Charge
Why Do You Need To Know About Charge?
Goals For This Lesson
Forces Between Charge
Using Charge
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Charge

       In this lesson you're going to learn about charge.  Before we start, here's a quick question for you.


Q1   Have you ever used charge?  Click on your answer.


You are using charge now, believe it or not.         To produce the light that the monitor emits, and to control the appearance of that light, voltage is used to control the path of charge that is emitted at the back of the monitor and which strikes the monitor screen.

 Here is what happens.

        Here are a few questions for you to answer.
Q2  If there is more positive charge on the top plate, which way will the charge move?

Q3 If the moving charge is positive, which way will it move compared to the direction the electron moves?

        Don't get the idea that monitors are the only devices that use charge.  Let's review a few other places where you may have used charge.         You use charge all the time.  You may not think about it much, but you do.  Here are some examples of times when you use charge.         Charge affects everything! What Do You Need To Learn About Charge?

        There are many things that you might need to know about charge.  Here is a partial list.


Goals For This Lesson

        This lesson introduces you to some simple concepts about charge.  At the end of the lesson, you want to be able to do the following.

Given a question involving charge
Be able to compute amounts of charge.
Be able to predict how charge moves - when charges attract and when they repel.

Forces Between Charges & Facts About Charge

        In this section you will begin by learning about charge - a basic electrical quantity.  We start with a short discussion of the force between charges.

        These tiny effects have gradually been studied and put to use, especially in the last century and a half.  Starting from observing these tiny effects, scientists and engineers have learned basic principles and discovered other electrical effects that have led to the industries we rely on today including the power industry, the electronic communication industry and the whole world of computers.

        The effects these forces have in the world are no longer tiny.  The major moving forces in society - the ability to communicate instantaneously and the ability to compute solutions to large problems - are directly attributable to what we know about electricity.  And, what we know about electricity starts with charge - the invisible quantity that produces electrical forces.

        But, there is another force.         The force law for charges is somewhat different because charge comes in two different types, positive and negative charge.
        The force law for charge is similar to the gravitational force law.  For two charges, q1 and q2, the force between them is:

F1,2 = q1 q2/(4peo r2)

Here, eo is a fundamental constant of nature, = ~8.885419x10-12 F/m.

        Like every other physical quantity, when you deal with charge you must account
 for units.

    When you use the force law expression:
F1,2 = q1 q2/(4peo r2)

Problem
P1   How many electrons does it take to produce -1 coulomb of charge?
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Consider this.

        If charge obeys an inverse square law it obeys a force law just like the gravitational force law.  The gravitational force law depends inversely upon the square of the distance between two masses, so mass plays a role somewhat similar to the role charge plays in the force law.

        Because of the similarity between the laws there are going to be some concepts that work the same in both cases.  There will also be some differences.  Two positive charges repel each other whereas two masses attract each other.  Charge comes in two varieties that we call positive and negative.  We don't know that that happens for masses.  Anti-matter probably does not have negative mass, although it interacts with matter explosively.  It doesn't look like two masses could repel each other.  The possibility of attraction and repulsion makes charge unique.


Questions

        If the force law between charged particles is the same as the force law between two masses, then what phenomena of gravitiation fields would you expect to be the same for charged particles?

Q4  The concepts of potential energy would be the same.

Q5  Just like two masses - like the earth and the moon - can orbit each other, charges can orbit each other.

Q6  Just like mass, charge is always positive.

Q7  Just like every particle has mass, every particle has charge.

Q8  Just like mass, two charged particles always attract each other.



        There's one last set of facts about charge that you should know.
Where Do You Use Charge?

        You may be tempted to think that charge is somewhat obscure and that you don't ever use charge.  You're wrong.  You use charge constantly, and you buy lots of things that store charge.


There is some late breaking news         In this lesson you have started to learn about charge, a basic electrical quantity.  However, charge in motion - charge flowing through a wire for example - is current, and that is something else you need to learn about.  Click here to go to the lesson on current. If you want to go directly to the lesson on voltage, click here.
Problem


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