Physics 1161 Lecture 2 Electric Fields

Physics 1161: Lecture 2, Slide 2 Three Charges Q=-3.5  C Q=+7.0  C Q=+2.0  C 6 m 4 m Calculate force on +2  C charge due to other two charges – Calculate force from +7  C charge – Calculate force from –3.5  C charge – Add (VECTORS!)

Physics 1161: Lecture 2, Slide 3 Electric Force on Electron by Proton What are the magnitude and direction of the force on the electron by the proton? + r = 1x m q=1.6x C e-

Physics 1161: Lecture 2, Slide 4 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

Physics 1161: Lecture 2, Slide 5 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 Field at A due to proton? + r = 1x m q=1.6x C A

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 5.Zero x y A B + +

Physics 1161: Lecture 2, Slide 7 Preflight 2.2 What is the direction of the electric field at point A? 1)Up 2)Down 3)Left 4)Right 5)Zero x y A B +

Physics 1161: Lecture 2, Slide 8 Preflight 2.3 What is the direction of the electric field at point B? 1)Left 2)Right 3) Zero x y A B +

What is the direction of the electric field at point C? 1.Left 2.Right 3.zero x y C + -

Physics 1161: Lecture 2, Slide 10 AB X Y Preflight 2.5 Charge A is 1) positive2) negative3) unknown

Physics 1161: Lecture 2, Slide 11 Preflight 2.6 Compare the ratio of charges Q A / Q B 1) Q A = 0.5Q B 2) Q A = Q B 3) Q A = 2 Q B AB X Y

Physics 1161: Lecture 2, Slide 12 AB X Y Preflight 2.8 The magnitude of the electric field at point X is greater than at point Y 1) True2) False

Compare the magnitude of the electric field at point A and B A B 1.E A > E B 2.E A = E B 3.E A < E B

Physics 1161: Lecture 2, Slide 14 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 1161: Lecture 2, Slide 15 E inside of conductor Conductor  electrons free to move – Electrons feel 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 1161: Lecture 2, Slide 16 AB X Y Preflight 2.10 "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 1161: Lecture 2, Slide 17 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 1161: Lecture 2, Slide 18 To Do ActivPhysics 11.1 – 11.3 – On the MasteringPhysics website Read sections 19-6 & 19-7 Do Prelecture 03 & Preflight 03. See you next Wednesday! Remember no class Fri or Mon!