Mapping E field Lines

Lab 3 Introduction

Lab 3 – Mapping E field Lines Due at the end of Week 3

- Lab 3 Review textbook Chapter 17 which involves mapping of E lines for a certain unknown charge distribution. Actually you will be mapping electrostatic (Coulomb) force lines since the electric field E is proportional to the Coulomb force by the equation (Equation E=F/q) but these are equivalent to the E lines with equivalent directions and proportional magnitudes. This simulation can be downloaded from within your course under “PH221 Simulations”; its file name is “EXP #3 E FIELD #3.ip”.

- The direction of the electric field (or Coulomb force – Total Force in this experiment) is determined by placing a small + test charge, green in color throughout these simulations, near a charged configuration and noting the direction that the test charged is accelerated. You are free to move the small green test charge around to various locations, while noting (1) the exact location and (2) the direction of the force on that charge caused by all other charges. On the graph paper on your answer sheet, record the E field direction at the appropriate locations for the green test charge. Try to connect the arrows in a line to form the electric field lines. These lines may not cross each other. For test charge locations, choose the corners of the squares all around the charged object(s).

Lab Week 3:  

What you are trying to do is a rough likeness of the type of E field you see when you have two positive charges present. Obviously, you could be at this for days moving the test charge around and see the force vector.  If you would, please provide at least FOUR lines of force in your report. That means you need to probably move the test charge around at least 15 to twenty times.

In week 3 we learn about how moving charges can produce magnetic fields. Magnets are ultimately produced by moving charge. Even permanent magnets have their fields due to orbital motion of electrons in the atoms that make up the magnet. Unlike the electric charge, there is no such counterpart called a magnetic charge in reality. That is, there is no magnetic mono-pole. All magnets have a North and South pole associated with them. Magnetic field lines always being and end on a N and S pole. Powerful magnets are used in the industrial lab just as powerful electric fields and voltages are used. One such application is in a tool called an evaporator. In my lab we have two such tools. They are used to deposit metals of various types onto silicon and silicon dioxide substrates called wafers. It is on these wafers we fabricate complex semi-conductor and optical circuits. The voltages can be used to accelerate metal ions using the concept of F = q*E where E * d = V. A ion can be accelerated in a linear or parabolic path as desired. Magnetic fields too are used to deflect ions from a linear path to enable these ions to be contained in a certain path trajectory by F = q*v*B. It is clear to see that if both and E and B field are present that the ion can be controlled in direction and controlled in the energy it has by adjustment of E and B. The evaporator use the B field to contain the path of the ion as you read about in the textbook where circular motion from setting m*v/r^2 = qvB. The result is a radial path for the ion to travel along. The E field gives the ion the energy to hit the substrate with such force it bonds with the substrate. The ions are produced by heating the source of the metal, usually a 15 CC sized portion of metal by an electron beam that melts or sublimes the metal into a gaseous state. Once in a gaseous state, the ions are accelerated and deflected as described above. The wafers are usually patterned or stenciled using a resist resin that has been applied and patterned using a process called lithography. Envision a wafer with such a pattern with hundreds and hundreds or many thousands of opening in the resist exposing the substrate the resin is applied to. Now envision the evaporated metal being deposited in a chamber where the E and B fields play a role on depositing the metal as described above. Some of the metal ions will deposit inside these openings in the resin and some of the metal will deposit on the resin. After the deposition is complete, the wafer is placed to soak in an acetone beaker where the solvent swells the resist from the sides of the opening and undercuts all the resist and lifts off the wafer leaving only the metal that directly deposited on the silicon or silicon dioxide to be left on the wafer. This result is how metal lines and indeed all metal deposition can be found to create lead lines, solders, heaters etc. These elements can be used for signal detection and manipulation and also be used to change the optical properties of the dielectric glasses such as silicon dioxide if they are "doped" or have a small amount of additional elements in them such as Phosphorus and Boron. This is but one small example of how some of the concepts you study here are used in an industrial lab.  I have attached a file that show the lithography of a photoresist. The resist is sensitive to ultraviolet light which either, depending on the resist type will break or cross link chemical bonds in the resist. What bonds are broken or not cross linked become susceptible to developer. The developer is used to remove resist, leaving an opening when metal can be placed by evaporation. Once the metal is evaporated, the wafer is placed in acetone which swells the resist, thus lifting the resist and metal away. Only the metal deposited in the non resist areas remains on the wafer. The second picture is a schematic of how metal evaporation works. Note the B field that changes the path of the metal ion that is produced either by melting or sublimation.

 

Lab 3