Electric Force Fields CA Content Standards: 5d Charged particles are the sources of electric fields and experience forces due to the electric fields from other charges. 5k* The force on a charged particle in an electric field is qE where E is the electric field at the position of the particle and q is the charge of the particle. 5l* How to calculate the electric field due to a point charge and recognize that static electric fields have as their source some arrangement of electrical charges.

The Action at a Distance Problem An electric charge changes the space around it such that: Other electrical charges in the space feel a force they did not feel prior to the placement of the charge (ELECTRICAL FORCE FIELD). Other electrical charges in the space have a potential energy they did not have prior to the placement of the charge (ELECTRICAL ENERGY FIELD). Today we discuss the Electrical Force Field Question: In what two ways does an electric charge change the space around it? What do we call these changes? Which change are we describing today?

Electrical Force Field Variables Electric Force (Fe) Electrical Field (E) Measurement of force on positively charged test particle Vector quantity w/ direction Units of Newtons Fe = qE Measurement of force/charge on positively charged test particle Vector quantity w/ direction Units of Newtons/Coulomb E=Fe/q Question: What two variables are used to measure the electric force field? Are these variables vectors or scalars? How is the direction of the variables determined? How do the two variables differ? Write the equation that relates Fe to E. F is simply the force and depends on the test charge size. E is electric field strength and depends only on the source charge size and the distance from the charge. This is same relationship as there was between gravitational force and the gravitational field strength variable (g). Units may help to make distinction between variables

Electrical Fields around Point Charges Strongest near charge Direction is away from positive charges and towards negative charges Lines can’t cross FE =SKc Q sourceQ test /d2 E= SKc Q source /d2 Question: Draw the electric fields for the following point charges: a) single positive charge b) single negative charge c) opposite point charges d) like point charges. For each indicate where the force field is the strongest. Activities: Short discussion on how representation for force fields must include direction and magnitude. Former is accomplished with arrow, later with spacing between lines. Draw fields around different charge arrangements. Discuss where field is greatest. For double charge situations put test charges at different places and ask for direction of force on charge. Show equations for each case for physics E.

Opposite Plate Charges FE and E remain constant between plates. Constant force results in charges moving w/ constant acceleration between plates. Fe = constant E = constant Question: Draw the electric field between two oppositely charged plates. What is true of Fe and E between the plates? Describe the motion of a positive test charge as it moves between the plates. Activities: Draw force line diagram for opposite plate charges. Discuss how gain of force from one plate makes up for loss of force from other plate as move from one side to another. This results in both the force and the force field strength being constant between the plates. Say that F is 2 N on a 1 C charge somewhere between the plates and then ask what is force if charge is somewhere else between plates. Discuss how charge between plates feels constant force which does NOT mean constant velocity. Rather the charge will have a constant acceleration like masses did when they were in free fall. So charge will speed up at constant rate as it approaches opposite plate. Write equations for this situation.

Electric Shielding Electric force and field inside a conductor is zero To shield objects from electric fields simply surround with a conductor Faraday Cages Question: What is the electric field inside of a conductor? How do you shield something from electric fields? What is a Faraday Cage and how does it protect the occupant? Activities: Show picture of threads aligning with electric fields in point and plate charge situations and then show shielding picture. Briefly discuss how if there were an inbalance in force the charges would move around until the imbalance went away. Radio demo Discuss electric field shielding in coax cable and cars in a thunder storm. Talk about how power supplies are often shielded. Discuss car commercial where lightning strikes car. Show Faraday cage picture. Finish with Dr. Electro video Show