Question 1

A uranium nucleus has 92 protons. How many neutrons does a uranium-235 nucleus have?

Accepted Answer

The number of neutrons in a nucleus is found by subtracting the number of protons from the mass number. For uranium-235, this is 235 - 92 = 143 neutrons.

Question 2

Which of the following is not a criterion to make a nuclear bomb?

Accepted Answer

The key components for making a nuclear bomb include fissionable material, the ability to produce excess neutrons, and the capability to sustain a nuclear chain reaction. The number of electrons is not a criterion for propagating the chain reaction in a nuclear bomb; rather, it is the neutrons that play a crucial role in sustaining the chain reaction.

Question 3

Radioactive isotope technetium-99m has a half-life of 6 hours. What fraction of a sample of technetium-99m remains after 24 hours?

Accepted Answer

After 24 hours, which is 4 half-lives (24 hours / 6 hours per half-life), the fraction remaining is (1/2)^4 = 1/16.

Question 4

What percentage of an atom's mass is in its nucleus?

Accepted Answer

The nucleus of an atom contains protons and neutrons, which account for more than 99.9% of the atom's total mass. The electrons, which orbit the nucleus, have much less mass and contribute minimally to the total mass of the atom.

Question 5

What two Japanese cities were attacked with nuclear bombs during World War II?

Accepted Answer

Hiroshima and Nagasaki were the two Japanese cities attacked with nuclear bombs by the United States in August 1945, leading to Japan's surrender and the end of World War II.

Question 6

Nuclear reactions are how much more energetic than chemical reactions?

Accepted Answer

Nuclear reactions release energy that is approximately 10 million times greater than that of chemical reactions due to the strong nuclear forces involved.

Question 7

The nuclear bombs developed during World War II used what principle?

Accepted Answer

The nuclear bombs developed during World War II, specifically the ones dropped on Hiroshima and Nagasaki, used the principle of nuclear fission. This process involves splitting heavy atomic nuclei into smaller, lighter nuclei, releasing a tremendous amount of energy.

Question 8

What is needed for a nuclear chain reaction?

Accepted Answer

A critical mass is the minimum amount of fissile material needed to sustain a nuclear chain reaction.

Question 9

In a well-designed water-cooled thermal fission reactor, the water is acting as a moderator for the neutrons. If some of this water is lost, the chain reaction in the reactor core will slow down because neutrons in the reactor core will

Accepted Answer

Water in a thermal fission reactor serves as a moderator, slowing down neutrons so they can more effectively cause fission in uranium-235 nuclei. If water is lost, neutrons remain too fast, making them more likely to be absorbed by uranium-238 without causing fission.

Question 10

When uranium undergoes fission, it releases an enormous amount of energy. That energy was stored in the uranium nuclei as

Accepted Answer

The energy released during uranium fission comes from the nuclear potential energy, which is the energy stored due to the strong nuclear forces acting between nucleons (protons and neutrons) in the nucleus.

Question 11

Fission chain reactions in uranium are sustained by neutrons, each of which is released by one fission and may induce a subsequent fission. Each fission also releases other particles, which include protons. However, proton-containing fission fragments are not as effective at causing subsequent fissions because they

Accepted Answer

Protons are positively charged and are repelled by the positively charged nuclei of uranium atoms, making it difficult for them to induce subsequent fissions.

Question 12

For a nuclear weapon to explode normally, the fissionable material in its core must be assembled very quickly. For technical reasons, a nuclear bomb developed by a terrorist group would probably not achieve such rapid assembly. As a result of its slow assembly, such a bomb would

Accepted Answer

For a nuclear weapon to detonate effectively, the fissionable material must be assembled rapidly to achieve a critical mass before the device can disassemble itself due to the initial stages of the nuclear reaction. Slow assembly would likely result in the device overheating and pushing itself apart, leading to a failure to reach critical mass and thus only producing a weak explosion, if any.

Question 13

The energy that is released by a nuclear weapon was put into the weapon's fissionable core in the process of

Accepted Answer

The energy released by a nuclear weapon originates from the process of assembling each of the core's fissionable nuclei from smaller nuclear pieces. This involves nuclear reactions that convert mass into energy, as described by Einstein's equation E=mc^2, where the binding energy of the nucleus is released.

Question 14

The fallout from a nuclear explosion and the waste from a nuclear reactor contain radioactive isotopes. The atoms of these isotopes

Accepted Answer

Radioactive isotopes have unstable nuclei, which makes them radioactive, but chemically, they behave the same as their stable counterparts. This means they participate in chemical reactions in the same way as non-radioactive isotopes of the same element.

Question 15

Most nuclear reactors in the United States are thermal fission reactors, reactors that slow their fission neutrons by sending those neutrons through materials known as moderators. Water is a common moderator. When a fast moving neutron enters water, it usually slows down because it

Accepted Answer

When a fast-moving neutron enters water, it primarily slows down through collisions with the water nuclei (hydrogen and oxygen), transferring most of its energy and momentum to those nuclei. This process is fundamental to the operation of thermal fission reactors, as it moderates the speed of neutrons to make them more likely to cause further fission reactions.

Question 16

The fallout from a nuclear explosion and the waste from a nuclear reactor contain radioactive isotopes. For example, iodine-131 (a fission byproduct) is radioactive while iodine-127 (normal iodine) is not. An atom of iodine-131 differs from an atom of iodine-127 because the iodine-131 atom has more

Accepted Answer

Iodine-131 differs from iodine-127 due to having more neutrons in its nucleus. Both isotopes have the same number of protons (since they are both iodine), but their mass numbers differ because of the different number of neutrons.

Question 17

When a free neutron collides with a uranium-235 nucleus and causes that nucleus to fission, the collision fragments contain a total of 2.5 free neutrons, on average, including the original free neutron. The critical mass for uranium-235 is about 50 kilograms. Suppose that another radioactive material, fundamentium-999, undergoes fission when struck by a neutron and the collision fragments contain a total of 0.5 neutrons, on average, including the original free neutron. What would be the critical mass of fundamentium-999?

Accepted Answer

For a chain reaction to be sustained, each fission event must produce at least one new neutron to continue the reaction. Since fundamentium-999 produces only 0.5 neutrons on average, it cannot sustain a chain reaction, implying an infinite critical mass.

Question 18

A uranium-235 nucleus undergoes fission during a nuclear explosion. If you were to collect all the fragments of that original uranium-235 nucleus, you would find that their total combined mass is

Accepted Answer

During a nuclear fission reaction, such as the fission of uranium-235, a small amount of the mass of the original nucleus is converted into energy according to Einstein's equation E=mc^2. This means the total mass of the fission fragments is less than the mass of the original nucleus.

Question 19

In many circumstances it is very important to have control of a fission nuclear chain reaction with a moderator. A moderator must therefore have a profound effect on the motion of

Accepted Answer

Moderators in nuclear reactors are used to slow down neutrons without capturing them, which increases the probability of these neutrons to cause further fission in the fuel. This is essential for sustaining a controlled nuclear chain reaction. Electrons, protons, and photons are not directly involved in the fission process in the same way as neutrons.

Question 20

Identify and briefly discuss three physical properties that isotopes of an element or compounds formed by isotopes of an element have in common.

Accepted Answer

The answer of Identify and briefly discuss three physical properties...

Quiz 15: Modern Physics