1 of 11 Concept Tests Final Review CTF 1 Calculator question B 10E5 105 A 1E5 C something else Answer 1E5 means 1 105 105 CTF 2 A cube is made larger and larger As the cube grows the ratio A increases Answer area volume B decreases C remains constant area 6 L2 6 decreases with increasing L 3 volume L L Phys1120 Dubson University of Colorado at Boulder 2 of 11 CTF 3 A box with mass M and charge Q is pushed along a rough surface at constant velocity by a uniform horizontal electric field of magnitude E The coefficient of kinetic friction between the box and the floor is What is the net force on the box A QE B g D QE Mg FN Mg C QE Mg E None of these M Q E v constant Answer the magnitude of the net force must be zero This is actually answer D FN Mg and QE FN Mg so the sum is zero But that was a dirty trick so I ll accept answer E as correct if you thought that the answer is zero CTF 4 A row of positive charges is stationary on the ground A person with a gaussmeter which measures the magnetic field is running to the right along the row of charges at the same height as the charges and in front of them in the diagram below Does the person measure a non zero B field A Yes B No v What is the direction of the B field which the moving observer measures A Up B Down C Forward direction D backward direction E no direction B field zero Answers In the frame of reference of the person the charges are moving to the left which constitutes a current to the left There is a B field in the person s frame of reference and its direction is up at the location of the person Phys1120 Dubson University of Colorado at Boulder 3 of 11 CTF 5 A charge q is located at the center of an gaussian cubical box as shown A student is asked What is the electric flux E though the top face of the cube Is the answer hard to calculate or easy to calculate A Easy B Hard q G G Qinside E Answer It s easy if you remember Gauss s Law v The total flux through dA o the entire box is q o By symmetry the flux through one face is 1 6 th the flux through the whole box Flux 1 6 q o z CTF 6 A capacitor with capacitance C is attached to a battery with voltage V What is the flux G G E da through the cubical volume shown The end faces of the cube are within the metal plates of the capacitor C A CV o B 2CV o C zero D None of these V Answer Zero In electrostatic situations the E field inside a conductor is zero Since E 0 on the right and left ends of the imaginary G cubical G G box the flux through those ends G is zero On the sides of the box E da 0 since E da So the total flux is zero Phys1120 Dubson University of Colorado at Boulder 4 of 11 Since z G G E da 0 then by Gauss s Law the total charge must be zero within the box must be zero so we have proven that the two plates must have equal and opposite charge densities CTF 7 Points A and B are distances r and 3r respectively from a point charge q What is the voltage difference between points A and B r 2r A q B 2kq A 3r D 3kq 4r B kq 3r C 8 kq 9 r E None Answer Voltage due to a point charge is kq r The voltage difference between points A and B is kq r kq 3r 2kq 3r CTF 8 Consider a point in empty space near several charges which might be positive negative or both Consider the following statements I The E field can be zero while the voltage is non zero II The voltage can be zero while the E field is non zero Which of these statements can be true A both can be true B neither can be true C only I can be true D only II can by true Answer Both can be true Phys1120 Dubson University of Colorado at Boulder 5 of 11 CTF 9 A capacitor has a voltage V across its plates An electron initially at rest is released from at a point very close to the negative plate of a capacitor and it accelerates toward the positive plate The electron has charge e and mass m There is no gravity in this problem What is the final kinetic energy of the electron just before it collides with the positive plate A m e V B 2eV C 1 2 mV2 D eV E None of these V Answer KE PE q V eV CTF 10 A resistor with resistance R is plugged into a 120VAC wall socket The graph below is either resistance R voltage V across current I thru or power P dissipated in the resistor vs time time What could the graph be A V only B I only C V or I only D V I or P E Some other combination Answer The graph could be V or I only Power is always positive P time Phys1120 Dubson I or V University of Colorado at Boulder 6 of 11 CTF 11 Consider the circuit shown with the switch initially open When the switch is closed the current I1 through resistor R1 I bat A increases B decreases C stays the same V R1 R2 I1 When the switch is closed the current from the battery Ibat A increases B decreases C stays the same Answers I1 stays the same I1 V R1 regardless of whether or not the switch is closed The battery current increases when the switch is closed CTF 12 When a resistor is plugged into a standard AC outlet the fuse blows and all the lights go out If we want to repeat this dangerous experiment and not have the fuse blow we need a resistor that is A larger B smaller C Same R but larger power rating Answer larger At constant voltage here V 120 VAC the power is P V2 R Larger resistance draws less power CTF 13 A charge q is released from rest at point in empty space were there may be a static E and or a static B fields There are no forces on the charge except for the forces due to the E and or B fields no gravity etc The charge is observed for a short while and is seen to move along a curved path Consider the following possibilities I There is only an E field present and no B field II There is only a B field present and no E field III There is both an E field and a …
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