Electrostatics Application

One common use of electrostatics is in laser printers. Laser printer use a process called xerography which applies some of the concepts used in electrostatics. This process involves a selenium coated aluminum drum, which is sprayed with positive charges from points on a device called a corotron. In the first stage, the conducting aluminum drum is grounded so that a negative charge is induced under the thin layer of positively charged selenium. In the next stage, the surface of the drum is exposed to the image of whatever is to be copied. Where the image is light, the selenium becomes conducting, and the positive charge is neutralized. In darker areas, the positive charge remains, and so the image has been transferred to the drum. The third and final stage takes a dry black powder, also known as toner, and sprays it with a negative charge so that it will be attracted to the positive regions of the drum. So, when a person wants to print on a blank piece of paper, it is fed through and the paper is given a greater positive charge than on the drum so that it will pull the toner from the drum. As the paper and toner passes through heated pressure rollers, it melts and permanently adheres the toner within the fibres of the paper.

http://cnx.org/content/m42329/latest/?collection=col11406/latest

Electrostatics Reflection

Electrostatics is the study of stationary electric charges or fields, as opposed to electric currents. This entails the forces acting on them and their behaviour in substances. Going back to Grade 9 Science, we reviewed insulators, conductors, and methods of charging objects. Insulators are materials in which the electrons are tightly bound to the nucleus, which means that they do not conduct electricity. Alternatively, conductors are materials in which the electrons in the outer part of the atom are free to move, which mean they conduct electricity. We learned of 3 ways to charge object, including friction, conduction and induction. Friction is when 2 objects are rubbed together and the electrons of 1 object transfer to the other. Conduction occurs when objects transfer electrons from touching, or making contact. Lastly, Induction is when a charged object is brought near, but does not touch a neutral object. Even though the objects do not touch, the charged object polarizes the neutral object and the electrons rearrange, therefore the object behaves as if it is charged, but is still electronically neutral.

The Law of Charges is that like charges repel and unlike charges attract. This is important to keep in mind when determining the direction a charges moves when in the presence of another charge. The Law of Conservation of Charge states that the net charge of an isolated system remains constant. The only way to change the net charge of a system is to bring in charge from elsewhere, or remove charge from the system. To describe the electrostatic interaction between electrically charged particles, we use Coulomb's Law. Coulomb's Law involves electric force (Fe), the universal constant (k), q (charge), and r (distance between the charges). Coulomb's law is Fe=kQq/r^2.

Next we learned about electric field, which is a vector quantity from which is determined the magnitude and direction of the force on a charged particle due to the presence of other charged particles. In comparison to gravitational force, they are both field forces, which means that no contact is necessary to influence an object. Although, a difference is that gravitational force is a weak force, while electrostatic force is a strong force. In addition, electrostatic force can be either attractive or repulsive due to the law of charges. Lastly, just as an object with mass has a gravitational field, a charged particle creates an electric field around it. To determine the direction of an electric field, we imagine that a small positive test charge is placed in the field, and depending on if the source is negative or positive, will move in different directions. To find the magnitude of an electric field, we can use the equation, E=kQ/r^2 or E=Fe/q, depending on whether the source charge is known, or the test charge is known.

An important skill we learned this unit was being able to draw a representation of an electric field. Electric field lines are used to show the electric field surrounding a charged particle.

http://www.alternativephysics.org/book/ElectricFields.htm

In the example above, we can see that the positive fixed charge has arrows coming out from it, while the negative charge has line going into it. We can also see that they are equal charges as the number of lines going in from the negative particle, equals the number of lines coming from the positive charge. In these diagrams, the number of lines also reveal the concentration of the electric field at a particular area. The more lines in an area, means the stronger the electric field. There are some specific rules for drawing these diagrams. Some of them are, that the lines must begin on positive charges, and end on negative charges, and no field lines can cross. 

The next concept we learned was electric potential energy. This is the energy stored in the system of 2 charges that are a certain distance apart. Electric PE reflects the work that must be done to move a small charge in an electric field created by a source charge. In this case, W (work) =∆PE (change in potential energy) which also equates to kQq/r. Because this depends on the distance or r, the PE is zero at an infinite distance. Unlike for electric force and electric field, we have to include the signs of the charges because unlike charges require negative PE as it takes work to separate the charges due to the attractive forces between the charges. 

Another slightly confusing topic we learned about was electric potential. Electric potential, also known as voltage is defined as the amount of electric potential energy per unit charge. The most difficult part about voltage is just recognizing that electric potential is the same as voltage when reading questions. The equation for electric potential is V (voltage) = PE/q, which also equals kQ/r. 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html

The last type of problem we learned to solve is regarding parallel plates as seen above. An electric field is established between the two plates when they become charged, and they are connected by a battery. As you can see, one plate is positively charged, and the other is negatively charged. In addition, the electric field goes from the positive plate to the negative plate. Since the field lines are parallel to each other, the electric field is uniform in comparison to the electric field between point charges. Because the electric field between plates does not vary with the distance (as is the case with point charges), we can use the equation E=∆V/d to find the magnitude of the electric field. 

As I mentioned previously, I think the most difficult thing to grasp is when to use each equation. Just knowing what each question is asking for is something that took awhile to understand, however became easier with practice.