1. January 30, 2010 Electric Force Review and then an Introduction to Electric Fields

2. Static Electricity Experiment Cut 2 20-cm strips of transparent tape (mass of each 65 mg). Fold about 1 cm tape over at one end of each strip to create a handle. Press both pieces of tape side-by-side on your lab table and rub your finger back and forth across the strips. Quickly pull the strips off the lab table. Hold the handles together and the strips will repel each other, forming an inverted “V”. Estimate the charge on each strip using a protractor. Assume the charges act as though they are at the center of mass of the strip. Hint: Begin by drawing a Free Body Diagram!

3. Two identical balls of mass m and charge q are hanging from strings of length L. Derive an expression for q in terms of m, , L, and fundamental constants. L  L m,q

4. The Electric Field A charge distribution changes the empty space around it such that this empty space will affect other charges brought nearby. We say that a charge distribution creates an electric field in the empty space surrounding it.

5. Electric Field This equation can be used to calculate the electric field a distance r away from a the center of a spherically symmetric charge distribution of q o Coulombs. Another charge q entering the electric field created by q o will experience a force F, which can be calculated by the equation F = qE.

6. Electric Field Direction The direction of the electric field at a point in space is the direction that a small positive charge at rest (a “test charge”) wishes to move if it is placed at that location. Thus, the electric field points away from positive charges and toward negative charges.

7. Electric Field Direction The electric field around point charges is spherically symmetric. When multiple point charges are present, the fields due to the individual charges “superimpose” to create a more complex picture. http://www.edumedia-sciences.com/m195_l2- electric-field.htmlhttp://www.edumedia-sciences.com/m195_l2- electric-field.html http://www.falstad.com/emstatic/ http://www.cco.caltech.edu/~phys1/java/phys1/EF ield/EField.htmlhttp://www.cco.caltech.edu/~phys1/java/phys1/EF ield/EField.html

8. Caution… Electric field lines are NOT VECTORS, but may be used to derive the direction of electric field vectors at given points. Electric field vectors are always tangent to the electric field line at any given point in space.

9. -q 1 q2q2 P Superposition problems involve vector addition. E2E2 E1E1 E