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HSC Physics · Module 8 flashcards
From the Universe to the Atom
49 cards. Work out each answer first, then open the card to check it and see the working.
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Chadwick identified the neutron by observing that radiation from beryllium bombarded with alpha particles …Show answer
Answer: knocked protons out of paraffin with energies that only a neutral particle of proton-like mass could explain
The radiation was uncharged, so it was not alpha or beta. Gamma rays could not transfer enough momentum to eject protons; a neutral particle of about proton mass could.
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A nuclear reaction has a mass defect of 0.10 u. How much energy is released? (1 u = 931.5 MeV) …Show answer
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In Thomson's cathode ray experiment, the rays were deflected towards the positively charged plate. This showed that cathode rays …Show answer
Answer: carry negative charge
Attraction towards the positive plate means the particles are negatively charged.
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Uranium-238 (Z = 92) undergoes alpha decay. The daughter nucleus is …Show answer
Answer: Thorium-234 (Z = 90)
An alpha particle removes 2 protons and 2 neutrons: A drops by 4 (238 → 234) and Z drops by 2 (92 → 90).
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Millikan's oil drop experiment established …Show answer
Answer: that charge comes in multiples of a fundamental unit, and its value
Every drop's charge was a whole-number multiple of 1.6 × 10-19 C. Combined with Thomson's q/m, this also gave the electron's mass.
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Thomson's 'plum pudding' model described the atom as …Show answer
Answer: a sphere of positive charge with electrons embedded in it
Thomson knew electrons were tiny and negative, and proposed they sat within a diffuse positive sphere to make the atom neutral.
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How much energy is equivalent to 1.0 kg of mass?Show answer
Answer: 9.0 × 1016 J
E = mc² = 1.0 × (3.0 × 108)² = 9.0 × 1016 J.
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In beta-minus decay …Show answer
Answer: a neutron becomes a proton and the atomic number increases by 1
n → p + e⁻ + antineutrino. The nucleus gains a proton, so Z rises by 1 while A is unchanged.
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A Maltese cross placed in a cathode ray tube casts a sharp shadow on the glass. This shows that cathode rays …Show answer
Answer: travel in straight lines
A sharp shadow forms only if the rays move in straight lines from the cathode.
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A major problem with Rutherford's model was that, according to classical physics, the orbiting electron should …Show answer
Answer: radiate energy continuously and spiral into the nucleus
An accelerating charge radiates. An orbiting electron is constantly accelerating, so it should lose energy and collapse into the nucleus within a fraction of a second.
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A chain reaction in uranium-235 is possible because each fission …Show answer
Answer: releases two or three neutrons that can cause further fissions
The emitted neutrons (slowed by a moderator) can be absorbed by other U-235 nuclei, triggering more fissions.
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In Thomson's charge-to-mass experiment, the electric and magnetic forces were balanced so that the rays were undeflected. This gave the speed of the rays as …Show answer
Answer: v = E/B
qE = qvB, so v = E/B. The speed was then used with the magnetic deflection alone to find q/m.
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Bohr's model successfully explained …Show answer
Answer: the Balmer series of hydrogen
It reproduced the hydrogen line wavelengths and the Rydberg constant, but failed for multi-electron atoms, line intensities and the Zeeman effect.
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The ground state of hydrogen is −13.6 eV. What energy is needed to raise the electron from n = 1 to n = 2?Show answer
Answer: 10.2 eV
E2 = −13.6/4 = −3.4 eV. ΔE = −3.4 − (−13.6) = 10.2 eV.
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The Geiger–Marsden experiment fired …Show answer
Answer: alpha particles at thin gold foil
Alpha particles from a radioactive source were directed at gold foil a few hundred atoms thick, with a scintillation screen to detect their scattering.
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Before Chadwick's discovery, the neutron was difficult to detect because it …Show answer
Answer: has no charge and so does not ionise or deflect in fields
Charged particles leave ionisation tracks and bend in fields. A neutral particle does neither, so it had to be inferred from its collisions with protons.
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A proton is made of quarks with the combination …Show answer
Answer: uud
up (+2/3) + up (+2/3) + down (−1/3) = +1. A neutron is udd (0).
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Millikan found that the charges on his oil drops were …Show answer
Answer: always whole-number multiples of 1.6 × 10-19 C
The charges clustered at integer multiples of a smallest value, showing that charge is quantised in units of e.
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A nuclide has a decay constant of 0.0231 year-1. Its half-life is …Show answer
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In the Sun, energy is released mainly by …Show answer
Answer: fusion of hydrogen into helium
The proton–proton chain combines four protons into a helium nucleus. The small mass lost appears as energy by E = mc².
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Thomson found the charge-to-mass ratio of cathode ray particles to be about 1800 times that of a hydrogen ion. He concluded that …Show answer
Answer: cathode ray particles are much lighter than the smallest atom
The large q/m, found to be the same regardless of gas or electrode material, pointed to a very light particle common to all matter — the electron.
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Schrödinger's model of the atom describes electrons as …Show answer
Answer: standing waves, with position described by a probability distribution
The wave function gives the probability of finding the electron at each point, replacing Bohr's definite orbits with orbitals.
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A fission reaction has a mass defect of 0.20 u. The energy released is about (1 u = 931.5 MeV) …Show answer
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One of Bohr's postulates states that the angular momentum of the electron is …Show answer
Answer: quantised in units of h/2π
L = nh/2π. This restricts the electron to particular orbits and so particular energies.
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The hydrogen spectral line at 486 nm is produced by the transition …Show answer
Answer: n = 4 → n = 2
1/λ = R(1/4 − 1/16) = 1.097 × 107 × 0.1875 = 2.06 × 106, λ = 486 nm — the blue-green Balmer line.
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A nucleus with a higher binding energy per nucleon is …Show answer
Answer: more stable
More energy per nucleon is needed to pull it apart, so it is more tightly bound and more stable.
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In the Geiger–Marsden experiment, the fact that most alpha particles passed straight through the foil showed that …Show answer
Answer: the atom is mostly empty space
Undeflected passage means most particles met nothing substantial on their way through hundreds of atoms.
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In the reaction 14N + 4He → 17O + X, the particle X is …Show answer
Answer: a proton
Mass numbers: 14 + 4 = 17 + 1. Atomic numbers: 7 + 2 = 8 + 1. X has A = 1, Z = 1: a proton.
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The neutrino was proposed by Pauli in order to …Show answer
Answer: account for the missing energy and momentum in beta decay
Beta particles were emitted with a range of energies, apparently violating conservation laws. A third, undetected particle carrying the remainder fixed this.
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The plum pudding model could not explain …Show answer
Answer: why some alpha particles bounce back from gold foil
A diffuse positive sphere cannot produce the strong, concentrated repulsion needed to reverse a fast alpha particle.
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Using En = −13.6/n² eV, the energy of the n = 3 level of hydrogen is …Show answer
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238U (Z = 92) emits an alpha particle, then two beta-minus particles. The resulting nuclide is …Show answer
Answer: 234U (Z = 92)
Alpha: 238 → 234, 92 → 90. Each β⁻ raises Z by 1 with A unchanged: 90 → 92. Result 234U.
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Thomson concluded that cathode ray particles were …Show answer
Answer: a universal constituent of all matter
The same q/m was obtained whatever gas or cathode material was used, suggesting one common particle.
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Control rods in a nuclear reactor …Show answer
Answer: absorb neutrons to control the reaction rate
Rods of boron or cadmium absorb neutrons. Inserting them reduces the number available to cause fission. (The moderator slows neutrons.)
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In Millikan's experiment, a charged oil drop was held stationary when …Show answer
Answer: the electric force balanced the weight
qE = mg. Knowing the drop's mass (from its terminal velocity) and E gave q.
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A radioactive sample has a half-life of 8.0 hours. After 24 hours its activity has fallen to …Show answer
Answer: 12.5%
24 h is three half-lives: ½ × ½ × ½ = 1/8 = 12.5%.
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In the Standard Model, mesons are composed of …Show answer
Answer: a quark and an antiquark
Baryons (such as protons and neutrons) have three quarks; mesons (such as pions) have a quark–antiquark pair.
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The diameter of the nucleus compared with the diameter of the atom is roughly …Show answer
Answer: 1/100 000
Nuclei are around 10-15 m across; atoms around 10-10 m.
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When a nucleus emits a gamma ray, its …Show answer
Answer: mass number and atomic number are unchanged
A gamma ray is a photon. It carries away energy from an excited nucleus but no nucleons or charge.
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Nuclear fusion requires very high temperatures because …Show answer
Answer: the nuclei must overcome their mutual electrostatic repulsion
Positive nuclei repel. Only at very high speeds (temperatures of millions of kelvin) can they get close enough for the strong force to bind them.
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Bohr's model avoided the collapse predicted by classical physics by postulating that …Show answer
Answer: electrons in stationary orbits do not radiate
Radiation occurs only during a transition between allowed orbits, not while an electron remains in one.
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Transitions of the hydrogen electron ending at n = 1 produce lines in the …Show answer
Answer: ultraviolet
The Lyman series (to n = 1) involves the largest energy gaps, giving UV photons. Balmer (to n = 2) is visible; Paschen (to n = 3) is infrared.
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The mass defect of helium-4 is 0.0304 u. Its binding energy per nucleon is about (1 u = 931.5 MeV) …Show answer
Answer: 7.1 MeV
Total BE = 0.0304 × 931.5 = 28.3 MeV. Per nucleon = 28.3/4 = 7.1 MeV.
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Combining Millikan's value for e with Thomson's value for e/m gives …Show answer
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Chadwick's analysis of collisions showed that the neutron has a mass …Show answer
Answer: about equal to the proton's
The recoil speeds of protons and nitrogen nuclei struck by the neutral radiation fitted a particle of roughly proton mass.
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In a particle accelerator, electric fields and magnetic fields are used respectively to …Show answer
Answer: accelerate and steer the particles
Electric fields do work on charges (increase speed); magnetic fields change direction without changing speed.
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The Lyman line for the n = 2 → n = 1 transition in hydrogen has a wavelength of about (R = 1.097 × 107 m-1) …Show answer
Answer: 122 nm
1/λ = R(1 − 1/4) = 8.23 × 106, λ = 1.22 × 10-7 m = 122 nm.
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In the fission reaction 235U + n → 141Ba + 92Kr + x n, the number of neutrons released, x, is …Show answer
Answer: 3
Mass numbers: 235 + 1 = 236 = 141 + 92 + x, so x = 3.
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In the Standard Model, the gluon is the exchange particle for …Show answer
Answer: the strong nuclear force
Photons carry the electromagnetic force, gluons the strong force, and W and Z bosons the weak force.
317 cards across Modules 5–8.
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