1 of 12 CTGAUSS 1 Two vectors A and B are perpendicular to each other A The dot product A B is equal to A AB B zero B C AB D Answer depends on the locations of the two vectors E Some other answer Answer zero CTGAUSS 2 A prism shaped closed surface is in a constant uniform electric field E filling all space pointing right The 3 rectangular faces of the prism are labeled A B and C Face A is perpendicular to the E field The bottom face C is parallel to E Face B is the leaning face The two triangular side faces are not labeled B A C E Which face has the largest magnitude electric flux through it A A B B C C D A and B have the same magnitude flux Answer A and B have the same magnitude flux 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 2 of 12 CTGAUSS 3 Two open surfaces are in an electric field as shown Surface A is a flat circular disk of radius R which squarely faces the charge Surface B is a hollow cup hemisphere of the same radius R The flat rim of the hemisphere is the same distance from the charge as the rim of the flat disk Which surface has the greater flux through it A A A B B B C Both surfaces have the same flux Answer Both surfaces have the same flux CTGAUSS 4 Two open surfaces are in an electric field as shown Surface A is a flat circular disk of radius R Surface B is a hollow cup hemisphere of the same radius R Which surface has the greater flux through it A A A B B B C Both surfaces have the same flux Answer A has greater flux 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 3 of 12 CTGAUSS 5 The flux thru an area r A A x r E E x x E y y field is y where the electric E Ex and Ey are constants is A B E C Ex A E x 2 E y 2 x A A x E y A z D Ey A E None of these Answer Ex A CTGAUSS 6 The net electric flux flowing through the closed cylindrical surface shown is A Zero B Positive C Negative Answer net flux is zero 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 4 of 12 CTGAUSS 7 Three closed surfaces enclose a point charge The three surfaces are a small cube a small sphere and a larger sphere all centered on the charge Which surface has the largest flux through it A Small cube B smaller sphere C larger sphere green yellow D Impossible to tell without more information E All three have the same flux pink Answer All three have the same flux CTGAUSS 8 To compute the E field around an infinite line of charge with charge per length a student draws the cylindrical gaussian surface of radius r and length L r L What is the LHS of Gauss s Law v v E da for this surface E 2 pr L A E 2 pr 2 B D E pr 2 E None of these Answer C E 2 p r 2 2 pr L E 2 pr L 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 5 of 12 CTGAUSS 9 The non zero electric field everywhere on a closed surface is constant meaning vector r E constant r E is everywhere constant in magnitude and direction Is the following calculation correct r r E da E da EA A Definitely correct B Definitely incorrect C Possibly correct possibly incorrect depends on details of the surface and E Answer This is a tough one I am pretty sure the answer is B Definitely incorrect CTGAUSS 10 A spherical shell with a uniform positive charge density on its surface is near a positive point charge Is the electric field inside the sphere zero A E 0 inside E B E 0 inside C Not enough info to answer Answer E 0 inside The total E field is the field due to the spherical shell of charge the field due to the point charge The field due to the spherical shell is zero inside the shell But the field due to the point charge is present inside the shell 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 6 of 12 CTGAUSS 11 A uniform infinite plane of negative charge creates a uniform E field of magnitude E perpendicular to the plane and pointing toward the plane as shown An imaginary gaussian surface in the shape of a right cylinder is shown This shape is sometimes called a pillbox The flux through surface is E E cap area A radius r L C 2A L r2 E A EA B 2EA D L r2E E None of these Answer None of these The answer is 2EA the flux is negative 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 7 of 12 CTGAUSS 12 A dipole sits near the origin We draw an imaginary Gaussian sphere radius r around it Gauss law says v v q enclosed E da e0 r Do we conclude that E 0 everywhere on that sphere A Yes E 0 everywhere B No E is not 0 at all points on that sphere Answer No E is not 0 at all points on that sphere The total flux through the sphere is zero but the E is non zero everywhere on the sphere In some places on the sphere the flux is positive in other places it is negative The total flux sums to zero CTGAUSS 13 Using Gauss s Law we determined that E 0 inside an insulating sphere with a uniform charge distribution on its surface What if we change the sphere to a cube Is E 0 inside A Yes E 0 everywhere inside B No E is not zero everywhere inside Answer No E is not zero everywhere inside With the sphere we were able to argue from symmetry that v v E da E A for the imaginary spherical surface In the case of a cube we cannot make the same symmetry argument regardless of which imaginary Gaussian surface we pick 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 8 of 12 CTGAUSS 14 A point charge q sits outside a solid neutral copper sphere of radius A What is the magnitude of the E field at the center of the sphere A E kq r2 r B E kq A2 q C E kq r A 2 D E 0 E None of these A Answer E 0 everywhere inside a metal in equilibrium Polarization charges form on the surface of the copper sphere which produce a field inside the copper which exactly cancels the field due to the point charge q CTGAUSS 15 A negative point charge with charge Q sits in the interior of a spherical metal shell The conducting metal shell has no net charge What is the …
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