1.Electrostatics Electric Field

The Electric Field is defined as the force on a test charge (small positive charge), divided by the charge: EE F F Electric field of a point charge +Q r

An electric field is defined as being present in any region where a charged object experiences an electric force. Web Links: Releasing a test chargeReleasing a test charge

Notes on E-field 1) The E-field points in the direction of force on a positive test charge 2) If a negative charge were placed in the E-field, what do you suppose would happen? 3) The E-field is a property of the fixed charges only (it is independent of the test charge) 4) E-fields add as vectors 5) Given the E-field value at a certain point, we can calculate the force F on any charge q 0 placed there: F = q 0 E

4.A proton is released in a uniform electric field, and it experiences an electric force of 3.75 × 10  14 N toward the south. What are the magnitude and direction of the electric field? E +southF

5.What are the magnitude and direction of the electric field at a point midway between a +7.0  C and a  8.0  C charge 8.0 cm apart? Assume no other charges are nearby. + - d q1q1 q2q2 E1E1 E2E2

[Extra] Four charges are placed in a square. There is a micro-coulomb charge placed at the (0 cm, 0 cm) mark, a 81.2 micro- coulomb charge placed at the (0 cm, 40.6 cm) mark, a micro-coulomb charged placed at the ( 40.6 cm, 40.6 cm) mark, and a micro-coulomb charge placed at the ( 40.6, 0cm) mark. Determine the strength of the electric field (to four significant digits) in the center of the square.

The field lines for a positive point charge. The lines flow out from the positive charge The field lines for a negative point charge. The lines flow in towards the charge The field lines for two positive point charges. The field lines flow away from both charges. The field lines for one positive point charge and one negative point charge. The field lines flow out of the positive charge and into the negative charge

The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge. The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge. E The electric field is stronger where the field lines are closer together. A BC E Electric field lines never cross. Electric field lines are drawn perpendicular to a surface outside of a conductor. Electric Field Applet.

To double the force on a given particle…

Electric field of two charges: The electric field between two closely spaced, oppositely charged parallel plates is constant.

Field lines indicate the direction of the field; the field is tangent to the line. The magnitude of the field is proportional to the density of the lines. Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge. Summary of field lines:

Electric Fields and Conductors

The electric field is perpendicular to the surface of a conductor The electric field is zero inside a conductor (hollow or solid) E=0  Any net charge on a good conductor is distributed equally on the surface.

Charged Conductors Any excess charge ends up on the surface of a conductor, independent of its shape Why do you think this happens? © Laura Fellman

What happens to a neutral conductor placed in an external electric field? “Shielding” E = 0 At equilibrium, the Electric Field at any point within a conducting material is zero. © Laura Fellman