© Vasily Smirnov/shutterstock (jumper)


Both of these photos show objects accelerating in fields.  The BASE jumper is accelerating to Earth due to the force of gravity acting through Earth's gravitational field.  The charged soap bubbles are accelerating away from the charged globe of the Van de Graaff generator due to the electrostatic force acting through the electric field of the Van de Graaff generator.  In both cases, potential energy stored in the system is being transformed into kinetic energy.

 


Read Unit Introduction
Read pages 560 to 562 and pay special attention to "Example 11.5" and to the connections to the previous lab activity.


Self-Check

Answer the following self-check (SC) questions then click the "Check your work" bar to assess your responses.

 

SC 7.

  1. Define electric potential energy .

  2. Solve "Practice Problem" 2 on page 561 of your textbook.

  3. Explain why it is important to indicate the reference position when stating a value for potential energy.

SC 8.  

When it comes to calculations, the textbook uses a particular method for defining the reference point for electric potential energy. Describe this method, and relate it to your experiences with the simulation in the previous lab activity.

 

   Self-Check Answer

SC 7.

  1. Electric potential energy is the energy stored in a system when work is done to move a charge from one location to another.

Given

 

Required

The work necessary to move the charge.

 

Analysis and Solution

 

Paraphrase

The work to move the charge is 1.60×10 −19  J.


  1. Potential energy is equal to the work done to change the position of an object. This only makes sense if you know the reference level where potential energy is defined to be zero. The reference level is the starting point for the displacement vector in the work calculation.

SC 8.  

The textbook uses the method where electric potential energy is defined to be zero at infinity. In other words, infinity is the reference level. When the simulation was allowed to run for nearly 20 minutes, the potential energy of the system dropped to a value of 1 J when the distance from the source was very large. It appears that the simulation uses the same method of defining the electric potential energy to be zero at infinity.