Electric Fields. Lyzinski Physics

Transcription

1 lectric Fiels Lyzinski Physics

2 Objects near the earth are attracte to the earth (an thus experience a force irecte towars the earth) because they lie within the earth s GRAVITATIOAL FILD. As you move away from the earth, the attractive forces become less because the fiel s strength ecreases. All objects (of equal mass) that lie on one of the concentric circles at the left experience the same force!

3 lectric Fiels Like the earth s gravitational fiel, every charge object creates an LTRI FILD aroun it. This is very similar to gravitational fiels aroun masses (such as the gravitational fiel of the earth). As you move away from a charge object, the strength of the electric fiel is reuce. Unlike the earth s gravitational fiel (which is ALWAYS attractive), electric fiels can be either attractive or repulsive. vs. q +q

4 lectric Fiels To unerstan electric fiels, we will place a VRY SMALL, positive test charge in the electric fiels, an see how it is affecte. lectric fiel line lectric fiel force Positive Point harge MOR Positive Point harge The more fiel lines, the stronger the electric fiel. Also, the closer the fiel lines, the stronger the fiel at this location.

5 Why oes the test charge nee to be positive? It oesn t have to be positive. However, if you use a negative test charge every electric fiel that we raw will have to be change Why oes the test charge nee to be very small? Since it is a charge object, it will create its own electric fiel. If its fiel is too big, it will HAG the electric fiel that it is trying to test

6 Some Important lectric Fiels + Small, POSITIV Test harge

7 More Important lectric Fiels Small, POSITIV Test harge

8 lectric Fiels (continue) Rules for lectric Fiel Lines Begin on + charges or at infinity. n on charges or at infinity. The number of lines rawn is proportional to the magnitue of the charge. Fiel lines VR cross each other (WHY???). lectric ipole

9 The Similarities continue Fiel Strength Gravitational Fiel F g g units g GmM R of F g m GM R kg m ( or s ) lectrical Fiel F e units kqq F q e kq of LTRI Fiel Strength

10 lectric Fiels (continue again) lectric Fiel Intensity, or lectric Fiel Strength () is how strong an electric fiel is. The closer the fiel lines, the stronger the intensity. F q e o k Qq q o o k Q (since is a vector, it requires a irection, like raially outwar, raially inwar, etc) APPLT ava/phys1/fiel/fiel.html

11 lectric Fiels (continue one last time) How woul you raw the exact electric fiel lines aroun two ore more charges. You can t just guess, right??? 1 q q This metho of combing vectors is calle VTOR SUPRPOSITIO!!!

12 Fin the T lectric Fiel strength (or intensity) at the lettere point below. #1: BASI lectric Fiel 3 m 8m F q o kqq q o o kq (8.999 m (3m) )(8 6) 7,991 A 7,991 [away from the charge in the irection of pt A] ee irection with. It s a VTOR!!!!

13 Fin the T lectric Fiel strength (or intensity) at the lettere point below. #: LIAR Variety 10 m 5 m 5m B from 5μμ charge kq kq [ ] [ ] m m (8.999 )(5 6) (8.999 )(5 7) [ ] [ ] (10m) (5m) [ ] from 57 charge [ ] 69.7 [ ] 5x 10 7 *** ote: otice that no signs were use in the electric fiel equation (the irections were accounte for with the vector arrows)

14 Fin the T lectric Fiel strength (or intensity) at the lettere point below. 3m 4 m 3m #3: D Variety 4 m kq (8.999 m (4m) )(3 6) cm 3m 4 m kq (8.999 m (8m) )(3 6) o kq (8.999 m (8.94m) )(3 6) o

15 ..continue / 41.4 / 107 / 3.57 o 41.1 / o / / 150. / 41.1 / [S 3.57 o ]

16 lectric Fiel quations F q e o k Q Use to fin the electric fiel at a point if you are given the force acting on the test charge. Use to fin the electric fiel at a point if you know how far the test charge is from the charge object.

17 The lectric Fiel Between Parallel Plates The electric fiel b/w two, oppositely charge parallel plates that are close together is UIFORM (constant) everywhere. The fiel lines are straight an equally space an perpenicular to the parallel plates. outsie the plates is zero (except for the slight bulge at the ens). epens only on the charge per unit area on each plate. oes OT epen on the istance b/w the plates (as long as the istance between the plates is VRY small compare to the surface area of the plates)!!!!

18 onuctors in lectrostatic quilibrium o net motion of charge occurs. What is the electric fiel on the insie of the conuctor? Where o all the excess electrons exist??? What is the irection of the fiel at the surface? Why?????

19 The n