Charged Object Moving in a Capacitor
Lab 4 which involves the vector characteristics of a moving charged object within a charged parallel plate capacitor.
• This simulation can be downloaded from within your course under “PH221.Simulations”; its file name is “#15 Charge and Cap.ip”.
A positively charged particle is moving horizontally when it enters the region between the plates of a capacitor as the simulation illustrates. (a) Draw (sketch) the
trajectory that the particle follows in moving through the capacitor. (b) When the particle is within the capacitor, which of the following four vectors, if any are
parallel (||) to the electric field E inside the capacitor: the particle’s displacement (), its velocity (v), its linear momentum (p), and its acceleration (a)? For
each vector, explain why the vector is or is not parallel to the electric field of the capacitor.
Run the simulation noting the particle’s trajectory (as indicated by the “tracking” or “strobes”) while inside the capacitor cavity. Also note the velocity and
acceleration vector, v and A, respectively arrows. Fill in the answers for the blanks in the Lab Answer Sheet at the end of this lab.
A capacitor is a charge storage device. A parallel plate capacitor consists of parallel conducting plates separated by an insulator. In this experiment, the insulator
is air and there is equal but opposite charges (+Q and –Q) placed on each conducting plate (See Figure 18.25, Section 18.6 and 18.7 in your textbook). This virtual lab
investigates the effects (if any) of a positively charged particle midway between the oppositely charged plates of a parallel plate capacitor moving in a + x-
direction. Just as in Lab 3, there will exist an electrostatic (Coulomb) force on the charged particle when it is inside the capacitor.
In this lab, you will investigate electric field (E) lines. Electric charges create an electric field in space surrounding them. Electric field lines essentially give
us a “map” of the direction and strength (magnitude) of the E field at various places in space. E lines are always directed away from positive charges and toward
negative charges (see Figure 18.23, textbook Section 18.7, and Lesson 3). In Figure 18.27 the absence of E lines indicates that the electric field is relatively weak
between the two positive charges. Since you will be performing in this experiment a virtual mapping of E lines for a certain charge distribution, carefully study
Conceptual Example 13 (“Drawing Electric Field Lines”) for the dos and don’ts of mapping E field lines.
Electric Field Lines Mapping Rules
-The E lines do not cross each other
-The closer the lines are together, the stronger the E in that region
-The field lines indicate the direction of E; the field points in the direction tangent to the field line at any point
-The lines are drawn so that the magnitude of the electric field, |E|, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer
the lines, the stronger the field
-E lines start on + charges and end on – charges; the number starting or ending is proportional to the magnitude of the charge.
Week 4 Lab – Besides running the lab and filling in the lab answer sheet, I want you to answer the following questions as well and include your answers with your
submitted reports. Remember you need to submit BOTH the lab sheet and the lab report please. Also, the lab assignment info is referencing an older version of the
textbook. The updated information you need to support this lab is: Chapter 17 in version 7 of the book with sections 17-1 through 17-3 is where you need to look. You
have this charge coming into the plates space with a horizontal velocity v. Just like when you throw a rock horizontally out from a cliff, you see the only
acceleration is caused by gravity in the vertical direction. Here, the acceleration is cause by the electric field between the plates. The potential difference relates
the work the field does on the charge. The electric field relates the force on the charge, and the potential difference the work on the charge.
Question 1 for week 4 lab: Does the charge begin to deflect immediately when entering the parallel plates? Why or why not?
Question 2 for week 4 lab: Using F = qE, F = ma, y = 1/2at^2, and x= v(0) * t, prove, meaning show the work that, y = qEx^2/2mv(0)^2. v(0) is the initial velocity in
the x direction entering the plates. Does this formula look familiar? Does the path look familiar? It should. You saw it back in physics part one when you studied 2D
kinematics motion of an object under free fall in 2D. It is parabolic path. You saw this when you studied projectile motion. Funny how motion under gravity and
electromagnetic forces produce similar phenomenon.
