Spiders ride electric fields to fly hundreds of miles.

https://www.youtube.com/watch?v=GRrUxi6d7so

Pre-lecture Study Resources

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

BoxSand Videos

Learning Objectives

Summary

Summary

Atomistic Goals

Students will be able to...

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BoxSand Introduction

Electric Fields  |  Field Model and Charges in an Electric Field

Field Theory

A field is a map. It tells you how navigating a region of space may go before you even get there. Imagine a room being warmed by a fireplace in the corner. I could walk to every point in the room, staring with position $\overrightarrow{r}=<0,0,0> m$, and record the temperature in a data table. That table represents the temperature field of the room. That scalar field could then be used to deduce heat transfers in the room. Fields are maps, where every point in space there is information (usually a scalar or a vector), that can be used to explain interactions. Fields are constructs, more than any other we've encountered. They are not observable in any conventional way but are rather a conceptual tool for the study of interactions.

An example of a vector field is a velocity field. Instead of every point in space containing one number, like in the temperature field example, every point in space contains three numbers which are the x, y, and z components of the velocity. A stream that narrows could possibly be described by the velocity field below.

This is an image of a velocity vector field of a stream with a small bump that narrows the stream in one part. It shows the flow of the water as vector arrows that shows the velocity increasing when the stream narrows.

Even though there are not vectors drawn everywhere in the stream does not mean the field is zero at those points, its merely due to the limitation of how many vectors are feasible (and useful) to draw. Now a rubber ducky floating down the river could have a somewhat predictable interaction with the river. It would speed up in the narrow region and possibly change directions if it entered from the bottom. Be careful, the temperature and velocity field examples are not the same as the electric field and potential. They are only used to get a loose feel for the concept.

The Electric Field

All charges create electric fields. We say the charge alters the space around it but this not true in any real way, that is what mass does. Charges rather create a field that can travel through the vacuum of space and influence other charges to feel a force. Consider the situation below where $Q$ is a positive charge that generates the vector field shown. That is the field owned by charge $Q$.

This is an image of a positively charged ion with vector arrows pointing away from the ion. The electric field is strongest closest to the ion with longer vector arrows and weaker further away from the ion with shorter vector arrows.

A second charge $q_0$, whose field is not being shown, resides in $Q's$ field. We call $q_0$ the test charge since we are not considering the field it creates. If the electric field from $Q$ is considered to be $\overrightarrow{E}$, then the force that field puts on the test charge follows the following equation.

Force on a Test Charge ($q_0$) in an Electric Field ($\overrightarrow{E}$)

$\overrightarrow{F_0}=q_0 \overrightarrow{E}$

The essence of an electric field lies in this example. The charge $Q$ generates a field that a second charge $q_0$ feels a force from and in that way they interact. This model gives $Q$ ownership to something that can exist without the presence of the second charge. After all, there would be no such thing as a Coulomb force if the second charge didn't exist. It is this reason some consider the electric field to be more fundamental and elegant than Coulombs Law.

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 18.4   |  Electric Field: Concept of a Field Revisted

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Equations, definitions, and notation icon Concept Map Icon
Key Terms Icon Student Contributed Content Icon

Additional Study Resources

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

B.U. Physics Icon

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.

This is an image of an interactive game

Other Resources


Resource Repository

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



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

B.U. Physics Icon

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.

Hyper Physics Icon

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!

The Physics Classroom Icon

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.

Clicker questions Icon

 

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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