Fission, Fusion, and Nuclear Decay
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AP Physics 2 › Fission, Fusion, and Nuclear Decay
In a fusion experiment, $^{2}\text{H}+{}^{3}\text{H}\rightarrow{}^{4}\text{He}+n+\text{energy}$. Which statement best explains why very high temperature is required?
High temperature is needed so electrons can form stronger chemical bonds that drive the reaction.
High temperature provides kinetic energy to overcome electrostatic repulsion so nuclei can get close enough to fuse.
High temperature ensures the total mass is conserved exactly, which causes energy to appear.
High temperature is unnecessary because fusion happens readily at room temperature when pressure is low.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Fusion requires extremely high temperatures because the positively charged nuclei must overcome their strong electrostatic repulsion to get close enough for the strong nuclear force to bind them. At high temperatures, nuclei have sufficient kinetic energy to approach within about 10⁻¹⁵ meters where nuclear forces dominate. The energy released comes from the mass defect when lighter nuclei form a more tightly bound heavier nucleus, converting mass to energy via E=mc². Choice B incorrectly attributes fusion to chemical bonding, which involves only electrons, not nuclear processes. Remember that fusion requires overcoming Coulomb repulsion between positive nuclei, which demands extreme temperatures like those in stellar cores.
A star’s core produces energy mainly by combining hydrogen nuclei into helium. Which statement best distinguishes this from fission in reactors?
Fusion splits heavy nuclei into lighter ones, whereas fission combines light nuclei into heavier ones.
Fusion is a chemical process involving electrons, whereas fission is a chemical process involving ions.
Fusion combines light nuclei and requires overcoming Coulomb repulsion, whereas fission splits heavy nuclei after neutron absorption.
Fusion occurs easily at low temperatures, whereas fission requires extremely high temperatures to start.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Fusion in stars combines light nuclei (like hydrogen) into heavier ones (like helium), requiring extreme temperatures to overcome Coulomb repulsion between positive nuclei. Fission in reactors splits heavy nuclei (like uranium) into lighter fragments, typically initiated by neutron absorption at relatively low temperatures. Both processes release energy through mass-energy conversion when products have higher binding energy per nucleon than reactants. Choice B reverses the definitions, incorrectly stating fusion splits and fission combines. Remember the distinction: fusion fuses light nuclei at high temperatures, while fission splits heavy nuclei often triggered by thermal neutrons.
A nucleus undergoes $\alpha$ decay: $^{238}\mathrm{U}\rightarrow^{234}\mathrm{Th}+^{4}\mathrm{He}$. Which change occurs to the parent nucleus?
Its mass number increases by 4 and its atomic number increases by 2.
Its mass number decreases by 2 and its atomic number decreases by 4.
Its mass number decreases by 4 and its atomic number decreases by 2.
Its mass number stays the same and its atomic number increases by 1.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Alpha decay occurs when a nucleus emits an alpha particle (helium-4 nucleus), which contains 2 protons and 2 neutrons. This emission decreases the parent nucleus's mass number by 4 (total nucleons lost) and atomic number by 2 (protons lost), as seen when uranium-238 becomes thorium-234. The energy for this decay comes from the mass difference between the parent nucleus and the combined mass of the daughter nucleus plus alpha particle. Choice A reverses the changes to mass number and atomic number, a common error when students confuse which number represents what. Remember that in alpha decay, the mass number drops by 4 and atomic number drops by 2, following the composition of the emitted helium nucleus.
In a bombarding experiment, $^{14}\mathrm{N}$ absorbs an alpha particle and becomes $^{17}\mathrm{O}$ plus another particle. Which statement best identifies the reaction type?
Fission, because a heavy nucleus splits into two medium nuclei and releases chemical energy.
Gamma decay, because only a photon is emitted and the nucleus composition changes.
Fusion, because two nuclei combine into a heavier nucleus after overcoming electrostatic repulsion.
Alpha decay, because the nucleus spontaneously emits $^{4}\mathrm{He}$ without any collision.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. When nitrogen-14 absorbs an alpha particle (helium-4), the nuclei combine to form a heavier nucleus, which is the defining characteristic of fusion. The reaction produces oxygen-17 plus a proton, conserving both mass number (14+4=17+1) and atomic number (7+2=8+1). Fusion requires overcoming the electrostatic repulsion between positively charged nuclei, which is why bombardment experiments use high-energy particles. Choice C incorrectly calls this alpha decay, which is emission rather than absorption of an alpha particle. The key insight is that fusion involves combining nuclei, not splitting or emitting particles.
A deuterium nucleus and a tritium nucleus undergo fusion to form $^{4}\mathrm{He}$ and a neutron. Which statement best explains why high temperature is required?
High temperature provides kinetic energy to overcome electrostatic repulsion so the strong force can bind the nuclei.
High temperature is needed to break chemical bonds so nuclei can share electrons and fuse.
High temperature is unnecessary because fusion of hydrogen isotopes occurs efficiently at room temperature.
High temperature is required because mass must be conserved exactly, preventing any energy release at low temperature.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Fusion of light nuclei requires extremely high temperatures because the positively charged nuclei must overcome their strong electrostatic repulsion to get close enough for the strong nuclear force to bind them. At high temperatures, nuclei have sufficient kinetic energy to approach within about 10^-15 meters, where the attractive strong force overcomes electrostatic repulsion. The fusion of deuterium and tritium releases energy because helium-4 has higher binding energy per nucleon than the reactants, converting mass to energy via E=mc². Choice B incorrectly invokes chemical bonds, which are irrelevant to nuclear processes. The key principle is that fusion requires overcoming Coulomb repulsion through high kinetic energy.
A nucleus emits an alpha particle, changing from $^{238}\mathrm{U}$ to $^{234}\mathrm{Th}$. Which statement correctly identifies the emitted particle?
A helium nucleus with 2 protons and 2 neutrons, reducing atomic number by 2 and mass number by 4.
A photon that carries chemical energy, leaving atomic number and mass number unchanged.
A neutron that triggers fusion at low temperature, increasing both atomic and mass numbers.
An electron from the nucleus, increasing atomic number by 1 while mass number stays the same.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. An alpha particle is a helium-4 nucleus consisting of 2 protons and 2 neutrons, which explains why uranium-238 (92 protons) becomes thorium-234 (90 protons) after alpha emission. The mass number decreases by 4 (from 238 to 234) because the alpha particle carries away 2 protons and 2 neutrons. Alpha decay occurs in heavy nuclei that have too many protons and neutrons, and emitting an alpha particle helps the nucleus move toward stability. Choice B incorrectly describes beta decay, confusing the type of particle emitted. The key is recognizing that alpha particles are helium nuclei that reduce both atomic and mass numbers.
In a reactor, a $^{235}\mathrm{U}$ nucleus absorbs a neutron and undergoes fission into two medium nuclei plus 3 neutrons. Which statement best explains why energy is released?
Mass is strictly conserved in the reaction, so no energy can be released.
The products have greater total binding energy per nucleon, so the mass defect increases and $E=mc^2$ is released.
The process is fusion because heavy nuclei combine at low temperature to form heavier nuclei.
The reaction releases chemical bond energy when uranium atoms rearrange their electron shells.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. In nuclear fission, a heavy nucleus like U-235 splits into two medium-sized nuclei after absorbing a neutron. The key insight is that the binding energy per nucleon is higher for medium-mass nuclei than for very heavy nuclei, meaning the products are more tightly bound. This increase in binding energy per nucleon corresponds to a greater mass defect (the difference between the mass of separated nucleons and the bound nucleus), and this "missing" mass is converted to energy according to Einstein's equation E=mc². Choice B incorrectly attributes the energy to chemical bonds, which involve electrons rather than nuclear processes. The strategy to remember is that nuclear energy comes from changes in binding energy, with more tightly bound products releasing energy.
In a hydrogen bomb stage, deuterium and tritium combine: $^2\text{H}+^3\text{H}\rightarrow,^{4}\text{He}+n$. Which statement best explains why energy is released?
Energy is released because the total mass of products must equal the total mass of reactants exactly.
Energy is released because the helium nucleus has greater binding energy per nucleon than the reactants.
Energy is released because the neutron forms a chemical bond with helium, lowering electron energy.
Energy is released because fusion occurs easily at low temperature when nuclei move slowly.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. In deuterium-tritium fusion, two hydrogen isotopes combine to form helium-4 plus a neutron, releasing tremendous energy. The helium-4 nucleus has much greater binding energy per nucleon than either deuterium or tritium, meaning its nucleons are bound much more tightly. This increase in binding energy per nucleon corresponds to a mass defect—the products have less total mass than the reactants—and this missing mass is converted to kinetic energy of the helium nucleus and neutron. Choice D incorrectly suggests fusion occurs easily at low temperature, when actually it requires extreme conditions to overcome electrostatic repulsion between positive nuclei. When analyzing fusion reactions, remember that nuclear energy comes from changes in binding energy, with products having higher binding energy per nucleon than reactants.
A nucleus emits an alpha particle: $^{238}\text{U}\rightarrow,^{234}\text{Th}+\alpha$. Which statement best explains why energy is released?
Energy is released because nucleons are created from energy, increasing total mass.
Energy is released because the products have greater total binding energy, so mass converts to kinetic energy.
Energy is released because electrons in uranium form stronger chemical bonds after the decay.
Energy is released because alpha decay is a type of low-temperature fusion of two heavy nuclei.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Alpha decay occurs when a heavy nucleus emits an alpha particle (helium-4 nucleus), transforming uranium-238 into thorium-234. Energy is released because the combined binding energy of the thorium nucleus plus the alpha particle exceeds that of the original uranium nucleus. This increase in total binding energy means the products are more stable, and the corresponding mass defect is converted to kinetic energy of the alpha particle and recoiling thorium nucleus. Choice D incorrectly calls alpha decay a type of fusion, when it's actually a decay process that reduces nuclear size rather than combining nuclei. To analyze nuclear reactions correctly, remember that nuclear energy comes from changes in binding energy—products with greater total binding energy have less mass than reactants.
A nucleus undergoes $\alpha$ decay: $^{238}\text{U}\rightarrow{}^{234}\text{Th}+{}^{4}\text{He}+\text{energy}$. Which statement best explains why energy is released?
Energy is released because electron sharing creates stronger chemical bonds in thorium.
Energy is released because the products have lower total mass, so $\Delta m c^2$ becomes kinetic energy.
Energy is released because fusion happens when a heavy nucleus emits an alpha particle at low temperature.
Energy is released because total mass is conserved exactly, so no energy conversion occurs.
Explanation
This question tests understanding of fission, fusion, and nuclear decay. Alpha decay occurs when a heavy nucleus emits an alpha particle (helium-4 nucleus), decreasing its mass number by 4 and atomic number by 2. The energy released comes from mass-energy equivalence: the combined mass of thorium-234 and helium-4 is less than the original uranium-238, and this mass defect (Δm) is converted to kinetic energy via E=Δmc². The products have greater binding energy per nucleon, making them more stable than the parent nucleus. Choice A incorrectly attributes energy to chemical bonding (a misconception confusing nuclear and chemical processes). Remember that nuclear energy comes from changes in binding energy, manifested as mass differences.