### Comparing electric force and gravitational force (practice) | Khan Academy

Practice: Relationship between electric force, charge, and distance analyzing the electric and gravitational interaction between a proton and an alpha particle. only really need to do the Qs, to see the charge (Q1 multiplied by Q2) E=k( Q1Q1/r^2), describe the relationship between electric force, charge, and distance. Forces between two electrically-charged objects can be extremely large. Metals are good conductors of electric charge, while plastics, wood, and rubber are not. When the ground connection is removed, the conductor will have a charge opposite in sign to that r is the distance between the charges.

The magnitude of the force and the distance between the two balloons is said to be inversely related.

### Electric charge and Coulomb's law

Coulomb's Law Equation The quantitative expression for the effect of these three variables on electric force is known as Coulomb's law.

Coulomb's law states that the electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects. In equation form, Coulomb's law can be stated as where Q1 represents the quantity of charge on object 1 in CoulombsQ2 represents the quantity of charge on object 2 in Coulombsand d represents the distance of separation between the two objects in meters.

The symbol k is a proportionality constant known as the Coulomb's law constant. The value of this constant is dependent upon the medium that the charged objects are immersed in.

In the case of air, the value is approximately 9. If the charged objects are present in water, the value of k can be reduced by as much as a factor of It is worthwhile to point out that the units on k are such that when substituted into the equation the units on charge Coulombs and the units on distance meters will be canceled, leaving a Newton as the unit of force. The Coulomb's law equation provides an accurate description of the force between two objects whenever the objects act as point charges.

A charged conducting sphere interacts with other charged objects as though all of its charge were located at its center. While the charge is uniformly spread across the surface of the sphere, the center of charge can be considered to be the center of the sphere. The sphere acts as a point charge with its excess charge located at its center. Since Coulomb's law applies to point charges, the distance d in the equation is the distance between the centers of charge for both objects not the distance between their nearest surfaces.

The symbols Q1 and Q2 in the Coulomb's law equation represent the quantities of charge on the two interacting objects. The sign on the charge is simply representative of whether the object has an excess of electrons a negatively charged object or a shortage of electrons a positively charged object.

While the practice is not recommended, there is certainly no harm in doing so.

This is consistent with the concept that oppositely charged objects have an attractive interaction and like charged objects have a repulsive interaction. If the charge of one of the objects is doubled, and the distance separating the objects is doubled, then what is the new force? The electrostatic force is directly related to the product of the charges and inversely related to the square of the separation distance. Doubling one of the charges would serve to double the force. Doubling the distance would serve to reduce the force by a factor of four.

The combined affect of these two variations would be to decrease the force by a factor of two - changing it from 0. If the charge of both of the objects is doubled and the distance separating the objects is doubled, then what is the new force? Doubling both of the charges would serve to quadruple the force. The combined affect of these two variations would be to not change the force at all; it remains as 0.

If the charge of one of the objects is increased by a factor of four, and the distance separating the objects is doubled, then what is the new force? Quadrupling one of the charges would serve to quadruple the force. The combined affect of these two variations would be to not alter the force at all; it would remain as 0.

There are three ways that objects can be given a net charge. Charging by friction - this is useful for charging insulators. If you rub one material with another say, a plastic ruler with a piece of paper towelelectrons have a tendency to be transferred from one material to the other.

For example, rubbing glass with silk or saran wrap generally leaves the glass with a positive charge; rubbing PVC rod with fur generally gives the rod a negative charge.

### Relationship between electric force, charge, and distance (practice) | Khan Academy

Charging by conduction - useful for charging metals and other conductors. If a charged object touches a conductor, some charge will be transferred between the object and the conductor, charging the conductor with the same sign as the charge on the object.

Charging by induction - also useful for charging metals and other conductors. Again, a charged object is used, but this time it is only brought close to the conductor, and does not touch it. If the conductor is connected to ground ground is basically anything neutral that can give up electrons to, or take electrons from, an objectelectrons will either flow on to it or away from it.

When the ground connection is removedthe conductor will have a charge opposite in sign to that of the charged object. An example of induction using a negatively charged object and an initially-uncharged conductor for example, a metal ball on a plastic handle. Electrons on the conductor will be repelled from the area nearest the charged object.

The electrons on the conductor want to get as far away from the negatively-charged object as possible, so some of them flow to ground. This leaves the conductor with a deficit of electrons.

## Inverse Square Law

The conductor is now positively charged. A practical application involving the transfer of charge is in how laser printers and photocopiers work. This is a good web page that gives a nice description of how a photocopier works: University of Delaware Why is static electricity more apparent in winter?

You notice static electricity much more in winter with clothes in a dryer, or taking a sweater off, or getting a shock when you touch something after walking on carpet than in summer because the air is much drier in winter than summer. Dry air is a relatively good electrical insulator, so if something is charged the charge tends to stay. In more humid conditions, such as you find on a typical summer day, water molecules, which are polarized, can quickly remove charge from a charged object.

Try this at home See if you can charge something at home using friction. I got good results by rubbing a Bic pen with a piece of paper towel. To test the charge, you can use a narrow stream of water from a faucet; if the object attracts the stream when it's brought close, you know it's charged.

## CHAPTER 22

All you need to do is to find something to rub - try anything made out of hard plastic or rubber. You also need to find something to rub the object with - potential candidates are things like paper towel, wool, silk, and saran wrap or other plastic.

Coulomb's law The force exerted by one charge q on another charge Q is given by Coulomb's law: Remember that force is a vector, so when more than one charge exerts a force on another charge, the net force on that charge is the vector sum of the individual forces.