1 of 13 Q21 1 Two uniformly charge spheres are attached to frictionless pucks on an air table The charge on sphere 2 is three times the charge on sphere 1 Which force diagram correctly shows the magnitude and direction of the electrostatic forces on the two spheres A B C D E Answer D By Newton s 3rd Law and by the form of Coulomb s Law Q21 2 Two protons are near each other Each feels an electrostatic repulsion of magnitude Felec and a gravitational attraction of magnitude Fgrav due to the other charge Felec As the charges are moved apart the ratio Fgrav A increases B decreases C remains constant Answer remains constant 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 2 of 13 Q21 3 An electric dipole consists of two equal and opposite charges Q and Q separated by some fixed distance d A charge q is brought near the dipole and is positioned so that the distances to the Q and the Q charges are identical as shown below A q D Q B Q d C E Some other direction What is the direction of the net electrostatic force on the q charge Answer to the right Be sure you can draw the vector diagram showing the forces on q Q21 4 What is the correct expression for the y component of the force on q due to the other charges Q and Q A y Q B r C q r Answer k Qq r2 Q x D Qq r2 Qq k 2 r k Qq r2 Qq k 2 2 r 2k E None of these Notice that the y component of the force is negative 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 3 of 13 Q21 5 Vector Review How many of these equation make no sense ur A j ur B 3i j 5j v ur A C 3 ur ur C A i ur C 4 i A 0 All make sense B 1 C 2 D 3 E 4 ur Answer The last one C 4 i makes no sense You can t add a number and a vector All the others makes sense Q21 6 Trig Review In a tilted xy coordinate system the weight vector mg is straight down The coordinates are tilted at an angle as shown What is Wy the y component of the weight mg A mg sin y B mg cos C mg sin D mg E 0 mg Answer mg cos x careful with the signs 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 4 of 13 Q21 7 An electric dipole Q and Q separated by a distance d is placed along the x axis as shown A positive test charge q is placed at position A to the right of the dipole Q Q q x A The positive test charge feels a force that is A zero B to the right C to the left If the test charge q is removed electric field at position A is A zero B to the right C to the left D There is no electric field at point A when the test charge is removed If a negative test charge q is used to measure the Electric field at A the electric field at position A is A zero B to the right C to the left Answers The force on q is to the left The E field at A points to the left regardless of whether the test charge is positive q or negative q Q21 8 Two charges Q and Q are located on the x axis as shown what is the magnitude of the electric field at point A Q Q R A kQ 1 1 R 2 9 C Zero Answer R A x R B kQ 1 1 R2 4 D kQ 1 1 R2 9 E None of these kQ 1 1 R 2 9 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 5 of 13 Q21 9 Two positive charges each of size Q are equal distances from the origin as shown What is the direction of the electric field at the point in empty space which forms a square with the two charges and the origin y Field here Q Q x A B C D E No Field there Answer B upper right ur ur ur ur ur Q21 10 Two vectors A and B are shown Consider the vector sum C A B What is Cy ur the y component of C A 3 y B 2 x A B C 2 D 4 E None of these don t know Answer Cy Ay By 1 3 2 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 6 of 13 Q21 11 A point in empty space is near 3 charges as shown The distances from the point to each of the three charges are identical Q y C Q 2Q A x B The direction of the electric field at that point is A Some angle less than 45o below the x direction B 45o below the x direction C along the y directions D Some other angle E The electric field at that point is zero Answer 45o below the x direction Be sure you can draw the E field diagram 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 7 of 13 Q21 12 Three charges of equal magnitude are arranged as shown What is the direction of the electric field at point x C BA B DA x x C E None of these An electron is fired into the region of the three charges from the lower right as shown What is the direction of the acceleration of the electron when it is at point x B C A x D E None of these Answers The direction of the E field at point x is straight up choice A The direction of the acceleration of an election is the direction of a F m qE m eE m vector quantities are in bold here which is the direction opposite of the E field So the direction of the acceleration is straight down None of these 1 14 2019 PHYS1120 Dubson Fa09 University of Colorado 8 of 13 Ch26 13 A charge Q is on the y axis at y d What is the magnitude of the E field at position x on the x axis A kQ d x y Q 2 B kQ x d 2 C x 2 kQ d 2 2 2 d kQ D x 2 d 2 x E None of these E What is Ex the x component of E at position x on the x axis d A E B E x 2 d 2 Answers E x x 2 d 2 C E B C D None of these x 2 d 2 x kQ Ex E cos E 2 2 2 x d x d 2 Ch26 14 What is the magnitude of the electric field at point x on the x axis due to the two Q …
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