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QUESTIONS & PROBLEMS

IDEAS TO IMPLEMENTION

How to answer a question: problem solving (t0_372.pdf)

View periodic table (cited Aug 2012)

Numerical values for constants and useful physical quantities)Don’t view a solution until a genuine effort has been made to answer the question or solve the problem.

Answers to problems

p1.23 08/12The debate as to whether cathode rays are charged particles or electromagnetic waves continued for many years. Which observation of cathode rays resolved this debate? Clearly distinguish the wave and particle properties.

p1.28 07/24(a) A negatively charged cylinder is fixed in position near a positively charged plate. Sketch the electric field lines between the cylinder and the plate.

(b) A tiny particle of mass 10-30 kg and charge +6×10-12 C is released at point Y. The particle initially accelerates at 7.0×1021 m.s-2. Calculate the electric field intensity at Y.

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p1.30 06/12A charged non-magnetic particle is moving in a magnetic field. What factors affect the magnetic force on the particle?

p1.34 4An electron is moving near a long straight wire. When a current is applied to the wire the electron experiences a force in the same direction as the current flow in the wire. What was the electron’s initial direction of motion? Explain.

p1.44 08/23Two parallel metal plates were in a magnetic field B = 1.2310-3 T. The plates were separated by a distance d = 22.5 mm. An electron was accelerated by a 10.5 kV before entering the region between the plates. (a) What was the speed of the electron upon entering the region between the plates?(b) What was the magnitude and direction of the force due to the magnetic field?(c) What was the magnitude and direction of the force due to the electric field if the electron continued on a straight path parallel to the plates?(d) What was the magnitude and direction of the electric field between the plates?(e) What was the voltage between the plates? Which plate was positively charged?(f) How was this set-up used by J. J. Thompson used to measure the q/m ratio for an electron?

p1.65 04/12A discharge is shown.

(a) What type of pattern is shown?(b) What end is the cathode?(c) Explain the pattern observed?(d) If the gas was changed, does the pattern change significantly?(e) If the pressure was changed, does the pattern change significantly?

p1.68 01/2A positively charged particle is moving with velocity v in a magnetic field. At this moment, what is the direction of the force on the positively charged particle? Explain. Describe the subsequent motion of the particle.

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uniform magnetic field out of pageB

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p1.71The diagram shows four discharge tubes and the patterns of striations observed in them.(a) Explain the patterns.(b) Account for the difference in the striation patterns.(c) List the tubes in order of decreasing pressure.

p1.75 7A charged non-magnetic particle is moving in a magnetic field. What would NOT affect the magnetic force on the particle?(a) The strength of the magnetic field.(b) The magnitude of the charge on the particle.(c) The velocity component parallel to the magnetic field direction.(d) The velocity component perpendicular to the magnetic field direction.Explain each alternative.

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p1.76 02/13(a) What is the name of the device shown below?(b) What is the purpose of the device?(c) What is the name and function of the parts labeled A to F?

In a special investigation, the voltage between the cathode and the anode isincreased so that an electron gains a velocity of 0.60 c, where c is the speed oflight. The electron starts from rest at the cathode.

(d) What was the accelerating voltage for the electron gun?

The deflection plates are separated by a distance of 18 mm and the voltage across them is 254 V.

(e) What is strength of the electric field between the plates? (f) What is the electric force acting on an electron?(g) What acceleration of the electron?

p1.79 10An electron travels at 2.0107 m.s-1 in a plane perpendicular to a 0.0100 T magnetic field.

(a) Describe the path of the electron.(b) Calculate the radius of the circular orbit.(c) Calculate the period of motion.(d) Calculate the frequency of the electron.

Hint: Centripetal force FC = m v2 / R

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A

B

C

D

E

E

F

p1.93 1A potential difference of 50 V is applied between two identical, parallel aluminum plates which are separated by a distance of 10 mm. In order to double this electric field strength, which new arrangement should be

used?

Explain.

p1.94 K

Crookes developed a gas discharge tube to investigate the electrical nature of matter. When a high voltage is applied to the tube, the glass behind the metal cross glows (fluoresces) and a shadow of the cross appears. (a) Explain the appearance of the shadow. If the cross is allowed to drop to the horizontal

position, describe and explain the pattern observed.(b) Explain the fluoresces of the glass.(c) When a magnet is moved towards the gas discharge tube, the shadow moves and is

distorted. Explain.(d) How did the use of discharge tubes by scientists contribute to the understanding of

atomic structure?(e) You have performed investigations using discharge tubes. Explain how you were able

to get them to function.

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p2.05 07/27Describe the experiment used by Hertz and his observation of the photoelectric effect. What did he do about these observations.

p2.06Describe how Einstein used the ideas of Planck to explain the photoelectric effect.

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p2.08An experiment was performed using a photocell. The surface was illuminated by light of different frequencies and the stopping voltage was measured.

Define each of the terms: stopping voltage, threshold frequency, threshold wavelength, work function of the surface.

(a) What is Einstein’s equation of conservation of energy explaining the photoelectric effect in terms of the stopping voltage and frequency?

wavelength (nm)

360 400 440 490 550 580

stopping voltage (V)

1.45 1.12 0.95 0.60 0.40 0.25

(b) Plot the stopping voltage versus the frequency of the light.

(c) Determine: the threshold frequency, threshold wavelength, the work function in eV and J, and the value of Planck’s constant.

(d) Another surface was used in the experiment. Its work function was 0.5 eV. Draw a line on the graph for this surface.

p2.13 05/12The family of curves below shows the relationship between the intensity of black body radiation and its wavelength for various Kelvin temperatures.

Who was the first to correctly explain this relationship?

What information do the graphs give you?

p2.33 07/27Scientists tried to explain observations of blackbody radiation using classical wave theory and then quantum theory. (a) What is meant by the term blackbody radiation?(b) What was a limitation of classical wave theory that could not explain

blackbody radiation?(c) How does quantum theory satisfactorily explain blackbody radiation?

p2.50 08/24How did Einstein’s theory of special relativity and his explanation of the photoelectric

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effect lead to the re-conceptualisation of the model of light?

p2.61 08/13What is the energy of a photon (joules and electron volts) of wavelength 550 nm?What is the frequency and what part of the electromagnetic spectrum for this photon?

p2.62 02/15A student carried out an experiment during which light of different frequencies wasshone onto a metal surface to produce electrons. The student measured the maximum kinetic energy of the emitted photoelectrons as the frequency of light was altered.

(a) How did the student measure the maximum kinetic energy?

(b) What is the mathematical relationship between the maximum kinetic energy of the photoelectrons and the frequency of the light incident on the metal surface? Define each term in the equation and its SI unit.

(c) How could the student best analyse the data to determine a value for Planck’s constant?

p2.66 KPhotoelectrons are emitted by a surface of a certain metal when the surface is illuminated by both violet light of wavelength 400 nm and green light of wavelength 550 nm but no photoelectrons are released from the surface by red light of wavelength 715 nm.(a) Calculate the frequencies and the photon energies of the three light beams. (b) Explain the differences in energies of the electrons released by the violet and green

light.(c) Explain why no electrons are released when illuminated by red light.

p2.70 03/14Heinrich Hertz used a set-up similar to the one shown to investigate the production and detection of electromagnetic radiation.

(a) In the diagram identify the transmitter, receiver, transmitter, spark gaps, high voltage source.

(b) List the components of the electromagnetic spectrum

(c) Explain the production and detection of the radio waves.

A glass sheet was placed between the transmitter and receiver. (d) Did the radio waves pass through the glass? Explain.(e) Were ultraviolet waves blocked when the glass sheet was in place?(f) Describe and explain the change in the spark length when the glass

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(g) How does this relate to the Photoelectric Effect?

p2.72 KAssess the impact of the discovery of the photoelectric effect of the development of the quantum model of light.

p2.90 05/23Explain how an understanding of blackbody radiation changed the direction ofscientific thinking in the early twentieth century.

p2.95 06/26Beginning in the late 19th century, observations and experiments on blackbody radiation and the Photoelectric Effect led physicists to revise their existing model of light. (a) What was the existing model for light? (b) What was the evidence for this model? (c) What model was introduced to explain blackbody radiation and the Photoelectric

Effect? (d) Use the above as an example to explain how scientists test, validate and revise

models.

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p3.13 08/15A block of silicon doped with boron is connected to a 10 V battery. What is the main way in which conduction occurs in the doped silicon block?

p3.18 KCompare the accepted models that are used to describe how an electric current flows in a metallic conductor at room temperature and a doped semiconductor at room temperature.

p3.22 KIn terms of the band theory explain the essential conditions for a substance to be a semiconductor.

p3.23 KDescribe how p-type semiconductors are produced.

p3.24 KDiscuss how shortcomings in available communications technology led to an increased knowledge of properties of materials with particular reference to the invention of the transistor.

p3.27

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12 V

n-Si

p-Siundoped

Si

sunlight

A solar cell can consist of an undoped silicon layer (~ 1mm thick) placed between very thin (~ 2 nm thick) of doped silicon.(a) Describe how one of the doped layers can be made.(b) What is the name of the effect that causes the production of electrons in the undoped layer?(c) Draw a labelled diagram to show how a potential difference is created across the cell by the movement of holes and electrons.

p3.30 02/14During the early 1950s most transistors were manufactured using germanium.(a) Why was germanium used instead of silicon?(b) Why is silicon used today instead of germanium?

p3.33 07/14(a) Summarize the property of a silicon p-type semiconductor? List a number of possible

doping atoms.(b) Summarize the property of a silicon n-type semiconductor? List a number of possible

doping atoms.

p3.39 04/25An example of a solar cell is shown. The solar cell is able to produce a current due to the photoelectric effect and the electrical properties of the n-type and p-type layers. Use this information to outline the process by which light shining on the solar cell produces an electric current that can light up a light globe.

p3.44 07/22 04/23Explain why solid state devices have largely replaced thermionic devices.In the past 50 years electrical technology has developed from the widespread use ofthermionic devices to the use of solid state devices and superconductors.List THREE disadvantages of thermionic devices that led to their replacement.Outline ONE advantage of using superconductors, with reference to TWOapplications.

p3.52 06/23(a) Draw labeled diagrams of the band structures of an insulator, a

semiconductor, and a conductor. With reference to your diagrams, describe the differences in electrical resistance between insulators, semiconductors and conductors.

(b) Explain how the addition of trace amounts of certain elements, such as phosphorus, can change the electrical resistance of semiconductors at a given temperature.

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p3.55Define the terms valance band and conduction band. Draw an energy level diagram for the semiconductor and diamond.

If a semiconductor has a forbidden band gap of 0.8 eV, what is the maximum wavelength of electromagnetic radiation that can be used to excite an electron from the valance band to the conduction band in this material? What part of the electromagnetic spectrum does this correspond to? Repeat the calculation of diamond, energy gap of 5.0 eV.

p3.57Explain why the resistance of a semiconductor decreases as its temperature is increased but for a metal it increases. Discuss conduction in metals, insulators and semicounductors.

p3.77Assess the impact of the invention of the transistor on society.

p3.90 05/13A doped silicon semiconductor has the following energy-level diagram.What element was most likely used to dope the silicon?

p3.92 05/15A current is passed along a square semiconductor rod as shown. Half of the current is carried by electrons and half by holes. A magnetic field is then applied to the rod at right angles to its axis. Describes the movement of the electrons and holes in the rod when the magnetic field is applied?

p3.95 05/25A student conducts an experiment using a photoelectric cell. Light is shone through a grid onto a metal surface. The metal is at earth potential and the grid is at 100 V, so that any electrons emitted from the surface produce a current in the external circuit. The student shines light sources of different photon energies onto the metal surface and records the current flowing for each. The light sources are adjusted so that their intensities are equal. The results are recorded in the table.

Photon energy (eV) Photo-current (μA) 0.5 0 0.9 0 1.2 0.5 DO PHYSICS ONLINE 10

1.70 2.8 1.75 4.0 1.90 8.0 2.20 9.2 2.50 9.4

(a) Plot the graph (X-axis – photon energy).(b) Draw the straight line of best fit in the region where the photo-current

varies greatest with photon energy.(c) From the line drawn on your graph, estimate the minimum energy

(work function) for photoelectric emission. What wavelength for the incident em radiation does this correspond to and what part of the em spectrum?

(d) The experiment is repeated, but the intensities of the light sources are doubled. Predict the results of this new experiment by drawing a second line on the graph.

(e) Explain the shape of the curves.

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p4.04 07/12 06/11The Bragg experiment used X-rays to investigate the structure of crystals. (a) What property of waves was the basis of their technique? (b) What single statement best describes the results of this experiment?

p4.14 KMagnetic levitation is being investigated and tested for use in a number of areas including transportation systems.(a) What is meant by the term magnetic levitation(b) Explain how magnetic levitation occurs in terms of the properties of the substances

involved.(c) Describe the benefits and limitations of magnetic levitations as used in maglev trains.

p4.20 07/23The table shows the critical temperature Tc at which some materials become superconducting.

(a) What is meant by the terms critical temperature and superconducting?(b) What are the critical temperatures in oC?

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Year discovere

d

Material Tc (K)

1941 Niobium nitride_ 161987 YBa2Cu3O7(YBCO)_ 921993 HgBa2Ca2Cu3O7 133

(c) What are scientists working in the area of superconductivity trying to achieve?

p4.44 05/24Explain how superconductivity occurs according to the BCS theory.

p4.49 08/14The Meissner Effect occurs when a magnet is released above a superconductor that has been cooled below its critical temperature and the magnet hovers above the superconductor. What is the best explanation for this?

p4.50 03/23Compare the model for the conduction of electricity in metals at roomtemperature with the model for conduction of electricity in superconductorsbelow the critical temperature.

p4.60 02/12Sketch a labeled graph to show how the resistance of a superconducting material changes as its temperature drops below its critical temperature.

p4.75 06/21(a) Superconductors offer a way of eliminating heating effects in many applications.

Evaluate the usefulness of using superconductors to eliminate energy losses in two different applications.

(b) Assess the impact on society and the environment of the potential applications of superconductors.

p4.78 The diagram shows two pieces of the same superconducting material above and below its critical temperature. The pieces are placed in a strong magnetic field. Complete the diagram to show the magnetic field in the region of both pieces. Explain your completed diagram.

p4.80Discuss the possible applications of using superconducting technology in computers.

p4.88 03/23A magnet can hover above a superconducting disk.

(a) What is meant by a superconductor?(b) Explain why the magnet is able to hover

above the superconductor.(c) Name this effect.(d) List two practical applications of the effect.

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superconductors in a magnetic field

above critical temperature

below critical temperature

MISCELLANEOUS PROBLEMS

m008 01/25A student carried out an experiment on the photoelectric effect. The frequency of theincident radiation and the energy of the photoelectrons were both determined frommeasurements taken during the experiment.

(a) Graph the results.(b) How could the reliability of the experiment be improved?(c) The intensity of the incident radiation was tripled. How does it change the graph?(d) Define the term critical frequency and estimate its value.(e) Define the term work function and estimate its value.

m013 06/13The temperature of a metal is reduced. What is the change in its electrical resistance and the reason for this change?

m020 02/27Discuss how energy savings can be achieved in each of the two applications of superconductors.

m045 05/14An FM radio station transmits at a frequency of 102.8 MHz.What is the energy, in joules and in eV of each photon emitted by the transmitter?What is the wavelength of the transmitted signal.

m047 06/27J. Plücker was the first to observe cathode rays within gas discharge tubes. He inferred that the rays were a form of electromagnetic radiation.(a) Describe a subsequent observation that led other scientists to argue that cathode rays

were charged particles.(b) Identify a potential hazard associated with performing experiments with discharge

tube.(c) Outline a safe work practice which addresses this hazard.

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m049 05/27Bubble chambers are used in conjunction with particle accelerators to photographically record the tracks of fast-moving charged particles. An intense magnetic field is applied at right angles to the path of the particles to deflect them according to their charge and momentum. The diagram shows a beam of protons travelling horizontally at 6.00107 m.s-1 and entering a liquid hydrogen bubble chamber in a vertical magnetic field of 1.82 T. Examination of the photograph taken by the camera, as sketched below, shows that the protons were deflected along a circular path of radius 0.350 m.

(a) Derive an expression for the momentum of a proton from the forces it experiences in this experiment.

(b) Calculate the mass of a proton in the bubble chamber.(c) Calculate the rest mass of a proton found from this experiment.

m050 05/11The discharge tube shown below contains a rotating paddle wheel that is free to move.The tube’s electrodes are connected to a high-voltage source.What does it tell you about cathode rays?

m065A cathode ray tube and transistor circuits in a conventional television rely on transformers. What type of transformer are needed for the cathode ray tube and transistor circuits? Explain.

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m072 05/26The diagram shows two parallel horizontal metal plates connected to a DC source ofelectricity. Suspended between the plates is a charged particle of mass 9.610-6 kg.

(a) Using conventional symbols, draw the electric field between the metal plates onthe diagram above.(b) Determine the magnitude of the electric field between the plates.(c) Determine the sign and magnitude of the charge on the particle if it is suspendedmotionless between the plates.

m076Describe how an investigation can be performed to demonstrate the production and reception of radio waves.

m080The electric deflecting plates in a cathode ray tube are 12 mm apart and a potential difference of 1234 V across them. A beam of positively charged particles moving at 2.67104 m.s-1 travels through the plates undeflected. (a) Sketch the experimental setup.(b) Calculate the electric field between the plates.(c) Calculate the magnetic field strength.

m082 08/21The work of scientists is influenced by external factors. Do you agree? Justify your answer with reference to the work of Einstein and Planck.

m088 04/14The minimum amount of energy needed to eject an electron from a clean aluminumsurface is 8.72 × 10–19 J. What is the work function for aluminum in eV? What is the maximum wavelength of incident light that can be shone on this aluminum surface in order to eject electrons? What is the threshold frequency? What part of the electromagnetic spectrum is the incident radiation?

m095 01/26Explain the concept of electrons and holes for conduction in semiconductors.

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m111 06/15When electromagnetic radiation shines on metals, photoelectrons may be emitted. The maximum kinetic energy of emitted photoelectrons is plotted against radiation frequency for four metals. Electromagnetic radiation of wavelength 187 nm shines upon an unknown metal and the maximum kinetic energy of the photoelectrons is found to be 2.5 eV. What is the unknown metal?

m115The critical temperature of mercury is 4.2 K. What does this mean? Compare how electric current is conducted through a sample of mercury at 293 K and 3 K. How does this differ from the conduction in a p type semiconductor.

m120 03/25When the laser light was shone onto a photocell, no current was detected. The intensity of the light was increased but still detected no current. Explain this observation.

m200 04/15The graph shows the intensity–wavelength relationship of electromagnetic radiationemitted from a blackbody cavity. In 1900, Planck proposed a mathematical formula that predicted an intensity–wavelength relationship consistent with the experimental data.The success of this formula depended on what hypothesis?Comment on the curve shown.

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m400 01/24Sir William Bragg and his son Sir Lawrence Bragg shared the Nobel Prize for physicsin 1915 for their work on X-ray diffraction and crystal structure analysis.

(a) Describe ways in which an understanding of crystal structure has impacted on science.

(b) Outline the methods of X-ray diffraction used by the Braggs to determine the structure of crystals.

m550In a modern cathode ray tube (CRO), the electrons are produced by thermionic emission from a hot filament forming the cathode. The heating of the filament by a low voltage DC power source causes electrons to be released from the hot filament with nearly zero kinetic energy. The electrons are then accelerated away from the filament by a large potential difference applied between the accelerating voltage plates. This forms a narrow electron beam with the electrons traveling at the same speed and impact onto the fluorescent screen to produce a bright spot in the centre.

The accelerating voltage was 5.00 kV. When a magnetic field was applied perpendicularly to the electron beam, the electrons (cathode rays) followed a circular path with a radius of 5.57 mm. Calculate the strength of the magnetic field.

m680Hertz used a high voltage source to generate a spark and realised that when another spark was produced at a receiving coil, energy must have been transferred. Early in his experiments Hertz made a chance observation when he could increase the strength of the spark in the receiving coils. Which of the following correctly describes what Hertz had done that led to this observation? Comment on each alterative. What was the significance of Hertz’s observation? (a) He had shone UV light on the receiving coil.(b) He had slightly increased the gap between the terminals of the receiving coil.(c) He had placed a glass plate between the transmitter and receiver.

m690List the properties of the photoelectric effect that could not be explained by classical theory of electromagnetic waves.

m700What was the connection between Hertz and the photoelectric effect?

m820 G950(a) Estimate the temperature of the surface of the Sun, given that the Sun emits light at a

peak wavelength around 550 nm.

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transmitter

receiver

high voltage source

spark gaps

induction coil

(b) Which star is hotter, a blue or a red star. Explain

m950An electron moving with a velocity of 4.56×106 m.s–1 enters a uniform magnetic field of strength 3.21×10-2 T at and angle of 42°.

(a) Describe the path of the electron traveling through the magnetic field.

(b) Calculate the radius of the path followed by the electron?

m955 03/12In a first-hand investigation that you performed, you used a discharge tube containing a Maltese Cross.

What information does the picture tell you?

m960An electron beam is a special discharge tube is shown. Show how the electron beam would be deflected by the magnet. What would happen if the S pole was brought near the electron beam. Explain your answers.

m964 02/11Describe the difference between an intrinsic semiconductor and an extrinsic semiconductor.

m965 03/26Describe Einstein’s contributions to Special Relativity and to Quantum Theory and how these contributions changed the direction of scientific thinking at the beginning of the 1900s.

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m970The flowchart represents a model of the scientific method used to show the relationship between theory and the evidence supporting it.

Analyse Einstein’s Theory of the Photoelectric Effect and the evidence supporting it as an application of this model of scientific method.

m990Max Planck and Albert Einstein were both personally affected by the years before, during and after WWII. They also had strong views about the role science in the period before the war. Discuss Planck’s and Einstein’s differing views about whether scientific research is removed from social and political issues.

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