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

Forces Between Charge

Using Charge

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

• These lessons have been designed to be read electronically, not on paper.
• To read them electronically means using a monitor.
• A monitor uses charge to make the screen emit light of various colors, and to control that light so that it forms letters and windows, etc.

• A caveat!  If you are reading this lesson using a flat panel display, then these statements are not true.  In that case imagine that you are using a CRT monitor.
• Here is a little pictorial model of what happens.  Click the appropriate phrase to show how the electrons (which carry charge!) move up, move down, or travel a flat path.  Note the following:
• When there is positive charge on the top plate there is also negative charge on the bottom plate.  The electron is simultaneously attracted to the positive charge and repelled by the negative charge.
• The same attraction/repulsion is obtained when the charges are reversed.

 Here is what happens.

• When you want a dot in the  top portion of the screen you put positive charge on the top plate and negative charge on the bottom plate, and the positive charge attracts the electrons and the negative charge repels the electrons as they fly by so they hit the screen above the center.
• When you want a dot in the bottom portion of the screen you put a positive charge on the top plate and a negative charge on the top plate to repel the electrons, moving them lower.
• With no charge on the plates, the electron travels a flat path.

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• You may have spoken of charging your car battery - or charging any other re- chargeable battery.  Charging a battery is doing exactly what the phrase says.  When you charge a battery you are putting charge into your battery, and the battery stores the charge for later use.
• When you discharge that battery you are also using charge.  Charge flows through the devices you attach to your battery - the lights in your car, the electronics that control your car, the CD player you put the charged batteries into, etc.

• Every time you run electronic gear - a TV, a stereo, a computer - from an AC wall plug outlet, you use a device called a power supply that stores charge in a capacitor.  That stored charge allows the electronic circuits you use to run during the very short times when the AC voltage goes through zero - and it does that 120 times a second on a 60hz power line.
• You not only use charge, but charge should be feared - at times.  When clouds get charged they can discharge by producing large lightning bolts that are very destructive.

• When there's a solar flare, the sun emits a stream of charged particles that pour down on the earth at a million miles per hour.  A flare can disrupt satellite communications affecting the whole world.
• Comets have a tail of charged particles and provide one of the most awesome sights in the heavens.

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

• Two charges can interact and produce forces on each other.  That effect is used when we deflect a stream of electrons to produce spots at different points on a monitor screen.  The same process occurs in ink-jet printers.  You need to learn about forces between charges.
• You need to learn about how charge flows.  Remember, it flowed into the battery and out of the battery.  Charge flow is important.
• You will need to learn about units for charge and charge flow, and you will need to learn about charge related energy concepts like voltage.

        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.

        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.

• Classical Greeks were the first to note that small pieces of material were attracted to rubbed amber.  That's the first recorded instance of an observation of force due to charge.
• You have seen electrical effects if you have noticed the attraction of small bits of paper to a recently used comb.
• Those effects are evidence of a force that exists - a force that is not a gravitational force.  That force is one of the fundamental forces of nature, and, along with gravity, it is one of the two forces that we humans can experience directly.

        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.

• There are two large forces that we can experience - gravitational forces and electromagnetic forces.
• Both of these forces act through space, sometimes over large distances.
• Gravitational effects cause the moon and planets to take elliptical orbits around a larger body. Mass causes this gravitational attraction. However, no one can give you a really good explanation of exactly what mass is except to say that it is a property of matter that causes this gravitational attraction.

• Some particles exhibit non-gravitational forces between them; forces that are much larger than gravitational forces.
• Not all particles experience this force, but those that do are said to possess a property called charge.
• Force due to charge obeys an inverse square law, of exactly the same form as the gravitational force.  Again, charge, like mass, is perhaps ultimately unexplainable, but some bodies possess it and are said to be "charged".

• Two like charges (two positive charges or two negative charges)  will repel each other, whereas two masses always attract each other.  This interactive demo gives an idea of how two unlike charges move.

• Proportional to the product of the two charges q₁ and q₂, and
• Inversely proportional to the square of the distance, r (in meters), between them.
• So, the force is given by an expression:

F_1,2 = q₁ q₂/(4pe_o r²)

Here, e_o 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.

• Charge is measured in coulombs.
• Coulombs are named after Charles Augustin Coulomb who was the first person to determine that the force law for charges was an inverse square law.
• Charge not only comes in two varieties, it also comes in discrete sizes. Electrons and protons each have the same size charge (but of opposite polarity) of magnitude 1.6x 10^-19 coulombs (+ or - as appropriate), where a coulomb is the MKS unit of charge.
• Note, the electron's charge is usually counted as negative, and the proton's charge as positive, although that is only a convention and there is no "lack" or "surplus" associated with negative and positive charges.

• Charge is measured in couloumbs (for q₁ and q₂).
• Force is measured in newtons (for F_1,2 ).
• Distance is measured in meters (for r).

• Remember that the charge on one electron is -1.6x 10^-19 couloumbs, so you just need to pile up enough electrons to get one couloumb.

        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.

        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.

• The charge on an electron is always the same.  It's has exactly the same value for every electron in the universe.
• The proton has the same absolute value of charge as the electron, but it has a positive charge, not negative.
• If you have just one electron and one proton (a hydrogen atom perhaps) then you have no net charge.  The two charges cancel!  And, they cancel exactly!
• Other fundamental particles also have exactly the same charge as an electron, although it can be either positive or negative.  The charge on an electron is a fundamental quantity - a constant of nature.

        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.

• When you plug an electrical device into a wall plug you use charget.  One example is a light bulb.  Charge flows from the wall plug, through the connecting wire and through the bulb.  In the process, the flowing charge heats up the filament in the bulb generating light - unless it is a fluorescent lamp, and then a different process creates the light.  Actually, when charge flows it is called current.  Click here to go to the lesson on current.
• If you own a car, you own a storage battery.  The battery stores enough energy to allow you to start your car. The battery stores energy by storing charge on the battery plates.  When you use the battery, charge flows out of the battery.  That's current flowing from the battery.
• Actually, when you buy a battery for a toy, a radio, a CD player, etc., you are buying stored energy, and the energy is stored as charge with potential energy.
• Batteries discharge (lose their charge) and some can be recharged (You can put charge back into them.).
• Every time you run electronic gear - a TV, a stereo, a computer - from a wall plug outlet, you use a device called a power supply that stores charge in a capacitor.  That stored charge allows the electronic circuits you use to run during the very short times when the AC voltage goes through zero - and it does that 120 times a second on a 60hz power line.

• Recently physicists have discovered more basic entities, quarks, that may have one third of the charge of an electron.  Again, it is exactly one third, and charge always comes in multiples of that quantity.  Quarks come in groups that have no net charge, and form some of the fundamental particles like protons and neutrons.  For those atomic particles charge always comes in integral multiples of the charge on an electron, or they have no net charge at all!

• Problem Basic1P01  - Charge Problem

• Charge
• Current
• Voltage
• Kirchhoff's Current Law
• Kirchhoff's Voltage Law
• Electrical Power & Energy
• Electrical Circuit Symbols