ELECTRIC POTENTIAL

 

 

 

How does ENERGY factor into the concept of the Electric Field?

    • In a gravitational field, work done by the force field creates a change in kinetic energy. Objects fall from high gravitational potential to low gravitational potential.

    • In a electric field, it is the work done to move a positive or negative charge of magnitude "q" against the electric field.

How is ENERGY related to the FORCE created by an electric field?

    • Change in potential energy (DU ) is the negative of the work done by the field force ( E).
    • For Gravity Fields, the motion of a small mass responding to the pull of gravity of a larger mass shows us the field lines of gravity.
    • For Electric Fields, the motion of a proton q (positive charge) pushed or pulled by a Coulomb Force of a much larger charge Q shows us the field lines for electric force.
    • There is a gain in potential if the work done by the field is negative.
    • There is a loss in potential if the work done by the field is positive.
    • Field lines always point from high potential to low potential.
    • In Potential Energy, two objects are involved-- the object creating the field and the object that responds to the field.

 

ELECTRIC POTENTIAL :    Electric potential is the change in potential energy per unit test charge. This is nothing that is very new. For Electric Fields, E is the force per unit test charge. Doing so focuses on the object that creates the field. This object clearly is a dominant feature in governing the behavior of lesser charged objects that come under the influence of the field that is created. The symbol for electric potential is V.

D V = D U/ q or DU = q D V ... The UNIT for V is the Volt. One volt = 1 Joule/ Coulomb

  

 

 

 

 

 

GRAPHIC RELATIONSHIPS--ELECTRIC FIELD LINES AND EQUIPOTENTIAL LINES :

  1. MAPPING THE E FIELD FROM EQUIPOTENTIALS: Pick a point in the region of the electric field lines and move so that your pencil draws a line that is always perpendicular to the electric field. See figure 1 below. Don't forget your arrows for your field lines. They point from high to low potential!

  2. SKETCHING EQUIPOTENTIAL LINES FROM ELECTRIC FIELD LINES:

  • Pick a point and and trace a line that crosses from one electric field line to the nest at right angles. See figure 2.

  • Repeat, starting with another point. Keep the path as smooth as possible and do not pick up your pencil at any time. Loops are not allowed!

  • To map a higher potential move closer to the positive charge and repeat the process.

  3.  ESTIMATING THE MAGNITUDE OF THE ELECTRIC FIELD: Dividimg potential changes (in VOLTS)         at any point by the spacing (in meters) between equipotential lines gives the size of the electric         field E at  that position (in Volts/meter ).

 
 
Figure 1 figure 2
 figure 3

 

 

       

POINTS TO REMEMBER ABOUT ELECTRIC POTENTIAL

  • No work is required to move a charge between points of the same potential. Simply stated, W = 0 when VA = V B.

  • The electric potential is constant everywhere in a conductor and equal to the potential at the surface.

  • The electron volt is defined as the energy that an electron (or proton) gains when accelerated through a potential of one Volt)       

  • 1 eV = 1.6 x 10 -19 Joules.

  • Just as F = qE, Work = q DV, so F & E are vectors but Work and Potential (V) are scalars!

  • When a problem asks for potential solve for V (Volts)! When a problem asks for potential energy or work, solve for DU or W (Joules).