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PHOTOELECTRICITY

1) Estimate the number of photons emitted by the lamp in each second. (ans: 1020 s-1)

2) Electromagnetic radiation of wavelength 2.50 10-7 m is shone on a piece of pure sliver.

The work function of silver is 4.73 eV. Calculate: (a) the energy transferred by each photon,(b) the threshold frequency of silver, (c) the maximum speed of the photoelectrons, (ans:

7.96 10-19 J, 1.14 1015 Hz, 2.96 105 ms-1)

3) Light with a wavelength of 500 nm is shone on a photocell and the stopping potential is

measured to be 1.00 V. calculate the work function, in eV, of the cathode of the photocell.

(1.49 eV)

4) Orange light with a wavelength of 6.00 102 nm is directed at a metallic surface with a

work function of 1.60 eV. Calculate

(a) the maximum kinetic energy, in joules, of the emitted electrons(b) their maximum speed

(c) the cutoff potential necessary to stop these electrons

(7.55 10-20

 J, 4.07 105 ms

-1, 0.472V)

5) Yellow – green light of wavelength 500 nm shines on a metal whose stopping voltage is

found to be 0.80 V. Find: (a) the speed of the fastest moving photoelectron produced, and

(b) the work function of the metal in both joules and electronvolts. (5.3 × 105 m s

−1, 1.68 eV)

6) Potassium has a threshold frequency of 5.4 × 1014

Hz, and when illuminated by ultraviolet

light of frequency 9.0 × 1014 Hz, photoelectrons with a stopping voltage of 1.5 V are

 produced. Determine a value for Planck’s constant. (6.7 × 10−34 J s)

7) Electromagnetic radiation of frequency 0.88 1015

 Hz falls upon a surface whose work

function is 2.5 eV. (i) Calculate the maximum k.e of photoelectrons released from thesurface. (ii) If a nearby electrode is made negative with respect to the first surface using a p.d

V , what value is required for V  if it is to be just sufficient to stop any of the photoelectrons

from reaching the negative electrode?

8) The maximum kinetic energy of photoelectrons ejected from a tungsten surface, by light

of wavelength 248 nm, is 8.6 10-20 J. What is the work function, in eV, of tungsten?

(ans: 4.46 eV)

9

Ultraviolet light of wavelength 280 nm is used in an experiment on photoelectric effect

with lithium ( 2.5 eV) cathode. Find (a) the maximum kinetic energy of the photoelectrons

and (b) the stopping potential. (ans: 1.9 eV, 1.9 V)

10) In a photoelectric experiment, it was found that the stopping potential decreases from

1.85 V to 0.82 V as the wavelength of the incident light is varied from 300 nm to 400 nm.

Calculate the value of the planck constant from these data. (answer: 4.12 × 10-15 eVs).

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11) A beam of 450 nm light is incident on a metal having work function 2.0 eV and placed

in a magnetic field of strength B. The most energetic electrons emitted perpendicular to the

field are bent in circular arcs of radius 20 cm. Find the value of B. (answer: 1.46 × 10-5

 T)

WAVE-PARTICLE DUALITY

1) What de Broglie wavelength is associated with a 0.10 kg ball moving at 19.0 m/s?

(3.5 10-34

 m)

2) Find the de Broglie wavelength of an electron that has been accelerated from rest through

a potential difference of 50 V. (1.7 × 10−10 m)

3) Determine the wavelength of a beam of electrons which is accelerated through a p.d of

100 V. Comment on whether the electron beam will be diffract after passing through a

crystalline solid.

4) Suppose an electron, a proton and a neutron have the same kinetic energy. Which particle

has the (i) shortest, (ii) longest de Broglie wavelength?

LINE SPECTRA

1) The diagram shows four energy levels of an

imaginary atom.(a) Calculate the ionization energy, in joules, of the

atom.

(b) Considering transitions between only these levels,(i) Calculate the shortest wavelength of the e.m

radiation (emitted or absorbed)

(ii) State the maximum number of possible electron

transitions. Hence sketch the spectrum of the

emission lines produced by downward electron

transitions.

2) The energy levels for atomic mercury are depicted in

the diagram on the left.

(a) Consider the mercury atom with its valence electron

in the ground state.Ultraviolet light with photon energies 4.9, 5.0 and 10.50

eV is incident

on some mercury gas. What could happen?

(b) Determine the wavelength of the light emitted afteran electron in an excited mercury atom makes the

transition from n = 3 to n = 1.

n=1-47 eV

n=2-22 eV

n=3-13 eV

n=4-9 eV

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3) Energy levels for an hydrogen atom is shown

on the left. A photon of energy 14.0 eV collided

with a hydrogen atom in the ground state.

(a) Explain why this collision will eject an

electron from the atom.(b) Calculate the energy of the ejected electron

in electronvolts and in joules.

(c) What is the momentum of the ejected

electron?

(d) Determine the wavelength of the ejected electron?

4) The energy levels for atomic mercury are shown

on the left diagram.

Electrons of energy 4.0 eV travel through a glass tube

containing mercury vapour.

(a) Will any photons be emitted from the mercuryatoms in the tube? Justify your answer.(b) Explain why there is a large increase in current

through the circuit when electrons ofenergy 14 eV pass through the vapour.

(c) What wavelength light will be emitted from the

tube when V = 6.2 V?

ANSWERS: (2) The 4.9 eV photon may be absorbed, promoting the electron from the

ground state to the first excited state. The 5.0 eV photon cannot be absorbed since there is no

energy level 5.0 eV above the ground state. The 10.5 eV photon may ionise the mercury

atom. In this case, the ejected electron will leave the atom with 0.1 eV of kinetic energy. 185

nm (in the ultraviolet)

(3) (a) Photon energy > ionisation energy i.e. there is enough energy to free the electron.

(b) 0.4 eV = 6.4 × 10 – 20 J, (c) 3.4 × 10 – 25 kg m s-1, (d) 1.9 × 10 – 9 m

(4) (a) Incident energy insufficient for any excitation. At least 4.90 eV required. (b) Electrons

have escaped mercury atoms and conduct current across tube. (c) 2.5 × 10 – 7 m

BAND THEORY

1) (a) Describe how the simple model of band theory is used to explain conduction in

intrinsic semiconductors.(b) Describe how the simple model of band theory is used to explain the temperature

dependence of metals and intrinsic semiconductors.

(c) Describe how the simple model of band theory is used to explain the dependence on lightintensity of the resistance of an LDR.

2) The diagram shows the energy levels of an imaginary atom:

(a) Sketch the energy bands for a solid formed by a lattice of such atoms, if thesolid is (i) insulator, (ii) instrinsic semiconductor, (iii) metal

(b) Using the band theory, explain why metals are good electrical conductorswhile insulators are poor electrical conductors.

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3) The electrical conductivity of thermistor increases with temperature. Would you expect

thermistors to be made of metals, insulators or intrinsic semiconductors? Explain your answer

using the band theory.

4) A hypothetical semiconductor has a conduction band of width 0.34 eV and a valence band

of width 0.22 eV. The forbidden band is 1.17 eV.(a) Distinguish between conduction band and valence band

(b) The electrical conductivity of the semiconductor can be increased by firing photons of a

 particular range of wavelengths onto the semiconductor. Explain how these incident photons

help to increase the electrical conductivity of the semiconductor.

(c) Determine the range of the wavelengths of the photons that will allow the process

described in (b) to occur. Comment on what will happen if a photon of a wavelength outside

this required range is incident upon the semiconductor.

X-rays

1) (a) The quality of an image produced using X-rays depends on sharpness and contrast.

State what is meant by, and briefly explain the causes of (i) sharpness, (ii) contrast.(b) A parallel beam of X-ray photons is produced by an X-ray tube with 80 keV across it.

The beam has its intensity reduced to one half of its original value when it passes through a

thickness of 1.0 mm of copper.

(i) Describe the energies of the X-ray photons in the beam.

(ii) Determine the linear absorption coefficient of the X-ray photons in copper. (iii)

Suggest, with a reason, the effect on the linear absorption coefficient if the beam is

comprised of 100 keV photons. (ans: 0.693 mm-1)

2) (a) Define for a material, (i) the linear attenuation coefficient, µ, (ii) the half thickness. (b) A monochromatic X-ray beam of intensity 6.0 Wm

-2 is incident on an aluminium sheet of

thickness 2.0 mm. For these X-rays, the half-value thickness of aluminium is 3.2 mm.

Calculate the intensity of the transmitted beam. (ans: 3.8 Wm-2

) 

3) (a) The intensity of a parallel monoenergetic X – ray beam after passing through a

thickness x of a medium is given by the equation   xe I  I 0

, where I 0 is the incident X-ray

intensity and is a constant. Give two factors that determine the value of  .

(b) A parallel monoenergetic X – ray beam passes through 2.5 cm of a material. Calculate the

value of if the emergent X-ray beam has an intensity of 0.42 of its initial value.

(ans: 34.7 m

-1

)

4) (a) Briefly explain the principles of the production of an X-ray beam.

(b) State how, in an X-ray tube, (i) the intensity of the X-ray beam may be increased,(ii) the X-ray beam may be made more penetrating, (iii) the unwanted ‘soft’ X-rays may be

removed from the beam.(c) The intensity of a parallel X-ray beam is reduced to one half of its initial intensity when it

 passes through a bone of thickness 0.40 cm. Calculate the thickness of bone necessary toreduce the beam intensity to one tenth of its initial value. (ans: 1.33 cm)

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5) X-rays of a specific energy are used to image a suspected leg bone fracture. The patient’s

leg is 15 cm thick and the bone is 5 cm thick. At this particular energy the linear attenuation

coefficient for tissue is 5 m-1

 and the linear attenuation coefficient for bone is 60 m-1

.

(a) Show that after the X-rays pass through the leg their intensity will have been reduced

about ten times more by the bone than by the tissue.

(b) Explain why an X-ray beam of this particular energy is suitable for detecting a legfracture.

6) The linear attenuation coefficients of X-rays in muscle and in bone are 0.89 cm-1 and 3.0

cm-1

 respectively. In a particular limb, a bone of thickness 1.5 cm is surrounded by muscle of

thickness 3.0 cm. A parallel beam of X-rays is incident on the limb. Calculate the fraction of

the incident intensity that is transmitted through the limb. (ans: 5.3 10-5)

NUCLEAR PHYSICS

m p = 1.007276u, mn = 1.008665u, me = 0.0005494,

1 u = 1.66 10-27 kg = 1.494 10-10 J = 934 MeV, 1 MeV = 1.6 10-13 J 

1) A carbon -14 ( C 146 ) nucleus has a mass of 14.00324u. Calculate its (i) mass defect, in u;

(ii) binding energy, in MeV; (iii) binding energy per nucleon, in MeV.

(ans: 0.109736 u, 102 MeV, 7.28 MeV)

2) Uranium-235 may undergo fission when bombarded by a neutron to produce Barium-141

and Krypton-92 as shown: n Kr  BanU    10

92

36

141

56

1

0

235

92  3  

The binding energy per nucleon of each nucleus is given in the table below:

Isotope binding energy per nucleon/MeV

Uranium-235 7.6

Barium-141 8.3

Krypton-92 8.5

Use the data in the table to calculate:

(i) the energy, in MeV, released in this fission reaction.

(ii) the mass equivalent of this energy

(iii) the energy, in J, released by 1 kg of uranium-235 fissioned.

(ans: 166.3 MeV, 2.97 10-28 kg, 6.82 1013J)

3) Fission of a Uranium -235 nucleus can be induced according to the equation:

The atomic masses of the nuclei are:U-235 nucleus=235.04277u, Mo-95 nucleus = 94.90553u, La-139 nucleus = 138.90534u

Mass of neutron = 1.00866u. Calculate the energy released in this reaction in joules. Any k.e

of the interacting particles can be neglected. (3.33 10-11

J)

4) Calculate the energy in MeV released by fusing two protons and two neutrons to form a

helium nucleus. The atomic mass of hydrogen and helium are 1.007825u and

4.002604u respectively. (28.3 MeV)

5) An -particle has a mass of 4.032u. Calculate its mass in Mev/c2. (3760Mev/c

2)

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6) A positron has the same mass as an electron. Calculate the energy released when an

electron and a positron annihilate. (1.64 10-13J)

7) The nucleus has a mass 10.01294u. Calculate its mass defect and binding energy.

Express the binding energy in MeV per nucleon. (1.11 10-28

 kg, 9.98 10-11

J, 6.24 Mev)

8) The nucleus has a mass 53.93962u. Calculate its binding energy in MeV per

nucleon. (8.51 MeV)

9) The main nuclear fusion reactions at the Sun’s core are summarised by this equation:

, where e+ is a positive electron (a positron)

(a) Fill in the missing numbers x, y, z .

(b) Calculate the energy released by the fusion of 1kg of hydrogen nuclei. (mass of He+ 

nucleus = 4.00260u, mass of e = 0.00055u, treat e as having zero mass) (ans: 5.72 10

14 J)

RADIOACTIVE DECAY

1) An isotope decays by β-emission to produce nuclide . Write down the nuclear equation

for this decay. Deduce the element of the isotope.

2) Radioisotope decays to nuclide in a series of radioactive decays. What is the

value of A if there are no beta decays.

3) Protactinium, Pa, decays to uranium by beta emission. The uranium produced is

itself radioactive and decays by alpha emission to thorium, Th. Deduce the atomic no. and

mass no. of thorium and protactinium.

4) The nuclide decays to the nuclide by 4 series of radioactive decay. What is the

value of A  if in each decay either an -particle or  β -particle is emitted

5) Beta radiation from a certain source can be stopped completely by a sheet of aluminium

3.0 mm thick. To a fair approximation, the ability of any sheet of material to stop beta

radiation depends only on the mass per square metre of a sheet of the material. Estimate the

thickness of lead sheet needed to stop the same beta radiation completely. ( aluminium = 2.7 x

103 kgm-3, lead = 11.3 x 103 kgm-3)

6) The diagram opposite shows the path followed by one alpha particle which passesclose to a gold nucleus N.

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(a) Add arrows to a copy of this diagram at points X and Y to show the direction of the forceon the alpha particle when it is at each of these points.

(b) The speed of the alpha particle was the same at points A and B. Sketch a graph showinghow the speed would vary with distance along the path from A to B.

(c) Draw a line on the same diagram to show the path which would be followed by an alpha

 particle which was travelling initially along the same line as before, but more slowly.(d) The evidence for a small, massive nucleus from Rutherford scattering might have been

less convincing if the alpha particles used had been of lower energy. Suggest how the

observations would have changed if lower energy alpha particles had been used.

LAW OF DECAY

1) (a) Calculate the decay constant in s-1 of a radioactive source that has a half life of 23

days.(b) If this source has an initial activity of 40 kBq what will its activity be 10 days later?

(ans: 3.5 10-7 s-1, 29580 Bq)

2) Calculate the activity of 1g of pure given that its half life is 4.5 109 year.

(ans: 12000 Bq)

3) The half life of is 5570 years. (a) What is its decay constant? (b) How many

disintegrations per second are obtained from 1 g of carbon if 1 carbon atom in 1012 is of

radioactive type? (c) After what time will the activity per gram have fallen to 3

disintegrations per minute? (ans: 3.94 10-12

s-1

, 0.198 s-1

, 11100 years)

4) Which of these has the higher activity: 2 1020 nuclei of isotope X, with half life 4 106

years, or 5 109

 nuclei of Y, with half life 6 hours?

5) The half life of is 7.1 s (a) Calculate the decay constant for . (b) Show that the

time taken for the mass of in a sample to decrease from 5.0 g to 1.0 g is approximately

16 s. (X is more active, 0.0976 s-1

, 16.5)

6) (a) The isotope has a half life of 8 days. One sample has an initial activity of 80 MBq.

Sketch a graph showing how the activity changes with time for the next 24 days. (b) Another

sample of the isotope has an initial activity of 56 MBq. Calculate the time for its activity

to fall to 20 MBq. (11.9 days)

7) The half life of a radioactive element is 40 days. Calculate the time taken for the activityto decay to 30% of its initial value. Find the number of emissions per second by 1 mg of the

substance after 30 days, given that the element has an atomic mass of 220.

(ans: 69.5 days, 3.26 1011 Bq)

8) At a certain instant, a piece of radioactive material contains 1012 atoms. The half  – life of

the material is 30 days.

(i) Calculate the number of disintegrations in the first second. (2.67 105 s-1)

(ii) How long will elapse before 104 atoms remain? (797.6 years)

(iii) What is the count rate at this time? (2.67ms-1)

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9) The decay constant of uranium-238 is 5.0 10-13 s-1. Calculate the activity and half-life of

2.0 mg of Uranium-238. (ans: 2.53 106

Bq, 1.39 1012

s = 43959 years)

10) The activity of a mass of C 146 is 5.0 108

Bq and the half-life is 5670 years. Estimate the

number of C 146

nuclei present. (ans: 1.29 1020)

11) Calculate the mass of  P 3015 which has an activity of 1015 Bq, given that its half-life is 2.5

minutes. (ans: 11 g) 

FUNDAMENTAL PARTICLES

1) (a) State the class of particles which includes protons and neutrons.

(b) A proton inside a nucleus decays into a neutron. Write an equation to represent this decay.

(c) State the composition of a proton in terms of quarks.

(d) Describe the decay of the proton in (b) in terms of quarks.

2) The type of decay for a number of caesium isotopes is shown in the table:

Isotope

Type of decay

(i) Explain the term isotope.(ii) State the interaction between quarks that gives rise to β decay. 

(iii) Describe the structure of a neutron in terms of quarks.

(iv) Describe decay in terms of the simple quark model.

(v) State two quantities that are conserved in a β decay. (vi) State the fundamental particles produced during the decay of .

(viii) State the number of protons in the nuclei produced during the decay of .

3) The table shows some of the isotopes of phosphorus and, where they are unstable, the type

of decay.

Isotope

Type of decay Stable

(i) State the difference between each of the isotopes shown in the table.

(ii) Describe the structure of the proton in terms of up (u) and down (d) quarks.(iii) Describe what happens in a beta-plus ( ) decay using a quark model.

(iv) State two quantities conserved in beta decay.(v) Examine the table of isotopes in the table and suggest what determines whether an isotope

emits or .

4) Particle production and annihilation are subject to conservation laws. Two of these laws

are conservation of mass-energy and conservation of momentum.

Free neutrons are unstable. A neutron may decay to become a proton with emission of an

electron. A student represents the decay by the following equation

(a) State, by reference to conservation laws, why the student’s equation is not correct.

(b) Write down the correct decay equation.

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{Family 1 lepton number is not conserved, Equation needs Family I lepton with no charge

and L = -1. fits the bill. }

Use conservation principles to find out if the following reactions are possible:

(i) , (ii) , (iii) , (iv)(v) , (vi) , (vii) ,

(viii) , (ix) , (x)

6) Up quarks have a charge of and down quarks have a charge of .

(i) State the number of each type of quark in a neutron.(ii) Explain in terms of charge why a neutron has this composition.

(b) (i) A neutron decays by β emission. Complete the follo wing decay equation, naming allthe particles produced in the decay.

neutron → β (electron) + .................................................................................. (ii) State and explain the change of quarks which occurs when this decay happens.

7) (a) State the combination of quarks that makes up a neutron.

(b) When a neutron decays, a down quark changes into an up quark as shown by the

following reaction.

(i) Show, in terms of the conservation of charge, baryon number and lepton number, that this

transformation is permitted.

(ii) State the products arising from the decay of an anti-down quark,

8) Leptons, mesons and baryons are three classes of sub-atomic particles.(a) Some classes of particles are fundamental; others are not. Circle the correct category for

each of these three classes.

leptons fundamental/not fundamental

mesons fundamental/not fundamental

 baryons fundamental/not fundamental

(b) Name the class of particles of which the proton is a member.

(c) By referring to the charges on up and down quarks explain how the proton has a charge of

+1e.

9) A negative pion ( ) is a meson with a charge of .

State and explain the structure of the in terms of up and down quarks.

10) A physicist, who is attempting to analyse a nuclear event, suggests that a particle and

a proton collided and were annihilated with the creation of a neutron, a particle, and a

 particle. and particles are mesons. The baryon and lepton numbers of both these mesons

are zero.

(a) Write down the equation that represents this interaction.

(b) Show, in terms of the conservation of charge, baryon number and lepton number, that this

transformation is permitted.

11) (a) A particle is made up from an anti-up quark and a down quark.

(i) Name the classification of particles that has this type of structure.(ii) State the charge on and baryon number of the particle.

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(b) A suggested decay for the positive muon is

Showing your reasoning clearly, deduce whether this decay satisfies the conservation rules

that relate to baryon number, lepton number and charge.

12) The equation for decay can be written as .

(i) For each particle, either give its quark composition or state that it is a fundamental particle.

(ii) Write a similar equation for decay.

(iii) Explain why these reactions can only be mediated by the weak interaction.

13) During an experiment into the energy spectrum of

 particles, the following graph was produced.

(a) Label each axis of the graph with appropriate units.

(b) State the significance of the figure 0.78.

(c) Explain why this energy spectrum of the particles led

to the suggestion that an additional undetected particle must be emitted during the nuclear decay process. State the

missing particle.

14) (a) State the names of the 2 classes of particle, each of which includes both the protonand the neutron.

(b) It is thought that, in certain circumstances, the proton has a slight probability of decayinginto a neutron, a positron and a third particle.

Write an equation to represent this reaction.

State the name of the third particle.(c) A free neutron is known to decay with a half-life of about 10 minutes.

In what situation are both neutrons and protons stable?(d) (i) State the quark composition of (i) the proton, (ii) the neutron.

(ii) In the reaction two quarks are created. These are a down quark

( ) and an anti-down quark ( ). Simplify this equation and using your answers to (d) (i),

write a quark equation.

(iii) Hence deduce the quark composition of the particle. (OCR Jun04)

15) Tritium-3 ( ) decays to helium-3 ( ) with the emission of a particle.

(i) Name the force responsible for this decay process.

(ii) Write a nuclear equation to represent this process.

(iii) Write a quark equation, in its simplest form, to represent this process. (OCR Jan04)

16) (a) The table of Fig. 6.1 shows four particles and three classes of particle.

hadron baryon lepton

 Neutron

Proton

Electron

neutrino

Indicate using ticks, the class or classes to which each particle belongs.

(b) The neutron can decay, producing particles which include a proton and an electron.

(i) State the approximate half-life of this process.

0.78

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(ii) Name the force which is responsible for it.

(iii) Write a quark equation for this reaction.

(iv) Write number equations which show that charge and baryon number are conserved in thisquark reaction.

(c) Fig. 6.2 illustrates the paths of the neutron, proton and electron only in a decay process of

the kind described in (b).

Fig. 6.2

Fig. 6.3 represents the momenta of the neutron, , the proton, and the electron, on avector diagram.

Fig. 6.3

(i) Draw and label a line on Fig. 6.3 which represents the resultant of vectors and .

(ii) According to the law of momentum, the total momentum of an isolated system remains

constant. Explain why the momentum is not the same as . (OCR Jan 05)

17) This question is about deducing the quark structure of a nuclear particle.

When a meson collides with a proton, the following reaction can take place

is a particle whose quark structure is to be determined.

The quark structure of mesons is given below.

Particle Quark Structure

(a) State and explain whether the original particle is a hadron or a lepton 

(b) State the quark structure of the proton.

(c) The quark structure of particle 3 is . Show that the reaction is consistent with the

theory that hadrons are composed of quarks.

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18) This question is about fundamental particles and conservation laws.

 Nucleons are considered to be made of quarks.

(a) State the name of the force (interaction) between quarks.

(b) Outline in terms of conservations laws, why the interaction is observed

 but the interaction has never been observed. (You may assume that mass-

energy and momentum are conserved in both interactions.)

19) This question is about the conservation laws that govern the production, decay and

interactions of fundamental particles.

Use the data in the table below to answer the following questions.

ParticleMass

(MeVc-2

)

Charge

Q

Baryon number

BLepton number L

 Neutron 939.6 0 + 1 0

Proton 938.3 + 1 + 1 0Antiproton ( 938.3 - 1 - 1 0

Electron ( 0.511 - 1 0 + 1

Antielectron ( 0.511 + 1 0 - 1

Pion ( 139.6 + 1 0 0

Pion ( 139.6 - 1 0 0

Lambda ( 1116 0 + 1 0

 Neutrino ( 0 0 0 + 1

Antineutrino ( ) 0 0 0 - 1

Gamma photon ( ) 0 0 0 0

The decay processes given below do not occur in nature. Determine and list the

conservation laws that are violated in these processes. For each suggest a possible correctdecay / interaction process.

Assume that the decaying / interacting particles are initially at rest.

(i) Neutron decay: .

Does not occur because:

Process which does occur is:

(ii) Lambda decay:Does not occur because:

Process which does occur is:(iii) Electron annihilates with a positron:

Does not occur because:

Process which does occur is:

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20) The diagram shows the main features of a bubble chamber photograph in which a pion

has collided with a stationary proton (reaction A). followed by two subsequent decays(reactions B and C).

The following information may be useful:

Particle Baryon number B Strangeness S

0 0

1 - 1

1 0

0 1

(a) Write an equation for reaction A.

(b) Show that charge, baryon number and strangeness are all conserved in reaction A.

(c) How is it possible for there to be more pions at the end than at the

 beginning of the reactions? Where did they come from?

(d) In addition to the quantities mentioned above, what else must be conserved in all three

reactions?

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MCQ ON CHANGE OF RESISTANCE WITH TEMPERATURE

1) The electrical conductivity of a metal decreases with increase in temperature becauseA the concentration of conduction electrons decreases

B the average distance between the metal ions increases

C the speed of the electrons in the metal increasesD the scattering of the conduction electrons by the metal ions increases

2) The electrical conductivity of an intrinsic conductor increases with increasing temperature.

The reason for this is that

A the energy required to excite an electron is less at higher temperatures

B the ratio of the number of electrons to the number of holes is greater at higher temperatures

C the probability of thermal excitation of electrons is greater at higher temperatures

D the drift velocity of the electrons and holes is greater at higher temperatures

3) A surge of current flows through a filament lamp when it is first switched on. The reason

for the surge is thatA when the lamp is switched on, the filament is cold and its resistance is much less than at its

working temperatureB the mains voltage may be at its peak value when the lamp is switched on and the current

will then be greater than its r.m.s valueC mains switches are spring-loaded and make sudden contact, not allowing time for the

current to increase gradually

D the parallel conductors in the mains cable act as a capacitor and this capacitor discharges

itself through the filament

4) The process of electrical conduction in a metal may be described by a model in which the

free electrons form a ‘gas’ and collide with the atoms of the metal. According to this model,

an increase in temperature causes the conductivity of the metal to decrease because

A the root mean square speed of the free electrons increases

B the number of free electrons decreases

C the mean time between collisions of electrons with atoms decreases

D the mean distance between atoms increases

5) The resistance of a semiconductor decreases rapidly with increasing temperature. Themain factor contributing to this effect is the rapid increase, with increasing temperature, of

A the speed of the random motion of the free charge carriers

B the concentration of the free charge carriersC the drift velocity of the free charge carriersD the frequency and amplitude of vibration of the atoms of the semiconductors

6) The resistance of a piece of pure silicon falls rapidly as the temperature rises because

A the ratio of positive to negative charge carriers increases

B the ratio of positive to negative charge carriers decreases

C random motions of the charge carriers are reduced

D the total number of charge carriers increases with temperature

Ans: (1) D, (2) C, (3) A, (4) C, (5) B, (6) D

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MCQ ON BAND THEORY

1)  The diagrams below show the two highest energy bands of 3 matters.

Which of the following is a possible matter for each set of energy bands as shown?

I II III

A Germanium Plastic Silver

B Tungsten Gold Silicon

C Wood Silicon Copper

D Nickel Germanium Water

2) Which statement about conduction of electricity in solids is correct?

A Free electrons are found both in the conduction band and in the valence band

B In a metal, there is a large energy gap between the conduction and valence bandsC The presence of impurities in an intrinsic semiconductor is used to increase its resistance

D In an intrinsic semiconductor, electrons travel in the opposite direction to holes (N07/37)

3) The diagram illustrates the upper energy bands in two different classes of solid at absolute

zero. The shaded areas represent occupied electron energy levels.

What are bands P and Q, and what are the classes X and Y of the solids? (N07/38)

band P band Q solid X solid Y

A conduction valence intrinsic semiconductor metal

B conduction valence metal intrinsic semiconductor

C valence conduction intrinsic semiconductor metalD valence conduction metal intrinsic semiconductor

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4) Which statement about the energy bands in an ideal intrinsic semiconductor is correct?

A The conduction band lies just below the valence band.B The number of electrons in the conduction band equals the number of holes in the

valence band.

C There is an energy gap of 5 eV to 10 eV between the valence and conduction bands.D There is a small overlap between the valence and conduction bands. (N09/38)

5) Which of the following statements below on intrinsic semiconductors is true?

A The total current flow is the sum of both ‘hole’ and ‘electron’ currents.  

B There are more electrons in the conduction band than there are holes in the valence

 band.

C The valence band is completely filled and the conduction band is partially filled.

D The valence band is completely filled and the conduction band is empty at room

temperature.

ANSWERS

(2) Holes behave like positive hence they travel in the opposite direction to electrons

under an applied electric field. Answer D 

Answer A is wrong because free electrons are found only in the conduction band. The

valence band is completely filled hence the electrons within the valence band are not

free to move to the higher occupied levels within the band.

Answer B is wrong because in a metal, the conduction and valence bands overlap. The

conduction band is partially filled hence there is no band gap.

Answer C is wrong because impurities reduce, not increase, the resistance ofsemiconductors.

(3) The higher energy band is the conduction band and the lower energy band the valen

At absolute zero, the valence band in a semiconductor is completely filled conduction

band is empty. There is a band gap between the two bands. On the other hand, for a

metal, the conduction band is partially filled hence there are plenty of empty energy

levels for the electrons to be excited to. Hence electrons flow freely in a metal under the

application of an electric field. Answer A

(4) The conduction band lies above the valence band.

The energy gap between the valence and conduction bands is about l eV.There is no overlap between the valence and conduction band except for conductors.

Only B is correct.

(5) A (1) C