbit hair in the lab, we’re generating 10’s of  C

Coulomb's Law Coulomb’s Law gives us a way to calculate the force between two charged objects F E = kq 1 q 2 /d 2 **Note: q 1 and q 2 represent charge magnitudes (we’ll talk about direction later) k is a constant = 8.99 x 10 9 N m 2 /C 2

Similarities to Gravity Recall how we calculated the gravitational force between objects: F G = Gm 1 m 2 /d 2 G is a constant = 6.67 x N m 2 /kg 2 Look familiar?

Different Constants G = 6.67E-11, k = 8.99e9 What does this tell us about the difference between gravitational forces and electrostatic forces?

Examine the configuration below. Which charge would exert the greatest force on the -2 charge?

Which list below ranks the charges in order of increasing force on the -2 charge? 1. A, B, D, C 2. A, C, B, D 3. D, C, B, A 4. C, A, B, D 5. D, B, C, A

Which arrow represents the direction of the net force on the -2 charge?

Calculate the electrostatic force on a +6  C charge by a -5  C charge, located 2m to its left F =.067 N, to the left

Balloon Demo

Calculate the electrostatic force between a proton in the nucleus of the atom (q = +1.60e-19C) and an electron (q = -1.60e- 19C) located in an outer energy level (d = 3e-11m) Calculate the electron’s acceleration

Levitation I once heard a person ask, couldn’t you make a person float using charges? Perhaps Imagine a person (m = 70kg) gathered - 10mC of charge by rubbing herself with rabbit fur What charge would we need to lift her off the ground?

What is this?

“Scaling Force” Recall Newton’s second law: The ratio of Force/mass is acceleration What is the analog for charged particles? Force/charge = We call this ratio the electric field

Electric Fields Like gravity, the electrostatic force is a non-contact force To conceptually deal with this, we talk about electric fields This is a region of space surrounding a charged particle that “carries” the electrostatic force

An electric field tells us the direction of the electrostatic force It also gives us a sense of the force magnitude

Drawing the Field Place a positive “test” charge near a charge, or charge configuration Determine the direction of the net force acting on that positive charge Draw an arrow in that direction (arrow length represents force magnitude) Move the charge to another place and repeat Field lines start/end at infinity (somewhat hard to draw…)

Negative charges?

With both of these configurations, what happens to field strength as I get further and further from the charge? Can you envision a scenario in which field strength remains constant?

Field Strength Imagine two, positive charges of charge q, separated by distance d. What is E for one of those two? E = k|q|/d 2 Units of N/C As they are vectors, fields add together as vectors do

Draw the Field for this charge configuration:

Find the field strength at point p (q = 5.0  C)

Is there a point at which E = 0?

Other configurations ms.php?sim=Charges_and_Fieldshttp://phet.colorado.edu/new/simulations/si ms.php?sim=Charges_and_Fields

Levitation Imagine a human being of mass 75kg If she were to somehow acquire an excess charge of 20  C by vigorously rubbing rabbit fur on a dry day, what field strength would be necessary to levitate her above the ground? Which way would the field point?

Calculate the acceleration of an electron (m = 9E-31kg, q = 1.6E-19C), placed in a field of strength 3E-7 N/C