Month: October 1997

Is critical mass the same for all fissionable materials?

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Is critical mass the same for all fissionable materials? — JG, Bateman, Australia

Critical mass is something of a misnomer because in addition to mass, it also depends on shape, density, and even the objects surrounding the nuclear fuel. Anything that makes the nuclear fuel more efficient at using its neutrons to induce fissions helps that fuel approach critical mass. The characteristics of the materials also play a role. For example, fissioning plutonium 239 nuclei release more neutrons on average than fissioning uranium 235 nuclei. As a result, plutonium 239 is better at sustaining a chain reaction than uranium 235 and critical masses of plutonium 239 are typically smaller than for uranium 235.

How is the super-critical mass achieved in nuclear weapons without it exploding …

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How is the super-critical mass achieved in nuclear weapons without it exploding prematurely? — JG, Bateman, Australia

Apart from obtaining fissionable material, this is the biggest technical problem with building a nuclear weapon. Although a fission bomb’s nuclear fuel begins to heat up and explode almost from the instant it reaches critical mass, just reaching critical mass isn’t good enough. To use its fuel efficiently—to shatter most of its nuclei before the fuel rips itself apart—the bomb must achieve a significantly super-critical mass. It needs the explosive chain reactions that occur when each fission induces an average of far more than one subsequent fission.

There are two classic techniques for reaching super-critical mass. The technique used in the uranium bomb dropped over Hiroshima in WWII involved a collision between two objects. A small cannon fired a piece of uranium 235 into a nearly complete sphere of uranium 235. The uranium projectile entered the incomplete sphere at enormous speed and made the overall structure a super-critical mass. But despite the rapid mechanical assembly, the bomb still wasn’t able to use its nuclei very efficiently. It wasn’t sufficiently super-critical for an efficient explosion.

The technique used in the two plutonium bombs, the Gadget tested in New Mexico and the Fat Man dropped over Nagasaki, involved implosions. In each bomb, high explosives crushed a solid sphere of plutonium 239 so that its density roughly doubled. With its nuclei packed more tightly together, this fuel surged through critical mass and went well into the super-critical regime. It consumed a much larger fraction of its nuclei than the uranium bomb and was thus a more efficient device. However, its design was so complicated and technically demanding that its builders weren’t sure it would work. That’s why they tested it once on the sands of New Mexico. The builders of the uranium bomb were confident enough of its design and too worried about wasting precious uranium to test it.

How do they split the first atom in an atomic bomb?

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How do they split the first atom in an atomic bomb? — N, Houston, Texas

Once the bomb has assembled a super-critical mass of fissionable material, each chain reaction that occurs will grow exponentially with time and lead to a catastrophic release of energy. But you’re right in wondering just what starts those chain reactions. The answer is natural radioactivity from a trigger material. While the nuclear fuel’s own radioactivity could provide those first few neutrons, it’s generally not reliable enough. To make sure that the chain reactions get started properly, most nuclear weapons introduce a highly radioactive neutron-emitting trigger material into the nuclear fuel assembly.

How does current flow and return in a home electric hot water heater? I only see…

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How does current flow and return in a home electric hot water heater? I only see two black hot wires and no white return wire. — DT, Waianae, HI

Your hot water heater is powered by 240 volt electric power through the two black wires. Each black wire is hot, meaning that its voltage fluctuates up and down significantly with respect to ground. In fact, each black wire is effectively 120 volts away from ground on average, so that if you connected a normal light bulb between either black wire and ground, it would light up normally. However, the two wires fluctuate in opposite directions around ground potential and are said to be “180° out of phase” with one another. Thus when one wire is at +100 volts, the other wire is at -100 volts. As a result of their out of phase relationship, they are always twice as far apart from one another as they are from ground. That’s why the two wires are effectively 240 volts apart on average.

Most homes in the United States receive 240 volt power in the form of two hot wires that are 180° out of phase, in addition to a neutral wire. 120-volt lights and appliances are powered by one of the hot wires and the neutral wire, with half the home depending on each of the two hot wires. 240-volt appliances use both hot wires.

How do airplanes work? What is the engineering behind how an airplane flies?

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How do airplanes work? What is the engineering behind how an airplane flies? — ZJ, Bangalore, India

An airplane supports itself in flight by deflecting the passing airstream downward. The plane’s wings push this airstream downward and the airstream reacts by pushing the wings upward. This action/reaction effect is an example of Newton’s third law of motion, which observes that forces always come in equal but oppositely directed pairs: if one object pushes on another, then the second object must push back on the first object with a force of equal strength pointing in the opposite direction. Even air obeys this law so that when the plane’s wings push air downward, the air must push the wings upward in response. In level flight, the deflected air pushes upward so hard that it supports the entire weight of the plane. Just how the airplane’s wings deflect the airstream downward to obtain this upward lift force is a marvel of fluid dynamics. We can view it from at least two perspectives: a Newtonian perspective which concentrates on the accelerations of the passing airstream and a Bernoullian perspective which concentrates on speeds and pressures in that airstream.

The Newtonian perspective is the most intuitive and where we will start. The airstream arriving at the forward or “leading” edge of the airplane wing splits into two separate flows that travel over and under the wing, respectively. The wing is shaped and tilted so that these two flows experience very different accelerations as they travel around the wing. The flow that goes under the wing encounters a downward sloping surface that pushes it downward and it accelerates downward. In response to this downward push, the air pushes upward on the bottom of the wing and provides part of the force that supports the plane.

The air that flows over the wing follows a more complicated route. At first, this flow encounters an upward sloping surface that pushes it upward and it accelerates upward. In response to this upward force, the air pushes downward on the leading portion of the wing’s top surface. But the wing’s top surface is curved so that it soon begins to slope downward rather than upward. When this happens, the airflow must accelerate downward to stay in contact with it. A suction effect appears, in which the rear or “trailing” portion of the wing’s top surface sucks downward on the air and the air sucks upward on it in response. This upward suction force more than balances the downward force at the leading edge of the wing so that the air flowing over the wing provides an overall upward force on the wing.

Since both of these air flows produce upward forces on the wing, they act together to support the airplane’s weight. The air passing both under and over the wings is deflected downward and the plane remains suspended.

In the Bernoullian view, air flowing around a wing’s sloping surfaces experiences changes in speed and pressure that lead to an overall upward force on the wing. The fact that each speed change is accompanied by a pressure change is the result of a conservation of energy in air passing a stationary surface—when the air’s speed and motional energy increase, the air’s pressure and pressure energy must decrease to compensate. In short, when air flowing around the wing speeds up, its pressure drops and when it slows down, its pressure rises.

When air going under the wing encounters the downward sloping bottom surface, it slows down. As a result, the air’s pressure rises and it exerts a strong upward force on the wing. But when air going over the wing encounters the up and down sloping top surface, it slows down and then speeds up. As a result, the air’s pressure first rises and then drops dramatically, and it exerts a very weak overall downward force on the wing. Because the upward force on the bottom of the wing is much stronger than the downward force on the top of the wing, there is an upward overall pressure force on the wing. This upward force can be strong enough to support the weight of the airplane.

But despite the apparent differences between these two descriptions of airplane flight, they are completely equivalent. The upward pressure force of the Bernoullian perspective is exactly the same as the upward reaction force of the Newtonian perspective. They are simply two ways of looking at the force produced by deflecting an airstream, a force known as lift.

Can lightning strike a flying airplane?

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Can lightning strike a flying airplane? — DC, Denver, CO

An object doesn’t have to be on the ground to be a target for lightning. In fact, most lightning strikes don’t reach the ground at all—they occur between different clouds. All that’s needed for a lightning strike between two objects is for them to have very different voltages, because that difference in voltages means that energy will be released when electricity flows between the objects.

If an airplane’s voltage begins to differ significantly from that of its surroundings, it’s going to have trouble. Sooner or later, it will encounter something that will exchange electric charge with it and the results may be disastrous. To avoid a lightning strike, the airplane must keep its voltage near that of its surroundings. That’s why it has static dissipaters on the tips of its wings. These sharp metal spikes use a phenomenon known as a corona discharge to spray unwanted electric charges into the air behind the plane. Any stray charges that the plane picks up by rubbing against the air or by passing through electrically charged clouds are quickly released to the air so that the plane’s voltage never differs significantly from that of its surroundings and it never sticks out as a target for lightning. While an unlucky plane may still get caught in an exchange of lightning between two other objects, the use of static dissipaters significantly reduces its chances of being hit directly.

Suppose I were to fall from an airplane that is cruising at about 30,000 feet. W…

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Suppose I were to fall from an airplane that is cruising at about 30,000 feet. What would kill me, the fall itself or the sudden deceleration as I intersect with the planet? — ZE, Woodinville, WA

In effect, you would be a skydiver without a parachute and would survive up until the moment of impact with the ground. Like any skydiver who has just left a forward-moving airplane, you would initially accelerate downward (due to gravity) and backward (due to air resistance). In those first few seconds, you would lose your forward velocity and would begin traveling downward rapidly. But soon you would be traveling downward so rapidly through the air that air resistance would keep you from picking up any more speed. You would then coast downward at a constant speed and would feel your normal weight. If you closed your eyes at this point, you would feel as though you were suspended on a strong upward stream of air. Unfortunately, this situation wouldn’t last forever—you would eventually reach the ground. At that point, the ground would exert a tremendous upward force on you in order to stop you from penetrating into its surface. This upward force would cause you to decelerate very rapidly and it would also do you in.

What is the difference between an elastic collision and an inelastic one? How do…

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What is the difference between an elastic collision and an inelastic one? How does an inelastic collision work and why?

When two objects collide with one another, they usually bounce. What distinguishes an elastic collision from an inelastic collision is the extent to which that bounce retains the objects’ total kinetic energy—the sum of their energies of motion. In an elastic collision, all of the kinetic energy that the two objects had before the collision is returned to them after the bounce, although it may be distributed differently between them. In an inelastic collision, at least some of their overall kinetic energy is transformed into another form during the bounce and the two objects have less total kinetic energy after the bounce than they had before it.

Just where the missing energy goes during an inelastic collision depends on the objects. When large objects collide, most of this missing energy usually becomes heat and sound. In fact, the only objects that ever experience perfectly elastic collisions are atoms and molecules—the air molecules in front of you collide countless times each second and often do so in perfectly elastic collisions. When the collisions aren’t elastic, the missing energy often becomes rotational energy or occasionally vibrational energy in the molecules. Actually, some of the collisions between air molecules are superelastic, meaning that the air molecules leave the collision with more total kinetic energy than they had before it. This extra energy came from stored energy in the molecules—typically from their rotational or vibrational energies. Such superelastic collisions can also occur in large objects, such as when a pin collides with a toy balloon.

Returning to inelastic collisions, one of the best examples is a head-on automobile accident. In that case, the collision is often highly inelastic—most of the two cars’ total kinetic energy is transformed into another form and they barely bounce at all. Much of this missing kinetic energy goes into deforming and heating the metal in the front of the car. That’s why well-designed cars have so called “crumple zones” that are meant to absorb energy during a collision. The last place you want this energy to go is into the occupants of the car. In fact, the occupants will do best if they transfer most of their kinetic energies into their airbags.

I am a huge figure skating fan and was wondering if you could explain to me the …

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I am a huge figure skating fan and was wondering if you could explain to me the physics of a triple axle jump? My friends and I are always asking ourselves how it’s done. — AF

While I don’t know the details of the jump, there are some basic physics issues that must be present. At a fundamental level, the skater approaches the jump in a non-spinning state, leaps into the air while acquiring a spin, spins three times in the air, lands on the ice while giving up the spin, and then leaves the jump in a non-spinning state. Most of the physics is in spin, so that’s what I’ll discuss.

To start herself spinning, something must exert a twist on the skater and that something is the ice. She uses her skates to twist the ice in one direction and, as a result, the ice twists her in the opposite direction. This effect is an example of the action/reaction principle known as Newton’s third law of motion. Because of the ice’s twist on her, she acquires angular momentum during her takeoff. Angular momentum is a form of momentum that’s associated with rotation and, like normal momentum, angular momentum is important for one special reason: it’s a conserved physical quantity, meaning that it cannot be created or destroyed; it can only be transferred between objects. The ice transfers angular momentum to the skater during her takeoff and she retains that angular momentum throughout her flight. She only gives up the angular momentum when she lands and the ice can twist her again.

During her flight, her angular momentum causes her to spin but the rate at which she spins depends on her shape. The narrower she is, the faster she spins. This effect is familiar to anyone who has watched a skater spin on the tip of one skate. If she starts spinning with her arms spread widely and then pulls them in so that she becomes very narrow, her rate of rotation increases dramatically. That’s because while she is on the tip of one skate, the ice can’t twist her and she spins with a fixed amount of angular momentum. By changing her shape to become as narrow as possible, she allows this angular momentum to make her spin very quickly. And this same rapid rotation occurs in the triple axle jump. The jumper starts the jump with arms and legs widely spread and then pulls into a narrow shape so that she spins rapidly in the air.

Finally, in landing the skater must stop herself from spinning and she does this by twisting the ice in reverse. The ice again reacts by twisting her in reverse, slowing her spin and removing her angular momentum. She skates away smoothly without much spin.

What is reverse osmosis and how it is used in the process of purifying seawater …

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What is reverse osmosis and how it is used in the process of purifying seawater for drinking water? — CS

In the form used for water desalination, reverse osmosis involves a special membrane that allows water molecules to pass through it while blocking the movement of salt ions. When water molecules are free to move between two volumes of water, they move in whichever direction reduces their chemical potential energy. The concept of a chemical potential is part of statistical physics—the area of physics that deals with vast collections of particles—and it depends partly on energy and partly on probability. Factors that contribute to a water molecule’s chemical potential are the purity of the water and the water’s pressure. Increasing the salt content of the water lowers a water molecule’s chemical potential while increasing the water’s pressure raises its chemical potential.

Because salty water has a lower chemical potential for water molecules than pure water, water molecules tend to move from purer water to saltier water. This type of flow is known as osmosis. To slow or stop osmosis, you must raise the chemical potential on the saltier side by applying pressure. The more you squeeze the saltier side, the higher the chemical potential there gets and the slower water molecules move from the purer side to the saltier side. If you squeeze hard enough, you can actually make the water molecules move backwards—toward the purer side! This flow of water molecules from the saltier water toward the purer water with the application of extreme pressure is known as reverse osmosis.

In commercial desalination, high-pressure seawater is pushed into jellyroll structures containing the semi-permeable membranes. The pressure of the salty water is so high that the water molecules flow through the membrane from the salty water side to the pure water side. This pure water is collected for drinking.