1.ElectrostaticsElectric Field

The Electric Field is defined as the force on a test charge (small positive charge), divided by the charge:

qFE

E E

F FqEF qEF

Electric field of a point charge

+Q

r

q

qFE

qrQqk 2

2rQkE

2rQqkF

An electric field is defined as being present in any region where a charged object experiences an electric force. 

Web Links: Releasing a test charge

Notes on E-field

1) The E-field points in the direction of force on a positive test charge2) 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 q0 placed there:

E F q0

F = q0E

4. A proton is released in a uniform electric field, and it experiences an electric force of 3.75 × 1014 N toward the south. What are the magnitude and direction of the electric field?

E

+ southF

qFE N/C 10x 2.34

C 10x 1.602N 10x 3.75 5

91

14

5. What are the magnitude and direction of the electric field at a point midway between a +7.0 mC and a 8.0 mC charge 8.0 cm apart? Assume no other charges are nearby.

+ -

d

q1 q2

E1

E2

21 EEE

22

21

2d

kq

2d

kqE

212 qqd

k4E

C x108.0C x107.0

m 08.0m/CN x108.9884 66

2

9

N/C x10 8.4E 7

[Extra] Four charges are placed in a square. There is a - 20.3 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 - 20.3 micro-coulomb charged placed at the ( 40.6 cm, 40.6 cm) mark, and a 10.15 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

B C

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…

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

Electric Fields and ConductorsThe electric field is perpendicular to the surface of a conductorThe 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

http://ap-physics.david-s.org