Peer Instruction: A User`s Manual
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1. TAALLA Z777777 7TITTIST TIITTITT7 TT 15 16 17 For the swinging pendulum shown below a make a diagram indicating the acceleration of the bob at positions P the end of the swing and Q the bottom of the swing and b draw force diagrams of the bob when it is at positions P and Q In each diagram show the net resultant force act ing on the bob if any lt A heavy pendulum bob is swinging back and forth when the string sup porting it suddenly breaks gnoring the mass of the string and air friction describe the subsequent motion of the bob if the break occurs when the bob is a at its lowest point and b at its highest point A block is placed on a planar sheet that is pivoted at one end The free side of the sheet is then raised very slowly as shown below When the sheet is first raised friction between block and sheet keeps the block from mov ing At a certain angle however the block begins to slide down the in clined sheet a If the sheet is kept at this angle will the acceleration of the block be zero constant or neither Why b If the condition u lt ls were not true what would happen to the block mi pivot WORK AND ENERGY 1 Two people push in opposite directions on a block that sits atop a friction less surface The soles of their shoes are glued to the frictionless surface If the block originally at rest at point P moves to the right without rotat ing and2. ly from a starting line to a finish line If A is initially at rest and B is ini tially moving to the right which mass has the larger change in momentum start finish A moving object collides with an object initially at rest a Is it possible for both objects to be at rest after the collision b Can one of them be at rest after the collision c Is it possible to have a collision in which all kinetic energy is lost Explain each answer and if you answer yes to any part cite an example You are to knock over a gigantic bowling pin at a fair by throwing a ball at it Y ou have a choice between two balls one that collides elastically and another that collides inelastically Which do you choose and why Suppose you are in an automobile accident a All other things being equal are you better off in a more massive car or alighter one Why b Is it better to be in a car that crumples on impact rather than one that holds together stiffly Why A very elastic superball is dropped from a height h onto the ground The ball rebounds at very nearly its original speed a Is the ball s momentum conserved during any part of this process b If we consider the ball and E arth as our system during what parts of the process is momentum con served c A nswer a and b for the case of a piece of putty that falls and sticks to the ground Suppose a golf ball is hurled at a heavy bowling ball initially at rest and
3. MAGNETISM 1 Can an electric or a magnetic field each constant in space and time be used to accomplish the actions described below E xplain your answers In dicate if the answer is valid for any orientation of the field s M ust any other condition be satisfied a move a charged particle in a circle b exert a force on a piece of dielectric c increase the speed of a charged particle d accelerate a moving charged particle e exert a force on an electron initially at rest A compass is placed in a uniform magnetic field W hat is the net force on the compass needle Two very long fixed wires cross each other perpendicularly They do not touch but are close to each other as shown E qual currents flow in the wires in the directions shown Indicate the locus of points where the net magnetic field is zero yl A charged particle having a certain kinetic energy enters a static mag netic field If no other forces act on the particle while it is within the field it has the same kinetic energy on leaving the field W hy A metallic bar moves to the right in a uniform magnetic field that points out of the page as shown a Is the electric field inside the bar zero E x plain b Is an external force required to keep the bar moving with con stant velocity E xplain In the following figure the bar moves to the right with a velocity v in a uni form magnetic field that points out of the page a Is the induced curr
4. bounces elastically from the bowling ball A fter the collision which ball has the greater momentum Which has the greater kinetic energy Why ROTATIONS 1 Suppose you are standing on the center of a merry go round that is at rest Y ou are holding a rotating bicycle wheel over your head so that its ro tation axis is pointing upward The wheel is rotating counterclockwise when observed from above along your axis of rotation a Suppose you now move the wheel so that its axis is horizontal What happens to you b What happens if you then point the axis of the wheel downward so that the wheel rotates clockwise as viewed from above The figure below shows the paths taken by several particles In a d the particles move with constant speed and in e the motion is back and forth With respect to the origins O state the direction of each angular momentum vector or indicate if the angular momentum is zero In each case is the angular momentum independent of time 3 A ball initially at rest rolls without slipping down an inclined plane as shown below a Make a diagram of the ball on the incline showing all forces act ing on the ball D escribe each force in words b Which force described in part a causes the ball to roll by creating a torque about its center N ow consider a block sliding down an identical inclined plane The block travels the same vertical distance as the ball before arriving at the bot tom c Wh
5. in the plane of the page in the directions in dicated by the arrows a Which of the tracks correspond to positively charged particles b If all three particles have the same mass and charges of equal magnitude which is moving the fastest c If all three particles are moving with the same speed and have charges of equal magnitude which has the greatest mass eS a Isit possible for a charged particle to move through a magnetic field without experiencing a force b Isit possible for a charged particle to be at rest in the simultaneous presence of a magnetic and an electric field c Isit possible for such a particle to move through the fields without ex periencing a force d What is the net force on a magnetic dipole ina uniform magnetic field INDUCTION AND MAXWELLS EQUATIONS 1 Inthe following figure a coil connected to a battery and a switch lies above a thin metallic ring When the switch is opened is the ring repelled by or attracted to the coil Le 2 The following figure shows the two ways two identical inductors may be connected in series In which case is the total inductance of the system larg er i e in which case is the total back emf of the series bigger Take into account the emf induced in each coil by the magnetic field of the other 3 Two identical coils are wound around a common iron core as in the fol A sinusoidally varying emf of the form E E sin tis applied to
6. isa positively charged insulator Bob C and a second rod B are in contact with each other and made from conducting material Rod B is fixed and C is suspended from a wire and free to swing Briefly describe what happens when A is brought near B 10 11 12 13 Two identical charges q are a distance L apart Is it possible to place a third charge q somewhere such that none of the charges experience any force If so where and what is q If not why not A point charge Q is at the center of a spherical conducting shell There is a point charge q outside the shell a Does q experience a force b Does Q experience a force c If there is a difference in the forces experienced by the charges reconcile your answer with N ewton s third law A hollow insulating cone is placed in a field of field strength E Whatisthe ratio of the flux through the conical surface A to the flux through the open cross section B of the cone SA yYyyYyYyYYYyYY Inthe figure below A is a solid metallic sphere and B a concentric metal lic shell Suppose A is charged positively and B is grounded a Qualita tively compare the magnitude and distribution of charges on A and B b sthere an electric field i outside B ii between A and B iii inside A E xplain B 14 15 16 17 Suppose a positive point charge q is located at the center of a spherical metal shell W hat charges appe
7. the left coil while the circuit on the right is left open i e the points P and Q are not connected a Is the induced emf across the right coil between the points P and Q in phase 90 ahead of 90 behind or 180 out of phase with the applied emf E Explain b Does the induced emf across the right coil induce an emf in the left coil What happens if a resistor is used to connect the points P and Q Explain 4 a A brass ring is placed on top of a coil of wire If a switch to a source of direct current is closed and charges start to flow in the coil the ring springs upward E xplain b The ring is then placed atop the coil once again and the switch opened The current in the coil rapidly dies out W hat happens to the ring now Explain 5 Write down the Maxwell equation most directly associated with the fol lowing statements and give a brief reason for each answer Be sure to de fine the quantities used in each equation a A n alternating emf is induced in a coil that rotatesin a uniform magnetic field b The lines of the mag netic field circle round a steady current c The static electric field inside a conductor is zero AC CIRCUITS 1 You are given a small box equipped with two electrical terminals U pon applying an alternating current to the two terminals you notice that the instantaneous potential difference across them and the instantaneous cur 10 11 12 rent through the box are in ph
8. INSTRUCTIONS FOR USE The Conceptual Exam Questions in this file can be copied into your exams or modified to suit your needs To locate a specific question e Use the bookmarks at left e Click on the subject of the desired question e Scroll down until you find the desired question To search for a specific word or phrase within this file e Use Find from the Tools menu To copy or modify the text in this file e Use Select Text from the Tools menu to copy the text you want use e Paste the copied text into your word processor or other application e Use Select Graphics from the Tools menu to copy the artwork KINEMATICS 1 6 Two stones are released from rest at a certain height one after the other a Will the difference in their speeds increase decrease or stay the same b Will their separation distance increase decrease or stay the same c Will the time interval between the instants at which they hit the ground be smaller than equal to or larger than the time interval between the in stants of their release A sa space shuttle burns up its fuel after take off it gets lighter and lighter and its acceleration larger and larger Between the moment it takes off and the time at which it has consumed nearly all of its fuel is its average speed larger than equal to or smaller than half its final speed Y ou are in an elevator that is rising at constant velocity Suddenly you
9. ar on a the outer surface and b the inner surface of the sphere c Suppose you bring an uncharged metal object near the sphere Will your answers to parts a or b above change Will the way charge is distributed over the sphere change Consider the electric field inside a charged rubber insulating balloon Is E necessarily zero inside the balloon if it is a spherical or b sausage shaped A ssume the charge to be distributed uniformly over the surface for each shape c H ow would the situation change if at all if the balloon were coated with a thick layer of conducting paint on its outside surface A positive charge moves in the direction of a uniform electric field D oes its potential energy increase or decrease D oes the electric potential in crease or decrease If the electric potential at some point is zero does it follow that there are no charges in the vicinity of that point E xplain DC CIRCUITS 1 4 Suppose a resistor and a capacitor are connected in series to a battery D oes the value of the resistor affect how much charge the capacitor stores If yes explain how If no what is the effect of the resistor Two light bulbs both operate from 110V but one has a power rating of 40W whereas the other has a rating of 75 W a Which bulb has the higher resis tance b Which carries the greater current The following circuit has three identical light bulbs connected to an ideal battery a H ow do the br
10. ase a What conclusions can you make about the resistance and or the capacitive and inductive reactance of the box b If you lower the frequency will the potential difference and the current remain in phase In order to run an ordinary household appliance from the cigarette lighter of a car you can buy a device that converts the car battery s 12 V dc to 110 V ac One part of the circuit in such a device makes the voltage alternating an other uses a transformer to increase the voltage D oes it make a difference in which order these two operations are carried out The same ac voltage is applied across a resistor and a capacitor and the same rms current is measured through each Do both dissipate the same amount of energy E xplain In a series RLC circuit the current lags the applied voltage Is the peak voltage greater across the capacitor or across the inductor Fluorescent light fixtures use an inductor called the ballast to limit the cur rent through the lamp Why is an inductor used instead of a resistor A light bulb designed to operate at 120 V rms is connected in series with an inductor a capacitor and a 120 V rms ac source a Isit possible for this bulb to be as bright as an identical one connected directly to the source b I sit possible for this bulb to be brighter than an identical one connected directly to the source A circuit containing a coil a resistor and a battery has achieved a steady state the cur
11. bove her head D oes the scale reading ever differ from the combined weight of the two bodies at any time during the lift E xplain Two blocks sitting on a frictionless table are pushed from the left by a horizontal force as shown below 12 13 14 a Draw a free body diagram for each of the blocks b E xpress in terms of the quantities given in the figure the force of contact between the two blocks c What is the acceleration of the blocks Suppose now a force of equal magnitude but opposite direction is applied to the block on the right m an d What is the force of contact between the two blocks in this case e W hat is the acceleration of the blocks f Do your answers to parts b through e make sense Verify Two teams A and B are competing in a tug of war Team A is stronger but neither team is moving because of friction D raw free body diagrams for team A for team B and for the rope The spring in configuration a is stretched 0 10 m How much will the same spring be stretched in configuration b a b In the situation below a person pulls a string attached to block A which is in turn attached to another heavier block B via a second string a Which block has the larger acceleration b H ow does the force of string 1 on block A compare with the force of string 2 on block B Explain your answers string 2 string 1 F
12. d the ped als work on the rear wheel there is no brake on the front wheel Identify all the action reaction pairs that exist for a horse pulling a cart Include Earth in your examination but ignore air resistance M ake sure your notation makes clear which force acts on which object What is the smallest force with which you can raise an object on Earth into the air a A car is stationary on a flat parking lot The force of gravity acts down ward and an equal and opposite normal force acts upward State the law according to which these forces are equal and opposite b When the car accelerates forward which force is responsible for this acceleration State clearly which body exerts this force and on which body the force acts A horse is pulling a wagon at constant velocity over a flat horizontal road Draw a free body diagram of all the forces acting on the horse and another one of all the forces acting on the wagon In your drawing indicate the rel ative magnitude of the forces and identify any third law force pairs A Iso identify forces of equal magnitude that are not third law pairs A basketball player is jumping vertically upward in order to land a shot Her legs are flexed and pushing on the floor so that her body is acceler ated upward a D raw free body diagrams of the player s body and E arth Show the relative magnitudes of the various forces and describe each in words i e contact gravitational etc and indicat
13. drop your Keys it so happens that when they strike the floor they are as high above ground level as when they left your hand The keys fall dead on the floor without bouncing M ake a single graph showing qualitatively the height above ground of both the keys and the elevator asa function of time starting from before the keys are released until after they strike the floor a Can astronauts floating in orbit tell which objects within their space ship would be heavy or light on E arth even though everything in the ship is effectively weightless E xplain b Could an astronaut use a weight less hammer in space to drive a nail through a piece of wood E xplain A battleship simultaneously fires two shells toward two enemy ships one close by A one far away B The shells leave the battleship at different angles and travel along the parabolic trajectories indicated below Which of the two enemy ships gets hit first Explain battleship K pE ee Cite an example of a body that accelerates while traveling at constant speed Is it possible for a body to be accelerating while traveling at con stant velocity E xplain NEWTON S LAWS 1 You are riding your bike along a flat country road Indicate the direction and relative magnitude of the frictional forces on the front and rear tires in the following situations a you are accelerating b you are pedaling along at a steady pace c you are braking Both the brake an
14. e which object exerts that force and on what Identify the action reaction pairs b R epeat this exercise for the situation immediately after the player s body breaks con tact with the floor c Finally consider in the same manner the situation at the top of the jump A toy train travels around a loop the loop track as shown below a Is there a normal force exerted by the track on the train at the instant the train is at the top of the loop b Why is it that riders feel weightless at the top of certain roller coasters 8 A monkey clings to a rope that passes over a pulley The monkey s weight 10 11 is balanced by the mass m of a block hanging at the other end of the rope both monkey and block are motionless In order to get to the block the monkey climbs a distance L measured along the rope up the rope a Does the block move as a result of the monkey s climbing b If so in which direction and by how much monkey A child sits on a pedestal that rests on the platform of a merry go round H e holds a bob suspended on a string D raw free body diagrams for the child the bob the string the pedestal and the platform Indicate the mag nitudes of the forces by the relative lengths of the arrows you draw D e scribe each force and indicate the third law pairs A weightlifter and a barbell are both at rest on a large scale The weightlifter begins to lift the barbell ultimately holding it motionless a
15. ends up at rest at point Q describe qualitatively how much work is done on the block by person 1 relative to that done by person 2 1 I lt 2 777 71777 777777 P Q a When an object s kinetic energy is increasing must its potential energy be decreasing E xplain b If a rocket engine delivers a constant thrust force on the rocket does it deliver more power as the rocket speeds up E xplain A marble is dropped from the top of a skyscraper Is the work done by E arth s gravitational force on the marble equal to smaller than or larger than that done by the marble s gravitational force on E arth E xplain Given that the gravitational force on a body is proportional to the body s mass why doesn t a heavy body fall more rapidly than a light body It is possible to have a non zero net force on an object that does no work on the object Give an example of this for a an object going in a straight line and b an object along a curved trajectory E xplain Suppose you re on a rooftop and you throw a ball downward to the ground below with an initial speed v You then throw an identical ball upward with the same initial speed v A fter rising this ball also falls to the ground At the moment each strikes the ground how do the speeds of the balls compare U se conservation of mechanical energy in your reasoning MOMENTUM amp COLLISIONS 1 Identical constant forces push two identical masses A and B continuous
16. ent clockwise or counterclockwise b If the bar moves to the left will the cur rent reverse 7 10 A very long line of positive charges moves in the direction of its long axis a Is an electric field produced b What about a magnetic field c If these exist sketch their direction s d Suppose we move a thin neutral metal wire consisting of static positive ions and free electrons instead of the line of positive charges A nswer the same questions for this case In the following figure a closed loop moves at a constant speed parallel to a long straight current carrying wire Is there a current in the loop If so is this current circulating clockwise or counterclockwise D _ Immm The square conducting sheet shown is moving at a uniform velocity v through a uniform magnetic field B that is perpendicular to the sheet a What happens to free charges inside the sheet when it begins to move What happens to these charges while the sheet is moving at a uniform ve locity b Do your answers to a depend on whether there are positive or negative or both free charges in the sheet c Draw the equipoten tials for the sheet in motion Charged particles passing through a bubble chamber leave tracks con sisting of small hydrogen gas bubbles These bubbles make visible the par ticles trajectories n the following figure the magnetic field is directed into the page and the tracks are
17. from a ceiling by nonconducting strings The conductorsare given charges q Q and q 3Q a A fter charging which of the angles 6 and 6 that the two strings make with the vertical is larger or are they equal The two conductors are now brought together and made to touch b Which of the two new angles 6 and 6 is the larger or are they equal c H ow do the angles 6 and 6 compare with the old angles 6 and 6 LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL LL Oae In the following figure the dashed line denotes a G aussian surface enclos ing part of a distribution of four positive charges a Which charges con tribute to the electric field at P b Is the value of the flux through the surface calculated using only the electric field due to q and q greater than equal to or less than that obtained using the field due to all four charges Ho 3 Oa S The following figure depicts three charges and a G aussian surface a Which charges contribute to the net flux through the G aussian surface b Which of the charges contribute to the field at a given point on the surface c Compare your answers to a and b and explain why they are the same or different Suppose the net charge enclosed by a surface is zero d D oes it follow that the field is zero at all points on the surface e Is the reverse true i e if the field is zero at all points on the surface is the net charge enclosed zero e 43 8 Sup
18. ich arrives at the bottom with more total kinetic energy W hy d Which arrives at the bottom with more linear momentum W hy OSCILLATIONS WAVES AND SOUND 1 Suppose you are at rest with respect to a source of sound and a strong wind is blowingin your direction D escribe the effect if any of the wind on a the observed frequency b the observed wavelength and c the wave speed 2 sit possible to move a source of sound relative to a stationary observer so that there is no shift in the sound frequency If so give an example 3 A spring hanging vertically with a mass m suspended from it elongates an amount Al If pulled a little farther and then let go the mass oscillates with a measured period T D iscuss how you could use this arrangement to de termine the value of g FLUIDS 1 Consider two identical ships one loaded with a cargo of cork which floats and the other empty Which ship displaces more water 2 A piece of cork floats in a pail of water that rests on the floor of an ele vator If the elevator accelerates upward how does the depth at which the cork floats change 3 Use the principle behind Bernoulli s equation to explain the following phenomena a smoke rises in a chimney faster when a breeze is blowing and b when a fast moving train passes a train at rest the two tend to be drawn together ELECTROSTATICS 1 In the figure below two uncharged conductors of identical mass and shape are suspended
19. ightnesses of the three bulbs compare b Which draws the most current c What happens to the brightness of A and B when C is unscrewed d What happens to the brightness of B and C if A is unscrewed Inthe following circuit you are to measure the potential difference across and the current through resistor R a Indicate how you would connect the volt meter and ammeter in the circuit b What resistances should an ideal volt meter and an ideal ammeter have in order to make accurate measurements A Vy 5 a When resistors are connected in parallel which of the following are the same for each current power potential difference b When resistors are connected in series which of the following are the same for each cur rent power potential difference 6 Suppose bulb A in the circuit shown is unscrewed from its socket a H ow do the brightnesses of the three remaining bulbs change b How do these brightnesses compare with each other A 7 Imagine that some node in a certain circuit does not obey K irchhoff s first law for instance the currents into and out of the node differ by some positive amount What happens to the potential at that node 8 When an RC circuit is connected to a battery the circuit draws current until the capacitor is fully charged Is the amount of energy stored in the capacitor larger than equal to or smaller than the energy delivered to the circuit by the battery Explain
20. pose two charged conducting spheres of different radii r4 and r gt r are connected by a conducting wire Which sphere has the greater charge density If the electric field E equals zero at a given point must the potential differ ence V also equal zero at that point Give an example to prove your answer A spherical rubber balloon has a charge uniformly distributed over its surface A s the balloon is inflated how does the electric field E vary a outside the balloon at some point well away from the surface b at the outer surface of the balloon c inside the balloon A ssume the balloon remains spherical during inflation a In the figure below a point charge q is placed midway between two identical point charges Q Isq in equilibrium If so is the equilibrium sta ble or unstable b A nswer the same two questions for the case where q is changed to q but Q remains positive A charged rod is placed near an uncharged nonconducting sphere sus pended from a nonconducting wire as shown below Neither the rod nor anything else touches the sphere a Will the rod and sphere exert forces on one another If so make a sketch showing the direction of the forces b Would your answer to a change if the sphere were made out of con ducting material c What is the total charge on the sphere after the rod is placed close to it d Do your answers to a and c violate Coulomb s law Explain F FFF 444444 q 0 RodA
21. rent has reached a constant value a D oes the coil have an inductance b Does the coil affect the value of the current in the circuit A lightbulb and an inductor are connected in parallel to an alternating source of emf a H ow does the brightness of the bulb vary as the fre quency of the ac source is increased from zero b If the inductor is re placed by a capacitor how does your answer change The sum of the peak voltages across the elements in a series RL C circuit is usually greater than the peak applied voltage Why Does this violate K irchhoff s voltage law Is the ac voltage applied to a circuit always in phase with the current through a resistor in that circuit If a capacitor is connected to a sinusoidally alternating source of emf does the displacement current between the capacitor plates lead or lag be hind the emf The following figure shows a phasor diagram for a series RLC circuit a Identify each of the three components 1 2 and 3 eg 1 R 2 L 3 C b Is the driving frequency above or below resonance 13 Step up transformers are used to increase potential differences The po tential difference however represents energy per unit charge and so a step up transformer increases the energy per unit charge H ow is this pos sible without violating the law of conservation of energy OPTICS 1 A thin converging lensis used to form a real image of a nearby object If the object is moved closer
22. to the lens a new real image is observed D oes the new image differ from the old one a in position relative to the lens and b in size If it does describe how B y changing the separation distance between lens and film a camera can focus on subjects at a variety of distances Suppose the proper lens film separation for taking an in focus picture of a distant object such as the moon is d To take an in focus picture of a nearby object will the prop er lens film separation be greater than equal to or less than d Explain using diagrams a D oes light traveling from one medium to another always bend toward the normal b A slight travels across the interface between two media does its frequency change c Its wavelength d Its velocity Explain your answers MODERN PHYSICS 1 M any houses have black asphalt roofs From an energy saving point of view is this the best roof finish on a a hot summer day and b a cold win ter night In either case if not what type of roof would be better In order for an atom to emit light must it first be ionized E xplain Do the emission and absorption spectra obtained from the same gas at room temperature contain the same number of lines E xplain Can a photon any photon be absorbed by a free electron H int con sider conservation of momentum and energy If an electron and a proton are accelerated from rest through the same po tential difference which has the longer wa
23. velength An electron in a hydrogen atom is in its ground state n 1 a If radia tion having a frequency greater than E 3 E h but less than E 4 E4 h isincident what happens b If instead a beam of electrons having kinetic energy greater than E 3 E 4 but less than E E 4 is used what happens Would a beam of protons in a proton microscope exhibit more or less diffraction than a beam of electrons of the same speed in an electron mi croscope E xplain O ne thousand photons uniformly distributed in wavelength between 1100 and 1600 nm exert pressure on a surface they strike Suppose a prism is used to remove all the photons having wavelengths below 1350 nm Will the radiation pressure be reduced by a less than half b half or c more than half Explain
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