1 of 9 Current Concept Tests CTR 1 Current is flowing through wires which are part of a circuit as shown What is the current in the wire at the bottom right A 1 A to the left B 1 A to the right C 3 A to the left D 3 A to the right E none of these 1A 2A I Answer 1 A to the right CTR 2 Consider the following four situations i An ion charge Q moves to the right ii A neutral hydrogen atom proton e and light electron e moves right iii A beam of electrons in a TV shoots right iv In an ionic solution massive positive ions flow right light negative electrons flow with equal speed to the left In which is the net current to the RIGHT A i only B iii only C iv only D i and iv only E None of these Answer i and iv only M Dubson Phys1120 University of Colorado at Boulder 2 of 9 CTR 3 A copper cylinder is machined to have the following shape The ends are connected to a battery so that a current flows through the copper A B C Which region A B or C has the greatest magnitude current density J A B C D all three have the same J Which region has the greater conductivity A B C D all three have the same Which region A B or C has the greatest magnitude electric field E A B C D all three have the same E Answers Region B has the largest current density J I A All 3 regions have the same current I so the region with the smallest A has the largest J All three regions have the same conductivity Conductivity is a property of the composition not of the shape of the sample Region B has the largest E field J E J is largest is region B from the 1st part of this question is constant for all regions so E must be max where J is max in region B M Dubson Phys1120 University of Colorado at Boulder 3 of 9 CTR 4 Salt water contains n sodium ions Na per cubic meter and n chloride ions Cl per cubic meter A battery is connected to metal rods that dip into a pipe filled with salt water The cross sectional area of the pipe is A A ammeter pipe full of salt water The magnitude of the drift velocity of the Na ions is v and the magnitude of the drift velocity of the Cl ions is v Assume v v What is the magnitude of the ammeter reading A enAv enAv B enAv enAv C enAv D enAv E zero Answer enAv enAv Notice that v and v are the magnitudes of the drift velocities so they are both positive by definition A current of charges to the right is equivalent to a current of charges to the left so the currents due to the 2 types of ions add They do not cancel M Dubson Phys1120 University of Colorado at Boulder 4 of 9 CTR 5 What is the correct way to connect the battery to the light bulb to make it light up A B C D hollow metal insulator metal E None of these will work Answer Circuit B is the only one that will work CTR 6 The instructor goes crazy and grabs the terminals of a fully charged 12V battery He will A immediately be killed B not be killed but he will start hollering because it really hurts C not feel anything This action is harmless Answer He will not feel anything His body s resistance R is very large mega ohm so the current through him I V R is very small M Dubson Phys1120 University of Colorado at Boulder 5 of 9 CTR 7 Which graph of voltage vs current shows the behavior of an ohmic resistor a resistor obeying Ohm s Law V V A I V B I V C I D I E None of these Answer Graph B The slope of this graph is V I R This graph shows constant slope so it shows constant resistance R Ohm s Law says that R constant CTR 8 Two cylindrical resistors are made of the same material same resistivity Resistor 2 is twice as long R2 The arrow shows the direction of current and has twice the diameter of resistor 1 What is the ratio R1 flow 1 2 A 2 B 4 C 1 2 D 1 4 M Dubson Phys1120 University of Colorado at Boulder 6 of 9 E 1 r L2 R A L A 1 1 Answer The ratio is 1 2 2 2 2 1 2 The area is proportional to radius 2 or diameter 2 r L R1 L1 A 2 4 2 1 A1 When the diameter is doubled the area increases by a factor of 4 CTR 9 A battery with emf or voltage V is attached to a resistor of resistance R The circuit diagram is shown below The point A is at zero volts B C V 5V R 10 A D The correct voltages at the points B C and D are A VB 0V VC 5V VD 0V B VB 5V VC 5V VD 0V C VB 5V VC 5V VD 0V D VB 5V VC 5V VD 0V E None of these Answer D In a circuit diagram a line represents a wire of zero resistance There is no voltage change along a zero resistance wire because Vwire I R wire I 0 0 So the voltage at B is the same as at C and the voltage at D is the same as at A CTR 10 Two light bulbs are connected to a battery in series in a chain one after another How does the current in upper light bulb A compare to the current in lower light bulb B A IA IB M Dubson Phys1120 Ibat RA IA RB IB V University of Colorado at Boulder 7 of 9 B IA IB C IA IB D answer depends on relative size of the two resistors M Dubson Phys1120 University of Colorado at Boulder 8 of 9 Answer IA IB Current is not used up in a resistor Just like water is not used up when it flows through your garden hose Every drop of water that enters your garden hose from the faucet eventually exits the other end And if the hose has no leaks and no bubbles the rate at which water enters the hose from the faucet in gallons per minutes is exactly the same as the rate at which it leaves This is because water is incompressible in order to make room for a new drop of water from the faucet one drop of water has to leave the other end This is how electrons flow through a resistor The flow of charges into the resistor is exactly equal to the flow of charges out of the resistor CTR 11 A light bulb is attached to a …
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