Coulomb s Law and Electric Fields

Transcription

1 Physics 102: Lecture 02 Coulomb s Law and Electric Fields Today we will get some practice using Coulomb s Law learn the concept of an Electric Field Physics 102: Lecture 2, Slide 1

2 Recall Coulomb s Law Magnitude of the force between charges q 1 and q 2 separated a distance r: Force is F = k q 1 q 2 /r 2 k = 9x10 9 Nm 2 /C 2 attractive if q 1 and q 2 have opposite sign repulsive if q 1 and q 2 have same sign Units: q s have units of Coulombs (C) charge on proton is 1.6 x C r has units of m F has units of N Physics 102: Lecture 2, Slide 2

3 Three Charges Calculate force on +2μC charge due to other two charges Draw forces Calculate force from +7μC charge Calculate force from 7μC charge Add (VECTORS!) Q=+7.0μC Q=+2.0μC 4 m 6 m F 7 Q=-7.0 μc Physics 102: Lecture 2, Slide 3

4 Physics 102: Lecture 2, Slide 4

5 Three Charges Calculate force on +2μC charge due to other two charges Calculate force from +7μC charge Calculate force from 7μC charge Add (VECTORS!) Q=+2.0μC F +7 4 m Q=+7.0μC 6 m Q=-7.0 μc Physics 102: Lecture 2, Slide 5

6 Three Charges F F F Calculate force on +2μC charge due to other two charges Calculate force from +7μC charge Calculate force from 7μC charge Add (VECTORS!) F = k q 1 q 2 /r (9 10 )(2 10 )(7 10 ) = N 25 3 = 5 10 N (9 10 )(2 10 )(7 10 ) = N 25 3 F N = Q=+7.0μC Q=+2.0μC 5 m 4 m 6 m F +7 F -7 Q=-7.0 μc Physics 102: Lecture 2, Slide 6

7 Adding Vectors F +7 +F -7 Q=+2.0μC F +7 5 m 4 m F -7 Q=+7.0μC 6 m Q=-7.0 μc Physics 102: Lecture 2, Slide 7

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9 Adding Vectors F +7 +F -7 y components cancel x components: F = F +7,x + F -7,x Q=+2.0μC F +7 F F +7,x = (3/5)F +7 F -7,x = (3/5)F -7 5 m 4 m F -7 F = (3/5)(5+5)x10-3 N=6 x 10-3 N Q=+7.0μC 6 m Q=-7.0 μc Physics 102: Lecture 2, Slide 9

10 Electric Field Charged particles create electric fields. Direction is the same as for the force that a + charge would feel at that location. Magnitude given by: E F/q = kq/r 2 Q p =1.6x10-19 C + r = 1x10-10 m E E = ( )( )/(10-10 ) 2 N = N/C (to the right) Physics 102: Lecture 2, Slide 10

11 Preflight 2.3 What is the direction of the electric field at point B? 1) Left 2) Right 3) Zero A y B x Physics 102: Lecture 2, Slide 11

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13 ACT: E Field What is the direction of the electric field at point C? A. Left Red is negative B. Right Blue is positive C. Zero y C x Physics 102: Lecture 2, Slide 13

14 Comparison: Electric Force vs. Electric Field Electric Force (F) - the actual force felt by a charge at some location. Electric Field (E) - found for a location only tells what the electric force would be if a charge were located there: F = Eq Both are vectors, with magnitude and direction. Add x & y components. Direction determines sign. Physics 102: Lecture 2, Slide 14

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17 Preflight 2.2 What is the direction of the electric field at point A? 1) Up 2) Down 3) Left 4) Right 5) Zero Red is negative Blue is positive A y B x Physics 102: Lecture 2, Slide 17

18 ACT: E Field II What is the direction of the electric field at point A, if the two positive charges have equal magnitude? A. Up Red is negative B. Down Blue is positive C. Right D. Left E. Zero A y B x Physics 102: Lecture 2, Slide 18

19 Electric Field of a Point Charge N/C N/C N/C N/C E This is becoming a mess!!! Physics 102: Lecture 2, Slide 19

20 Physics 102: Lecture 2, Slide 20

21 Closeness of lines shows field strength - lines never cross # lines at surface Q Arrow gives direction of E - Start on +, end on - Electric Field Lines Physics 102: Lecture 2, Slide 21

22 Preflight 2.5 X A Y B Charge A is Field lines start on positive charge, end on negative. 1) positive 2) negative 3) unknown Physics 102: Lecture 2, Slide 22

23 Preflight 2.6 X A Y B Compare the ratio of charges Q A / Q B # lines proportional to Q 1) Q A = 0.5Q B 2) Q A = Q B 3) Q A = 2 Q B Physics 102: Lecture 2, Slide 23

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25 Preflight 2.8 X A Y B The electric field is stronger when the lines are located closer to one another. The magnitude of the electric field at point X is greater than at point Y 1) True 2) False Density of field lines gives E Physics 102: Lecture 2, Slide 25

26 ACT: E Field Lines B A Compare the magnitude of the electric field at point A and B 1) E A >E B 2) E A =E B 3) E A <E B Physics 102: Lecture 2, Slide 26

27 E inside of conductor Conductor electrons free to move Electrons feels electric force - will move until they feel no more force (F=0) F=Eq: if F=0 then E=0 E=0 inside a conductor (Always!) Physics 102: Lecture 2, Slide 27

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29 Preflight 2.10 X A Y B "Charge A" is actually a small, charged metal ball (a conductor). The magnitude of the electric field inside the ball is: (1) Negative (2) Zero (3) Positive Physics 102: Lecture 2, Slide 29

30 Demo: electric field lines Physics 102: Lecture 2, Slide 30

31 Recap E Field has magnitude and direction: E F/q Calculate just like Coulomb s law Careful when adding vectors Electric Field Lines Density gives strength (# proportional to charge.) Arrow gives direction (Start + end on -) Conductors Electrons free to move E=0 Physics 102: Lecture 2, Slide 31

32 To Do Do your preflight by 6:00 AM. Physics 102: Lecture 2, Slide 32