# Electric Potential and Voltage

### Objectives

1. Describe the concept of electric potential energy in terms of the energy expended in moving a charged particle within a surrounding (external) electric field.

2. Define the term “electric potential difference” or “voltage” between two points in an electric field, and its corresponding unit of measure, the “volt”.

3. Define an equipotential surface.

4. Recognize the electric potential energy as energy stored in the electric field produced by charges.

5. Calculate the electric potential difference between any two points in the vicinity of a group of point charges or two points within a uniform electric field

 This section addresses, in whole or in part, the following Georgia GPS standard(s): S8P2. Students will be familiar with the forms and transformations of energy. c. Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, sound) and their characteristics. S8P5. Students will recognize characteristics of gravity, electricity, and magnetism as major kinds of forces acting in nature. b. Demonstrate the advantages and disadvantages of series and parallel circuits and how they transfer energy. c. Investigate and explain that electric currents and magnets can exert force on each other.

 This section addresses, in whole or in part, the following Benchmarks for Science Literacy: Magnetic forces are very closely related to electric forces and can be thought of as different aspects of a single electromagnetic force. Moving electric charges produce magnetic forces and moving magnets produce electric forces. The interplay of electric and magnetic forces is the basis for electric motors, generators, and many other modern technologies, including the production of electromagnetic waves. Electrical energy can be produced from a variety of energy sources and can be transformed into almost any other form of energy. Moreover, electricity is used to distribute energy quickly and conveniently to distant locations.

 This section addresses, in whole or in part, the following National Science Education Standards: Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways. Electrical circuits provide a means of transferring electrical energy when heat, light, sound, and chemical changes are produced. In most chemical and nuclear reactions, energy is transferred into or out of a system. Heat, light, mechanical motion, or electricity might all be involved in such transfers. In some materials, such as metals, electrons flow easily, whereas in insulating materials such as glass they can hardly flow at all. Semiconducting materials have intermediate behavior. At low temperatures some materials become superconductors and offer no resistance to the flow of electrons. All energy can be considered to be either kinetic energy, which is the energy of motion; potential energy, which depends on relative position; or energy contained by a field, such as electromagnetic waves.

Electric Potential and Voltage

An electric charge will experience an attractive or repulsive electric force inside an electric field of a charged object.

A positively charged particle will tend to move away from a positively charged sphere and accelerate from the sphere due to the repulsive force between like charges. Therefore, work must be done to push or move this positively charged particle, from point to point in the electric field, towards a positively charged sphere.

Also, separating two unlike charges will require work since unlike charges attract.

Work done to overcome the repulsive or attractive electric force on the charged particle is similar to the mechanical work done in compressing or stretching a spring.

The energy expended to perform work (i.e. moving the charge in a direction opposite the electric force on the charge) increases the electric potential energy of the charged particle. When the charge is released, this potential energy is converted to kinetic energy when the charge moves and accelerates in the direction of the electric force that acts on it.

After reading this content summary, review the relevant chapter and the relevant sections (Chapter 9) in your textbook to learn more about the highlighted topics (in bold face, underline, or italics). Click on the hyperlinked words to review the related information online. The learner should answer the Review Questions for each section after covering the relevant material. Be sure to complete all online practice exercises and activities to check your understanding before proceeding to the next topic.

Vocabulary

electric potential
voltage
volts (V)
q

Electric Potential

The electric potential energy changes when a positively charged particle is moved from point to point inside the electric field of a positively charged sphere.

Pushing the charged particle toward the sphere will require increasing the energy needed to overcome the increasing repulsion. So, the electric potential energy increases.

The electric potential (V) is defined as the electric potential energy (U) per charge (q). Thus:

Increasing the amount of charge moved results in increased electric potential energy needed to move the larger charge. But the ratio of the potential energy per charge (i.e., electric potential) remains the same.

The electric potential difference () between the initial and final positions of the charged particle inside the electric field is defined as the electric potential energy difference per charge required to move charges between the initial and final positions.

This electric potential difference between two points is the energy transfer per coulomb of charge in moving the charge, or the work that the charge can do because of its position in the electric field.

This electric potential difference is also often called the voltage, measured in unit of volts (V).

1 volt (V) = 1 joule (J) of energy transferred per 1 coulomb (C) of charge.

This difference in electric potential (i.e., voltage) between two points along a conductor provides the force that moves charge through the conductor.

Points closer to a positively charged sphere have a higher electric potential than points farther away. This is because the increased amount of energy expended (as work) to move a (positive) test charge toward the positively-charged sphere is stored as the electric potential energy. Thus, when released, positive charges move freely from points of higher potential to points of lower potential.

Work is required to move electrons away from the positively charged sphere. When released, these electrons freely move from a point at lower potential (farther away from the sphere) toward a point at a higher potential (nearer the positive sphere).

Two points at the same electric potential have zero voltage. Therefore, no force is required (and no energy expended) to move charges between those points.

Points of equal electric potential are called equipotential points.

Review Questions

1.      What is the difference between electric potential and electric potential energy?

2.      How do electric positive and negative charges move between two points of different electric potentials?

3.      What is the electric potential difference (or voltage) between two points if 12.0 joules of energy are required to move 1.0 coulomb of charge between those two points?

4.      How much energy is needed to move electric charges between two points along an equipotential surface?

Activity/Assignments

a.      Click on this simulation: Electric Potential due to point charges and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

Content provided by Mr. Martin O. Okafor, Georgia Perimeter College

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 9, 2007
Modified May 18, 2007