Electricity & Magnetism Lecture 3: Electric Flux and Field Lines Today’s Concepts: A) Electric Flux Gauss’ Law B) Field Lines Electricity & Magne/sm Lecture 3, Slide 1 Your Comments What the heck is epsilon 0? IT’S JUST A CONSTANT k = 9 x 109 N m2 / C2 −12 2 2 εο = 8.85 x 10 C / N·m I don't understand electric flux, how it's derived and the formula. I also need someone to explain εo or ε not. Cause I don't know what that is. “Calculang Electric Field from Arc of Charge!” [hint] “Why is gauss' law so important? Why is flux a useful value?” Electricity & Magne/sm Lecture 3, Slide 2 more comments I find the idea of represenCng electric field as a finite number of lines N a bit absurd. man Coulomb was a lazy Frenchman, he puts me to sleep. Tesla, Weber, Faraday, those guys would help me build a flux capacitor and go back to the 60s to party with Jerry Garcia... im having troubles in wri[en homeworks , and integrals.. and can you please explain more about flux and gauss' law please~! To be honest, the prelecture and homework are talking 2 different things A big comment... I'd to see us go over or just even have a glimpse of some exam-calibur quesCons both mulCple choice and wri[en to get a feel of what midterms are like. As well as if Neil's marking is like how a TA's (especially for wri[en quesCon or will you grade harder? like an iron-fisted physic professor? We’ll skip checkpoint quesCons most ppl get right. Many examples will have symbols, not numbers. Read acCvity guide before class—know your mission. Get busy right when you come in. My Comments ! You will need to understand integrals in this course!!! ! Forces and Fields are Vectors ! Always Draw a Picture First. What do the Forces/Fields Look Like? Prelecture AcCvity Guide Exam? E E E Infinite line of charge Finite line of charge Finite line of charge (non-constant λ) (constant λ) Homework WORKS FOR ALL! E Arc of charge (constant λ) Electricity & Magne/sm Lecture 3, Slide 3 Electric Field Lines DirecCon & Density of Lines represent DirecCon & Magnitude of E Point Charge: DirecCon is radial Density ∝ 1/R2 Electricity & Magne/sm Lecture 3, Slide 4 Electric Field Lines Dipole Charge DistribuCon: DirecCon & Density much more interesng. Electricity & Magne/sm Lecture 3, Slide 5 CheckPoint: Field Lines - Two Point Charges 1 Compare the magnitudes of the two charges o |Q1| > |Q2| o |Q1| = |Q2| o |Q1| < |Q2| o There is not enough informaon to determine the relave magnitudes of the charges. Electricity & Magne/sm Lecture 3, Slide 6 CheckPoint: Field Lines - Two Point Charges 2 What do we know about the signs of the charges from looking at the picture? o Q1 and Q2 have the same sign o Q1 and Q2 have opposite signs o There is not enough informaon in the picture to determine the relave signs of the charges Electricity & Magne/sm Lecture 3, Slide 7 CheckPoint: Field Lines - Two Point Charges 3 Compare the magnitudes of the electric fields at points A and B. o |EA| > |EB| o |EA| = |EB| o |EA| < |EB| o There is not enough informaon to determine the relave magnitudes of the fields at A and B. Electricity & Magne/sm Lecture 3, Slide 8 Point Charges “Telling the difference between posiCve and negave charges while looking at field lines. Does field line density from a certain charge give informaon about the sign of the charge?” −q +2q What charges are inside the red circle? −-QQ Q 2Q −-Q −-2Q −-QQ +Q +Q +2Q +Q A B C D E Electricity & Magne/sm Lecture 3, Slide 9 Clicker Question: Same Sign, Diff. Magnitudes Which of the following field line pictures best represents the electric field from two charges that have the same sign but different magnitudes? A B Equipotential lines not Field LInes C D Electricity & Magne/sm Lecture 3, Slide 10 Electric Flux “Counts Field Lines” “I’m very confused by the general concepts of flux through surface areas. please help” Φ = E~ dA~ S · ZS Flux through Integral of E~ dA~ surface S on surface S · Electricity & Magne/sm Lecture 3, Slide 11 CheckPoint: Flux from Uniformly Charged Rod An infinitely long charged rod has uniform charge density of λ, and passes through a cylinder (gray). The cylinder in case 2 has twice the radius and half the length compared to the cylinder in case 1. Compare the magnitude of the TAKE s TO BE RADIUS ! flux through the surface of the cylinder in both cases. A. Φ1 = 2 Φ2 B. Φ1 = Φ2 C. Φ1 = 1/2 Φ2 D. None of these Electricity & Magne/sm Lecture 3, Slide 12 CheckPoint Results: Flux Unif. Charged Rod Compare the magnitude of the flux through the surface of the cylinder in both cases. A.Φ1 = 2 Φ2 B. Φ1 = Φ2 C. Φ1 = 1/2 Φ2 TAKE s TO BE RADIUS ! D. None of these The first cylinder encloses twice the amount of charge as the second flux doesn’t depend on length or radius because case 1 have more surface area that is parallel to the electric field line than case 2 so case 2 is half case 1 Electricity & Magne/sm Lecture 3, Slide 13 CheckPoint Results: Flux Unif. Charged Rod “The flux is proporConal to the Area that the field is passing through. A.Φ1 = 2 Φ2 Although the radius is twice as long in B. Φ = Φ the second case, it's length is half as 1 2 long. Both cases have the same surface C. Φ1 = 1/2 Φ2 area that the field passes through, so D. None of these the fluxes are equal. ” Φ = E~ dA~ S · ZS E constant on barrel of cylinder TAKE s TO BE RADIUS ! E perpendicular to barrel surface (E parallel to dA) RESULT: GAUSS’ LAW Case 1 Case 2 Φ proporConal to charge enclosed ! Electricity & Magne/sm Lecture 3, Slide 14 Direction Matters: For a closed surface, points outward Q Φ = E~ A~ = enclosed S · ✏ I 0 Electricity & Magne/sm Lecture 3, Slide 15 Direction Matters: For a closed surface, points outward Φ = E dA < 0 S · IS Electricity & Magne/sm Lecture 3, Slide 16 Clicker Question: Trapezoid in Constant Field y Label faces: 1: x = 0 4 3 2: z = +a 3: x = +a 1 2 4: slanted x Define Φn = Flux through Face n dA z E A) Φ1 < 0 A) Φ < 0 A) 2 Φ3 < 0 A) Φ4 < 0 B) Φ1 = 0 B) Φ = 0 Φ = 0 2 B) 3 B) Φ4 = 0 C) Φ1 > 0 C) Φ > 0 C) 2 Φ3 > 0 C) Φ4 > 0 Electricity & Magne/sm Lecture 3, Slide 17 Clicker Question: Trap. in Constant Field + Q y Label faces: 1: x = 0 4 3 2: z = +a x a 1 3: = + +Q 2 4: slanted x Define Φn = Flux through Face n Φ = Flux through Trapezoid Add a charge +Q at (−a, a/2, a/2) z How does Flux change? (“more negave” is “decreases”, “less negave” is “increases”) A) increases A) Φ1 Φ3 increases A) Φ increases Φ decreases B) 1 B) Φ3 decreases B) Φ decreases Φ remains same C) 1 C) Φ3 remains same C) Φ remains same Electricity & Magne/sm Lecture 3, Slide 18 Gauss Law E E E dA Q dA E E dA dA E dA E E E Qenclosed ΦS = E~ A~ = · ✏0 closedI surface Electricity & Magne/sm Lecture 3, Slide 19 CheckPoint: Flux from Point Charge (Sphere) 1 A posi/ve charge (blue) is contained inside a spherical shell (black). How does the electric flux through the two surface elements, dΦA and dΦB change when the charge is moved from posi/on 1 to posi/on 2? o dΦA increases and dΦB decreases o dΦA decreases and dΦB increases o Both dΦA and dΦB do not change Electricity & Magne/sm Lecture 3, Slide 20 CheckPoint Results: Flux Point Charge (Sphere) 1 A posi/ve charge (blue) is contained inside a spherical shell (black). How does the electric flux through the two surface elements, dΦA and dΦB change when the charge is moved from posi/on 1 to posi/on 2? o dΦA increases and dΦB decreases o dΦA decreases and dΦB increases o Both dΦA and dΦB do not change Electricity & Magne/sm Lecture 3, Slide 21 CheckPoint: Flux from Point Charge (Sphere) 2 A posi/ve charge (blue) is contained inside a spherical shell (black). How does the flux ΦE through the en/re surface change when the charge is moved from posion 1 to posion 2? o ΦE increases o ΦE decreases o ΦE does not change Electricity & Magne/sm Lecture 3, Slide 22 CheckPoint Results: Flux Point Charge (Sphere) 2 A posi/ve charge (blue) is contained inside a spherical shell (black). How does the flux ΦE through the en/re surface change when the charge is moved from posion 1 to posion 2? o ΦE increases gauss law refers to the total charge enclosed, regardless of where it is. o ΦE decreases o ΦE does not change Electricity & Magne/sm Lecture 3, Slide 23 Think of it this way: +Q +Q 1 2 The total flux is the same in both cases (just the total number of lines) The flux through the right (leg) hemisphere is smaller (bigger) for case 2. Electricity & Magne/sm Lecture 3, Slide 24 Things to notice about Gauss Law Qenclosed ΦS = E~ A~ = · ✏0 closedZ surface If Qenclosed is the same, the flux has to be the same, which means that the integral must yield the same result for any surface.