Physics 1161 Lecture 2 Vectors & Electric Fields

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!)

Three Charges Q=-3.5  C Q=+7.0  C Q=+2.0  C 6 m 4 m Resolve each force into x and y components Add the x-components & the y-comp. Use Pyth. Theorem & Trigonometry to express in R,θ notation 53 o

Three Charges Use Pyth. Theorem & Trigonometry to express in R,θ notation φ Since resultant is in first quadrant, θ = φ

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 Toward the left e-

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

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 Toward the right 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 + +

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 + +

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 + 30% 0% 30% 40%

Preflight 2.3 What is the direction of the electric field at point B? 1)Left 2)Right 3) Zero x y A B + 70% 30%

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

Electric Field Applet 1/EField/EField.html 1/EField/EField.html

AB X Y Preflight 2.5 Charge A is 1) positive2) negative3) unknown Field lines start on positive charge, end on negative. 80%0%20%

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 # lines proportional to |Q| AB X Y 20%30% 40%

AB X Y Preflight 2.8 The magnitude of the electric field at point X is greater than at point Y 1) True2) False Density of field lines gives E The electric field is stronger when the lines are located closer to one another. 10%90%

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 18 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 19 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!)

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 40%10%50%

Physics 1161: Lecture 2, Slide 21 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