1 of 7 Exam I Review RI 1 Three equal mass charges are released from rest at the positions shown on the xaxis Which mass has the largest initial acceleration Q Q Q A B C A A B B C C D 2 of the masses have the same acceleration x E Some other answer Answer B The particle which feels the largest E field due to it neighbors will have the largest force Since all the masses are equal the largest force produces the largest acceleration RI 2 Four charges are arranged as shown all a distance r from the origin What is the magnitude of the E field at the origin in units if kQ r2 y Q 2Q x 2Q 4Q A 1 B 2 C 3 D 4 E None of these Answer None of these The answer is 5 The magnitude of the E field is 5kQ r2 PHYS1120 Dubson 9 15 2009 University of Colorado at Boulder 2 of 7 RI 3 A line of charge has length L and total charge Q What is the relation between dx and dQ A dx Q L dQ B dQ L Q dx C dQ Q L dx D None dE point A h dx dQ 0 x L At point A what is the magnitude of the field dE due to the element of charge dQ A k dQ k dQ h2 x2 B C h2 x2 k dQ h2 x2 What is the correct expression for sin A x h2 x2 h B C h2 x2 h h2 x2 x D What is the correct expression for dEx k Q L dx A h 2 x2 3 2 Answers dQ Q L dx dE x dE sin k x Q L dx k h Q L dx B h2 x2 k dQ h2 x2 k x Q L dx dE h PHYS1120 Dubson 9 15 2009 2 x2 h2 x2 C sin h 2 x2 3 2 x h2 x2 3 2 University of Colorado at Boulder 3 of 7 RI 4 A point charge Q is brought near a large metal plate and the field lines are as shown Does the plate have a non zero net charge A Yes B No Is the magnitude of the charge on the plate greater less or the same as the charge Q A greater B less C same Answers Yes the plate has a net negative charge because field lines terminate on it but no field lines begin on it The plate has a smaller magnitude negative charge than the point charge There are 8 fields lines coming out of the point charge but only 5 field lines entering the plate z RI 5 If the electric field over a surface be zero G E is non zero everywhere on a closed surface S can the flux G G E da 0 A Yes B No S G E is constant everywhere on a closed surface S can you conclude G G G E B No that v da E v da A Yes If the electric field S S JG Answers Yes A simple example is a constant E field E E x The E field is non zero G G yet E da 0 for any closed surface in the field z S NO In order to pull the E outside the surface integral you must first have that JG G JG G E da everywhere on the surface so that E d a E da and then you must have magnitude of the field E constant over the surface PHYS1120 Dubson 9 15 2009 University of Colorado at Boulder 4 of 7 RI 6 A long straight line of charge has charge per length Does Gauss s law allow you to G G E compute the flux v da through the spherical surface of diameter D centered on the S line A Yes B No D Consider now the open hemispherical surface shaped like a cup centered on the line as G G E shown Does Gauss s law allow you to compute the flux da through this surface S Taking the outward normal direction as positive A Yes B No D 2 Answers G G Q enc E Yes v S da o and Qenc D so G G D E v S da o Yes Apply Gauss s Law to the closed hemisphere The surface integral over the flat portion is zero since E field perpendicular to surface there So we can G G G G D E write E da v da 2 o pencup PHYS1120 Dubson 9 15 2009 University of Colorado at Boulder 5 of 7 RI 7 The electric field throughout a region of space is given by the formula JG E A y x B x y where x y are the coordinates of a point in space and A B are JG constants What is E x A Ay B Bx C Ax D By E None of these Answer JG Ay E x A y x B x y x A y x x B x y x Ay Since x y 0 and x x 1 RI 8 An insulating 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 B E 0 inside E C Not enough info to answer Answer E inside is NOT zero Etotal Eshell Epoint Inside the shell Eshell 0 but the field due to the point charge is still present This is not a metal shell so there is no polarization of the charge on the shell and no screening RI 9 A spherical shell of charge has a non uniform distribution of charge over its surface Is the electric field everywhere within the shell zero A Yes B No E 0 Answer No The E field inside a uniform spherical shell of charge is zero But if the charge is non uniform the E field will not cancel inside the sphere RI 10 Consider a point charge Q off center within a spherical metal shell Does Gauss s Law allow you to compute the total charge on the inside surface of the shell PHYS1120 Dubson 9 15 2009 University of Colorado at Boulder 6 of 7 A Yes B No Does the total charge on the inside surface of the shell depend on the total net charge of the whole shell A Yes B No Q Consider a dipole within a spherical metal shell Is there a non zero surface charge density on the inside surface of the shell A Yes B No Q Q Answers Yes you can compute the total charge on the G G inside surface Consider the flux v E da over a spherical surface within the metal S Since the E field must be zero everywhere within the conductor then G G v E da Qenc o 0 Since Qenc 0 there must be a charge Q spread over the inside S surface of the conductor Since the point charge …
View Full Document
Unlocking...