Atoms are comprised of charges. The positively charged nucleus is attracted to the negatively charged electrons. Molecules, which are comprised of atoms, also bind together due to this charge related force of attraction. Solids and liquids, comprised of molecules, also bind together due to the electric force of charges. All forces we experience, except gravity, are due to the electric force.

Crash Crouse physics introduces us to charges.

https://www.youtube.com/watch?v=TFlVWf8JX4A

 

Pre-lecture Study Resources

Watch the pre-lecture videos and read through the OpenStax text before doing the pre-lecture homework or attending class.

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Learning Objectives

Summary

Summary

Atomistic Goals

Students will be able to...

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BoxSand Introduction

Electric Phenomena  |  Coulomb's Law

Electric Force

To understand the effects of objects with a distribution of charges exerting a force on each other, lets start with the force between two point charges. This is known as Coulomb's Law. The below equation reads, the magnitude of the electric force of particle 2 on 1 is equal to the magnitude of the electric force of 1 on 2 (Newton's 3rd law). That force is equal to the electric constant $k$ times the magnitude of $q_1$, times the magnitude of $q_2$, divided by the absolute distance between $q_1$ and $q_2$ squared.

$|\overrightarrow{F}^E_{21}| = |\overrightarrow{F}^E_{12}| = \frac{k|q_1||q_2|}{|\Delta \overrightarrow{r}_{12}^2|}$,   where $k=8.99\times 10^9 N \cdot m^2/C^2$

The direction of the net force is determined by drawing a Free-Body Diagram and using the fact that unlike charges attract and like charges repel.

This is an image of a free body diagram of the forces on an charged atom. It shows three different charges in a triangle formation with the first ion with a positive charge of four microcoulombs at seventy three degrees from the horizontal with a distance of fifteen centimeters is another ion with a charge of negative six microcoulombs. From the first ion at a distance of ten centimeters horizontally to the right is the third ion with a charge of negative five microcoulombs. To the right of this scenario is a force body diagram of the first ion showing the force vectors from the second and third ion as well as the net force vector.

Here you see an example of the FBD for $q_1$ interacting with $q_2$ and $q_3$. Since 1 is positively charged and 2 and 3 are both negatively charged, 1 is attracted to both 2 and 3. So both $F_{12}$ and $F_{13}$, acting on particle 1, are towards the other particle. The magnitude of $F_{13}$ is larger than $F_{12}$ even though $q_2$ has a greater charge. That is due to $q_3$ being closer to $q_1$ than $q_2$. With the electric force being inversely proportional to the distance between the particles and only proportional to the charge to the 1st power, being closer means a greater force in this case.

To find the force on a charged particle you typically have to do the following:

  1. Draw a FBD of forces acting on the particle of interest using whether they attract or repel and a rough scaling of how large due to the relative charge magnitudes and distances.
  2. Determine the distance between the particle of interest and each other particle.
  3. Determine the magnitude of each electric force.
  4. Use vector summation to add up the components of each electric force and find the net force. You may have to use an angle found using geometry of the physical setup to find a component of the force.


Key Equations and Infographics

A representation with the words net charge of a system on the top. There is an equation that shows that the net charge of a system is equal to the elementary charge multiplied by the total number of protons minus the total number of electrons. This is also written in words below.

 

A representation with the words electric force on the top. There is an equation that shows that the magnitude of the electric force of particle two on one is equal to the magnitude of the electric force of one on two which is equal to the constant k times the magnitude of q one, times the magnitude of q two, divided by the absolute distance between q one and q two squared. This is also written in words below.

 

A representation with the words force on a test charge in an electric field on the top. There is an equation that shows that the force exerted on a test charge residing in an electric field is equal to the test charge multiplied by the electric field. This is also written in words below.

 

A representation with the words electron slash proton charge on the top. There is an equation that shows that the elementary charge of a proton is equal to positive one point six, zero, two by ten to the nineteenth power. The elementary charge of an electron is equal to negative one point six, zero, two by ten to the nineteenth power. This is also written in words below.

Now, take a look at the pre-lecture reading and videos below.

OpenStax Reading


OpenStax Section 18.3  |  Coulomb's Law

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Additional Study Resources

Use the supplemental resources below to support your post-lecture study.

YouTube Videos

Pre-Med Academy is back with three well put together videos on our main subjects, Electric Charge, Electric Force, and Coulomb's Law

Youtube: Pre-Med Academy - Electric Charge

Youtube: Pre-Med Academy - Electric Force

Youtube: Pre-Med Academy - Coulomb's Law

Step By Step Science has several example problems,  here are a few on using Coulomb's Law. There are more in this section's content repository.

 

Youtube: Step By Step Science - Force between Two Charges

 

Youtube: Step By Step Science - Triangular Arrangment

 

Youtube: Step By Step Science - Proton and Electron

 

Other Resources

This link will take you to the repository of other content related resources .

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Simulations


This simulation by Duffy helps you with Coulombs Law and Newton's 2nd law

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The colorado PhET simulation is everything you ever needed in life, that you didn't know that you needed. We present to you, John Travoltage.

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Charge transfer PhET simulation with baloons and a sweater.

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For additional simulations on this subject, visit the simulations repository.

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Demos


Rub a plastic cup against carpet, hair, or fur, and observe the effects of the net charge now on the cup.

This is a gif of a charged plastic cup near a dripping faucet of water. It shows that the water molecules moving towards the cup even though the cup doesn’t touch the water because the water molecules are polar in charge and are attracted to the plastic cup.

For additional demos involving this subject, visit the demo repository

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History


Oh no, we haven't been able to write up a history overview for this topic. If you'd like to contribute, contact the director of BoxSand, KC Walsh (walshke@oregonstate.edu).

Physics Fun

Surface tension is a polarization force between atoms. It's responsible for holding a water droplet together.

This is a gif of a drop of water being dropped into a larger body of water. It shows that the ball of water does not immediately fall into the body of water but stays as a sphere momentarily before the surface tension breaks and joins into the rest of the water.

Play with John Travolta's leg and arm to get a Travoltage.

Other Resources




Boston University's Page on electric charge and Coulomb's law is neat reference with a couple of example problems

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PPLATO is a complete resource with a lot of information, and several practice questions per subject. This webpage covers electric charge, the electric field, and electric potential, frocus on the electric charge, we'll get back around to field and potential later.

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The Hyperphysics and their reference card for Coulomb's Law and the electric force.
 

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The Physics Classroom has an entire chapter on static electricity. The last lesson of the chapter is on Electric Fields so save that for later!

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Resource Repository

This link will take you to the repository of other content related resources .

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Problem Solving Guide

Use the Tips and Tricks below to support your post-lecture study.

Assumptions

 

Checklist

 

Misconceptions & Mistakes

  • If you have two charges QQ and qq, and the magnitude of charge QQ is greater than that of charge qq, the two charges still apply an equal and opposite force to each other. Regardless of the relative magnitudes, Newton's 3rd Law still holds.
  • While you can induce a dipole in both insulators and conductors, the atomic nature of the dipole is different for the two cases.

Pro Tips

 

Multiple Representations

Multiple Representations is the concept that a physical phenomena can be expressed in different ways.

Physical

Physical Representations describes the physical phenomena of the situation in a visual way.

 

Mathematical

Mathematical Representation uses equation(s) to describe and analyze the situation.

A representation with the words net charge of a system on the top. There is an equation that shows that the net charge of a system is equal to the elementary charge multiplied by the total number of protons minus the total number of electrons. This is also written in words below.

 

A representation with the words electric force on the top. There is an equation that shows that the magnitude of the electric force of particle two on one is equal to the magnitude of the electric force of one on two which is equal to the constant k times the magnitude of q one, times the magnitude of q two, divided by the absolute distance between q one and q two squared. This is also written in words below.

 

A representation with the words force on a test charge in an electric field on the top. There is an equation that shows that the force exerted on a test charge residing in an electric field is equal to the test charge multiplied by the electric field. This is also written in words below.

 

A representation with the words electron slash proton charge on the top. There is an equation that shows that the elementary charge of a proton is equal to positive one point six, zero, two by ten to the nineteenth power. The elementary charge of an electron is equal to negative one point six, zero, two by ten to the nineteenth power. This is also written in words below.

Graphical

Graphical Representation describes the situation through use of plots and graphs.

 

Descriptive

Descriptive Representation describes the physical phenomena with words and annotations.

 

Experimental

Experimental Representation examines a physical phenomena through observations and data measurement.

 

Practice

Use the practice problem sets below to strengthen your knowledge of this topic.

Fundamental examples

 

(1) A point charge with charge $q_1 = 1.8 nC$ is located at the origin. A point charge with charge $q_2 = 3.6 nC$ is located a distance $r_{12} = 6 nm$ away. (a) What is the magnitude of the coulomb force from charge 1 on charge 2? (b) Is this an attractive or repulsive force?

(2) A point charge with charge $q_1 = - 1.8 nC$ is located at the origin. A point charge with charge $q_2 = 3.6 nC$ is located a distance $r_{12} = 6 nm$ away. What is the magnitude of the coulomb force from charge 1 on charge 2?

(3) A point charge with charge $q_1 = - 1.8 nC$ is located at the origin. A point charge with charge $q_2 = 3.6 nC$ is located a distance $r_{12} = 6 nm$ away. A point charge with charge $q_3 = 5 nC$ is located at a position $r_3 = <2,2> nm$. What is the magnitude of the coulomb force from charge 1 on charge 2?

Solutions found HERE

Short foundation building questions, often used as clicker questions, can be found in the clicker questions repository for this subject.

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Practice Problems

BoxSand practice problems - Answers

BoxSand's Conceptual problems

BoxSand's multiple select problems

BoxSand's quantitative problems

Recommended example practice problems 

  • OpenStax, Period and Frequency also linked in Charges & the Electric Force sections.
  • Large set of questions on electric charge and force. External PDF
  • Coulomb's Law Review, External PDF

For additional practice problems and worked examples, visit the link below. If you've found example problems that you've used please help us out and submit them to the student contributed content section.

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