Ch 23: Electric Force & Field; Gauss' LawWorksheetSee all chapters
All Chapters
Ch 01: Intro to Physics; Units
Ch 02: 1D Motion / Kinematics
Ch 03: Vectors
Ch 04: 2D Motion (Projectile Motion)
Ch 05: Intro to Forces (Dynamics)
Ch 06: Friction, Inclines, Systems
Ch 07: Centripetal Forces & Gravitation
Ch 08: Work & Energy
Ch 09: Conservation of Energy
Ch 10: Momentum & Impulse
Ch 11: Rotational Kinematics
Ch 12: Rotational Inertia & Energy
Ch 13: Torque & Rotational Dynamics
Ch 14: Rotational Equilibrium
Ch 15: Angular Momentum
Ch 16: Periodic Motion
Ch 17: Waves & Sound
Ch 18: Fluid Mechanics
Ch 19: Heat and Temperature
Ch 20: Kinetic Theory of Ideal Gasses
Ch 21: The First Law of Thermodynamics
Ch 22: The Second Law of Thermodynamics
Ch 23: Electric Force & Field; Gauss' Law
Ch 24: Electric Potential
Ch 25: Capacitors & Dielectrics
Ch 26: Resistors & DC Circuits
Ch 27: Magnetic Fields and Forces
Ch 28: Sources of Magnetic Field
Ch 29: Induction and Inductance
Ch 30: Alternating Current
Ch 31: Electromagnetic Waves
Ch 32: Geometric Optics
Ch 33: Wave Optics
Ch 35: Special Relativity
Ch 36: Particle-Wave Duality
Ch 37: Atomic Structure
Ch 38: Nuclear Physics
Ch 39: Quantum Mechanics

Concept #1: Electric Charge

Example #1: Charge of Atom

Practice: How many electrons make up −1.5 × 10−5 𝐶?

Example #2: Electrons In Water (Using Density)


Hey guys, let's do another example about electric charge, okay? Water weighs one kilogram per liter has a molecular weight of 18 grams per mole and has 10 electrons per molecule.

Part A how many electrons does two liters of water have? An Part B, what charges, what charge do these electrons represent? So, how many electrons do we find in Part A? what is the charge of those electrons, okay? So, for Part A. First, what we want to do is we want to figure out how to get from liters which is what we're given, we're given two liters of water to number of electrons, this will tell us how to solve the problem, we need to create a sort of map to the solution, okay? Let's start with liters, right? Because that's what's given to us, what can we go to next? Well, we're told that there's a conversion between kilograms and liters that we can say for every liter of water it has a mass of one kilogram. So, we know how to go from liters to kilograms, next we have grams to moles. Now, we don't have kilograms to moles but we know right away that one kilogram is 1,000 grams. So, we can easily go from kilograms to grams and then using the conversion go from grams to moles. Now, our last conversion is electrons per molecule, we don't have our number of molecules yet, we have in moles but we can use Avogadro's number to convert moles to molecules. Now, using our last conversion factor, we can go from molecules to number of electrons. So, this right here is our map, that's going to guide us through this problem, okay? So, let's start doing these conversions, 2 liters of water times 1 kilogram per liter is 2 kilograms. So, our water has a mass of 2 kilograms now right away, we know that that's equivalent to 2,000 grams, okay? So, we've done this step and this step. Now, we need to go from grams to moles, okay? 2,000 grams times 18 grams per one mole is about 111 moles. So, we've done the next step. Now, we need to go from moles to molecules using Avogadro's number 111 moles times 1 mole per 6 times 10 to the 23 molecules is 6. Seven times 10 to the 25 molecules of water, okay? So, we've done this step, the last step is simply to figure out how many electrons are represented by this much water as many molecules of water, we know that it's 10 electrons per molecule. So, it's very simple, we just multiply this number by 10, 6.7 times 10 to the 26 electrons, okay? And we followed our map successfully from liters which was given to us to electrons, okay? Now, part be, what charge does this amount of electrons represent? Well, each electron has a charge e, the elementary charge and we have some number of electrons in which we figured out in part A. So, multiplying these together will tell us our total charge our number is 6.7 times 10 to the 26 and the elementary charge is what? Remember, guys you need to know this 1.6 times 10 to the negative 19 coulombs multiplying those together we get a total charge of 1.07 times 10 to the eighth, cool? Okay.

Practice: How many electrons do you have to add to decrease the charge of an object by 16uC?