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Transcript
Physics 102: Lecture 02 Coulomb’s Law and Electric Fields • Today’s lecture will cover Textbook Sections 16.3-6 Physics 102: Lecture 2, Slide 1 Recall Coulomb’s Law • Magnitude of the force between charges q1 and q2 separated a distance r: F = k q1q2/r2 k = 9x109 Nm2/C2 • Force on nucleus of Hydrogen from eQp=1.6x10-19 C + F Qe = -1.6x10-19 C - r = 1x10-10 m F = (9x109)(1.6x10-19)(1.6x10-19)/(10-10)2 N = 2.3x10-8 N (to the right) Physics 102: Lecture 2, Slide 2 Three Charges • Calculate force on +2mC charge due to other two charges Q=+2.0mC 4m – Calculate force from +7mC charge – Calculate force from –3.5mC charge – Add (VECTORS!) Q=+7.0mC Physics 102: Lecture 2, Slide 3 6m Q=-3.5 mC 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/r2 q=1.6x10-19 C + r = 1x10-10 m E= Direction? Physics 102: Lecture 2, Slide 4 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 5 ACT: E Field What is the direction of the electric field at point C? 1) Left 2) Right 3) Zero y C x Physics 102: Lecture 2, Slide 6 ACT: E Field What is the direction of the electric field at point C? 1) Left Red is negative 2) Right Blue is positive 3) Zero Away from positive charge (right) Towards negative charge (right) Net E field is to right. y C x Physics 102: Lecture 2, Slide 7 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 = qE • Both are vectors, with magnitude and direction. Add x & y components. Physics 102: Lecture 2, Slide 8 Preflight 2.2 What is the direction of the electric field at point A? 1) Up 2) Down 3) Left 4) Right 5) Zero A y B x Physics 102: Lecture 2, Slide 9 Preflight 2.2 What is the direction of the electric field at point A? 1) Up Red is negative 2) Down Blue is positive 3) Left 4) Right 5) Zero A y B x Physics 102: Lecture 2, Slide 10 ACT: E Field II What is the direction of the electric field at point A, if the two positive charges have equal magnitude? 1) Up 2) Down 3) Right 4) Left A y 5) Zero B x Physics 102: Lecture 2, Slide 11 ACT: E Field II What is the direction of the electric field at point A, if the two positive charges have equal magnitude? 1) Up Red is negative 2) Down Blue is positive 3) Right 4) Left 5) Zero A y B x Physics 102: Lecture 2, Slide 12 Preflight 2.5 X A Charge A is Y B Field lines start on positive charge, end on negative. 1) positive Physics 102: Lecture 2, Slide 13 2) negative 3) unknown Preflight 2.5 X A Charge A is Y B Field lines start on positive charge, end on negative. 1) positive Physics 102: Lecture 2, Slide 14 2) negative 3) unknown Preflight 2.6 X A Y B Compare the ratio of charges QA/ QB # lines proportional to |Q| 1) QA= 0.5QB Physics 102: Lecture 2, Slide 15 2) QA= QB 3) QA= 2 QB Preflight 2.6 X A Y B Compare the ratio of charges QA/ QB # lines proportional to Q 1) |QA|= |0.5QB| Physics 102: Lecture 2, Slide 16 2) |QA| = |QB| 3) |QA| = |2 QB| Preflight 2.8 X A Y B The magnitude of the electric field at point X is greater than at point Y 1) True Physics 102: Lecture 2, Slide 17 2) False 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 Physics 102: Lecture 2, Slide 18 2) False Density of field lines gives E ACT: E Field Lines B A Compare the magnitude of the electric field at point A and B 1) EA>EB Physics 102: Lecture 2, Slide 19 2) EA=EB 3) EA<EB ACT: E Field Lines B A Compare the magnitude of the electric field at point A and B 1) EA>EB Physics 102: Lecture 2, Slide 20 2) EA=EB 3) EA<EB E inside of conductor • Conductor electrons free to move – Electrons feels electric force - will move until they feel no more force (F=0) – F=qE: if F=0 then E=0 • E=0 inside a conductor (Always!) Physics 102: Lecture 2, Slide 21 E inside of conductor • Conductor electrons free to move – Electrons feels electric force - will move until they feel no more force (F=0) – F=qE: if F=0 then E=0 • E=0 inside a conductor (Always!) Physics 102: Lecture 2, Slide 22 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 Physics 102: Lecture 2, Slide 23 (2) Zero (3) Positive 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 Physics 102: Lecture 2, Slide 24 (2) Zero (3) Positive Recap • E Field has magnitude and direction: – EF/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 25 To Do • Read Sections 17.1-17.3 • Extra problems from book Ch 16: – Concepts 12-18 – Problems 11, 13, 25, 31 • Do your preflight. See you next time! Physics 102: Lecture 2, Slide 26