physics

Light can't penetrate even a millimeter into plutonium, so why is a neutron able to travel centimeters into plutonium?

A neutron has no charge and only interacts with nuclei that are so small that they're hard to hit.

Lead, with 82 electrons per atom, is an excellent absorber of X-rays. Why?

Almost any X-ray matches the energy of one of lead's many electrons and thus can cause efficient photoelectron emission.

Which has more mass: a positron or an antiproton?

An antiproton has more mass than a positron.

A medium-sized nuclei with too many neutrons to be stable is likely to undergo beta decay. What emerges from that nucleus during beta decay, and what happens to the nucleus as a result?

An electron and an antineutrino emerge from the nucleus but the proton remains behind in the nucleus and increases its atomic number by one.

Unlike heavy water, ordinary water occasionally absorbs neutrons. Why does a thermal fission reactor using an ordinary water moderator need more enriched uranium than a reactor using heavy water?

An ordinary water moderator requires more enriched uranium so that the neutrons needed for the chain reaction are more likely to be absorbed by the enriched uranium instead of the water whereas a heavy water moderator doesn't absorb any neutrons.

What occurs whenever a charged particle accelerates?

Bremsstrahlung

Control rods are usually installed on top of the reactor where their weights tend to pull them into the core. Why is this arrangement much safer than putting the control rods at the bottom of the reactor?

By installing the control rods on top of the reactor, if the fission rate becomes too high, the control system can simply drop the control rods into the core to decrease the fission rate.

One of the most accurate atomic clocks is the hydrogen maser. This devise uses excited hydrogen molecules to duplicate 1.420-GHz microwave photons. In the maser, the molecules have only two states: the upper maser state and the lower maser state (which is actually the ground state). To keep the maser operating, an electromagnetic system constantly adds excited state hydrogen molecules to the maser and a pump constantly removes ground-state hydrogen molecules from the maser. Why does the maser require a steady supply of new excited state molecules?

If the excited states are outnumbered by the ground states, more photons will be absorbed than emitted and the maser will not amplify passing photons.

When two medium-sized nuclei are stuck together during an experiment at a nuclear physics lab, the result is usually a large nucleus with too few neutrons to be stable. The nucleus soon falls apart. Why could more neutrons make it stable?

Neutrons would experience the attractive nuclear force while diluting the repulsion between protons.

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

Nuclear fission

Naturally occurring copper has two isotopes, 63Cu and 65Cu. What is different between atoms of these two isotopes?

Nuclei of 65Cu atoms contain two more neutrons than those of 63Cu.

If electrons were not Fermi particles, any number of them could occupy a particular orbital. How would these electrons tend to arrange themselves among the orbitals in an atom?

The electrons would occupy 1S orbital.

One of the first lasers used synthetic ruby as its laser medium. However, a ruby laser is a three-state laser; its lower laser state is its ground state. Why does that arrangement make the ruby laser relatively inefficient?

The ground state systems absorb much of the amplified light before it can leave the ruby.

Why can't a CD player use a light-emitting diode (LED) in place of its diode laser?

The incoherent light from an LED will not focus as well because of the independent photons emitted.

An X-ray technician can adjust the energy of the X-ray photons produced by a machine by changing the voltage drop between the X-ray tube's cathode and anode. Explain.

The larger the voltage drop between cathode and anode, the more kinetic energy each electron has by the time it reaches the anode and the more energetic the X-rays it can produce.

A laser light show uses extremely intense beams of light. When one of these beams remains steady, you can see the path it takes through the air. What makes it possible for you to see this beam even though it isn't directed toward you?

The laser beams experience deflections in the air which makes it possible for you to see the beam of light.

To make the beams at a laser light show even more visible, they are often directed into mist or smoke. Why do such particles make the beams particularly visible?

The mist or smoke particles are larger than the air molecules and thus scatter light better than air molecules which makes the beams particularly visible.

The stronger the magnetic field used in MRI, the larger the fraction of protons that align their spins with the field. Why does this increased alignment make it easier for the MRI to study the protons?

The more spin-aligned protons in a patient, the more radiative transitions the MRI system can produce and the more easily it can detect tissue.

As a nuclear reactor operates, its fuel rods gradually accumulate neutron-absorbing fission fragments. As those fragments accumulate, what must the reactor operators do to keep the chain reaction going?

The operators must increase the probability of each fission neutron causing a subsequent fission, for example, by pulling the control rods farther out of the reactor.

A fission chain reaction can occur at room temperature, so why must hydrogen be heated to astronomical temperatures to get its nuclei to fuse?

The protons in hydrogen nuclei repel one another tremendously at short distances; therefore, they need to be moving very quickly with gigantic amounts of thermal energy for them to touch.

Once fallout from a nuclear blast distributes radioactive isotopes over a region of land, why is it virtually impossible to separate many of those radioactive isotopes from the soil?

The radioactive isotopes have unstable nuclei and are chemical indistinguishable from ordinary elemental constituents of the soil.

Which of the following does a moderator not have an effect on?

electron speed

If an atom contains seven electrons and those electrons have as little energy as possible, how many different orbitals do they occupy in that atom?

5 orbitals

A uranium nucleus has 143 neutrons. How many protons does a uranium-235 nucleus have?

92

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

neutrons.

A Radioactive isotope has 1/64 of the original material left after 10 minutes. What is its half-life?

2 minutes

Nuclear reactions are how much more energetic than chemical reactions?

10 million times

X-rays were discovered in

1895.

If most of the highest energy valence levels in a diode laser's p-type anode weren't empty, it would become relatively inefficient and probably wouldn't emit laser light at all. Why not?

Electrons in the highest valence levels can absorb the laser light while undergoing radiative transitions to the conduction levels and that absorption would trap the laser radiation and reduce the diode's ability to emit laser light.

In which direction does the electric field point in the middle of a p-n junction?

From n-type half to p-type half.

The most troubling radioactive isotopes in fallout are those with half-lives between a few days and a few thousand years. Why are those with much shorter or much longer half-lives less of a problem?

It's possible to wait out isotopes with short half-lives because they decay away quickly and long half-life isotopes decay so slowly that it's unlikely they'll decay during a person's lifetime.

On a rainy day you can often see oil films on the surfaces of puddles. Why do these films appear brightly colored?

Light reflects from the top and bottom surfaces of an oil film and the two reflections interfere with one another and the type of interference, constructive or destructive, depends on the film's thickness as well as the light's wavelength.

While some laser media quickly lose energy via the spontaneous emission of light, others can store energy for a long time. Why is a long storage time essential in lasers that produce extremely intense pulses of light?

Long storage allows energy to be deposited into the laser medium over a longer period of time which means that conventional light sources can be used to store the energy and huge amounts of total energy can be accumulated.

Magnetic resonance imaging (MRI) differs from computed tomography imaging in that it involves no "ionizing radiation". What electromagnetic radiation is used in MRI, and why aren't the photons of this radiation able to remove electrons from atoms and convert those atoms into ions?

MRI uses radio waves or microwaves and photons of those two types of electromagnetic radiation have far too little energy to cause chemical damage to molecules.

Magnetic resonance imaging isn't good at detecting bone. Why not?

Option A: MRI detects hydrogen and bone contains little hydrogen.

Some of the X-rays emitted by an X-ray imaging machine are fluorescence X-rays and are formed when individual atoms in the machine undergo radiative transitions. While the mercury atoms in a fluorescent lamp also undergo radiative transitions, those mercury atoms emit ultraviolet light, not X-rays. The reason for this difference is that

Option B: the electrons in the imaging machine atoms experience much larger changes in energy during their radiative transitions than the electrons in the mercury atoms do.

When two sheets of glass lie on top of one another, you can often see colored rings of reflected light. How do the nearby glass surfaces cause these colored rings?

Partial reflections of light from back of one sheet of glass and from the front of the next sheet interfere with one another and the type of interference, constructive or destructive, depends on the spacing between the glass as well as the wavelength of the light.

If you're wearing polarizing sunglasses and want to see who else is wearing polarizing sunglasses, you only have to turn your head sideways and look to see which people now have sunglasses that appear completely opaque. Why does this test work?

Polarizing sunglasses normally block horizontally polarized light, so when you look at someone's eyes when they are wearing polarizing sunglasses, you see only vertically polarized light. Tilting your head sideways will allow your sunglasses to block vertically polarized light and thus you will see no light coming from the eyes of other people wearing polarizing sunglasses.

Thermal energy can shift some of the electrons in a hot semiconductor from valence levels to conduction levels. What effect do these shifts have on the semiconductor's ability to conduct electricity?

Semiconductor's ability to conduct electricity increases.

The uranium fuel in a thermal fission reactor must be replaced every so often. When that fuel is removed, it's still mostly uranium. Why can't that uranium be cleaned and reused until the uranium is completely consumed?

The reactor is consuming only the 235U and the uranium that is left in the fuel is mostly 238U which is not fissionable.

A fission bomb can be used to initiate fusion in hydrogen as part of a hydrogen bomb, but a power plant can't use a fission reactor to initiate fusion in hydrogen as a way to generate electricity. Why not?

The role of the fission bomb is to provide the astronomical temperatures needed to bring protons close enough for the nuclear force to stick them together but a fission reactor does not achieve those temperatures.

Electric charge is strictly conserved in our universe, meaning that the net charge of an isolated system can't change. Why doesn't the production of an electron-positron pair in a patient cause a change in the patient's net charge?

When an electron-positron pair is created, there is no net change in charge because the electron has a charge of -1 while the positron has a charge of +1, so their sum is 0 charge.

When a large nucleus is split in half during an experiment at a nuclear physics lab, the result is usually two medium-sized nuclei with too many neutrons to be stable. These nuclei eventually fall apart. Why don't these smaller nuclei need as many neutrons as they received from the original nucleus?

With fewer protons in the two medium-sized nuclei as a result of being split in half, the diluting effect of neutrons matters less.

Why does increasing the current passing through an LED affect the brightness of its light?

With more current passing through the LED, the population of electrons in excited conduction states increases and the rate of radiative transition in the LED increases.

If a nuclear reaction adds an extra proton to the nucleus of 58Fe (a stable isotope of iron), it produces 59Co (a stable isotope of cobalt). Will this change in the nucleus affect the number and arrangement of the electrons in the atom that's built around this nucleus? Why or why not?

Yes, the 59Co nucleus requires one more electron than 58Fe to form a neutral atom, so the electron arrangement will change.

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

are repelled as they try to approach other uranium nuclei.

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

be traveling too fast and will be absorbed by uranium 238 nuclei.

When an x-ray photon is absorbed in a tumor, it may cause the tumor cell to die because the photon

carries enough energy to damage molecules and cause chemical injury to the cell.

In a common X-ray tube, electrons accelerate from the

cathode to anode.

Laser light is

coherent

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

collides with the water nuclei and transfers most of its energy and momentum to those nuclei.

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

have unstable nuclei but are chemically indistinguishable from stable atoms.

A laser light beam can travel much farther than a flashlight without spreading out because laser light

is coherent

The photoelectric effect is what makes X-ray imaging possible because

it involves electromagnetic radiation knocking an electron all the way out of an atom.

Laser light exhibits much stronger interference effects than light from a flashlight. This is because

laser light is both coherent and monochromatic.

LED light is not damaging to your eye like a laser can be because

laser light is coherent while LED light is not.

"Laser" stands for

light amplification through stimulated emission of radiation.

LED" stands for

light emitting diode.

Imaging x-rays go easily through living tissue but not so easily through bone. This is primarily because the calcium atoms in bone have

many more electrons than the atoms in tissue have.

The X-rays used in medical imaging travel through tissue relatively easily but are mostly absorbed by bone. This is because

the atoms in bone generally have more tightly bound electrons than those in tissue and are thus able to absorb higher energy photons.

A laser diode's wavelength and color depend primarily on

the band gap of its anode.

A laser's output color depends primarily on

the energy gap between the upper and lower laser levels.