Rich countries are science rich,Poor countries are science poor"

I don't necessarily agree with this statement such that the development/progress and even the failure of a certain country to be rich would not depend on its science or technological advancement.It is because there are also factors that could be attributed to the development and even the failure of a certain country such as government leaders(the kind of government and how its leaders manage and made use of their talents to achieve the desired success), natural resources(the availability of natural resources and how it is used efficiently and effectively,manpower or the people living that country(the persistence and perseverance of its constituents out of their talents/skills to become an asset and not a burden of the country).We all know that Philippines,our dear country belongs to the Third World Counties with that we could perhaps say that our country is a science poor one but we could merely observe that we are struggling to be updated and not left behind by the technological advancement that the other country has.And we can't deny the fact that though our country is poor,we are still a step ahead to some rich countries as far as science is concerned.Therefore we cannot totally equate a country's success/richness to its technological advancement or science.

Reaction:
Rich countries are science rich
Poor countries are science poor

I agree with the statement "rich countries are science-rich and poor countries are science-poor". This is due to the fact that science principles are behind the success of all the products and services that the people make use of which makes a country rich. This goods and services are direct product of science and technology.
Science and technology make a country rich. The growth of science has brought not only changes in ideas about the physical world but also a transformation of society. If a country is rich in science and technology, the people benefit from these by the efficient products and enhancement of processes.

Amusement Park Physics

How do physics laws affect amusement park ride design?

Roller Coaster

For many people, there is only one reason to go to an amusement park: the roller coaster. Some people call it the "scream machine," with good reason. The history of this ride reflects a constant search for greater and more death-defying thrills.

How does a roller coaster work?
What you may not realize as you're cruising down the track at 60 miles an hour is that the coaster has no engine. The car is pulled to the top of the first hill at the beginning of the ride, but after that the coaster must complete the ride on its own. You aren't being propelled around the track by a motor or pulled by a hitch. The conversion of potential energy to kinetic energy is what drives the roller coaster, and all of the kinetic energy you need for the ride is present once the coaster descends the first hill..

Once you're underway, different types of wheels help keep the ride smooth. Running wheels guide the coaster on the track. Friction wheels control lateral motion (movement to either side of the track). A final set of wheels keeps the coaster on the track even if it's inverted. Compressed air brakes stop the car as the ride ends.

Wooden or steel coaster: Does it make a difference?
Roller coasters can be wooden or steel, and can be looping or nonlooping. You'll notice a big difference in the ride depending on the type of material used. In general, wooden coasters are nonlooping. They're also not as tall and not as fast, and they don't feature very steep hills or as long a track as steel ones do. Wooden coasters do offer one advantage over steel coasters, assuming you're looking for palm-sweating thrills: they sway a lot more. Tubular steel coasters allow more looping, higher and steeper hills, greater drops and rolls, and faster speeds.

Carousel

Carousels are not considered "thrill machines" by any stretch of the imagination. Still, carousels are as reliant on the laws of motion as their more exciting cousins, the roller coasters. It's theoretically possible that, allowed to spin out of control, a carousel could gain enough speed so that the riders would be thrown off. Thankfully, runaway carousels are not the least bit common.

Are some horses moving faster than others?
With all of its beauty and seeming simplicity, the carousel is a delicate balance of motion and forces. All of the horses move through one complete circle in the same amount of time. The horses on the outside of the carousel have to cover more distance than the inside horses in the same amount of time. This means the horses on the outside have a faster linear speed than those at the hub.

What if they're galloping?
On some carousels, the horses go up and down in a galloping motion simulating what it might be like to ride a real horse. For these carousels, the ride designer had to approach the problem of movement around the central axis differently. In a normal carousel, each horse maintains a constant acceleration, radius, and tangential speed (speed tangent to the circular path of the carousel). If you add a gallop to some of the horses, you must consider the forces needed to change that horse's position upward or downward as it goes around the track. In designing with these forces in mind, you also need to take into account the mass of the horse and its rider.

Bumper Cars

Newton 's third law of motion comes into play on the bumper cars. This law, the law of interaction, says that if one body exerts a force on a second body, the second body exerts a force equal in magnitude and opposite in direction on the first body. It's the law of action-reaction, and it helps to explain why you feel a jolt when you collide with another bumper car.

How do bumper cars work?
Bumper car rides are designed so that the cars can collide without much danger to the riders. Each car has a large rubber bumper all around it, which prolongs the impact and diffuses the force of the collision.

The bumper cars run on electricity, carried by a pole on the back of the car that leads up to a wire grid in the ride's ceiling. This grid carries the electricity that runs the car. Electrical energy carried to the cars from the grid is converted to kinetic energy, some of which is converted to heat.

What happens to the drivers?
When bumper cars collide, the drivers feel a change in their motion and become aware of their inertia. Though the cars themselves may stop or change direction, the drivers continue in the direction they were moving before the collision. This is why it's important to wear a seat belt while driving a real car, since otherwise you could suffer injury being thrown forward in a collision.

The masses of the drivers also affect the collisions. A difference in mass between two bumper car riders will mean that one rider experiences more change in motion than the other (or more of a jolt). The type of collision, velocity of the cars, and mass of the individual drivers all come into play in bumper car collisions.

Free Fall

Galileo first introduced the concept of free fall. His classic experiments led to the finding that all objects free fall at the same rate, regardless of their mass. According to legend, Galileo dropped balls of different mass from the Leaning Tower of Pisa to help support his ideas.

A freely falling body is an object that is moving under the influence of gravity only. These objects have a downward acceleration toward the center of the earth. Newton later took Galileo's ideas about mechanics and formalized them into his laws of motion.

How do free-fall rides work?
Free-fall rides are really made up of three distinct parts: the ride to the top, the momentary suspension, and the downward plunge. In the first part of the ride, force is applied to the car to lift it to the top of the free-fall tower. The amount of force that must be applied depends on the mass of the car and its passengers. The force is applied by motors, and there is a built-in safety allowance for variations in the mass of the riders.

After a brief period in which the riders are suspended in the air, the car suddenly drops and begins to accelerate toward the ground under the influence of the earth's gravity. The plunge seems dramatic. Just as Galileo and Newton explain in their theories of free fall, the least massive and most massive riders fall to the earth with the same rate of acceleration. If the riders were allowed to hit the earth at that speed, coming to a sudden stop at the end of the ride, there would certainly be serious injuries. Ride designers account for this by building an exit track. The car is attached to this track, which gradually curves toward the ground. A stretch of straight track allows the car to slow down and brake, producing a controlled stop at the bottom, that keeps passengers from getting injured.

Pendulum

Pendulum rides are a little like the swing sets you might remember from your childhood. Swings give you a feeling of flying in a controlled manner. You pump your legs to provide enough force to increase the height of the swing's arc, and enjoy the increased velocity of the downward swing. When you stop pumping, the swing gradually slows and then stops.

What causes the feeling of "weightlessness" on pendulum rides?

Riders often experience near-weightlessness as they approach the top of a pendulum ride. If the ride is the type that makes a complete 360-degree circle, they experience a feeling of complete weightlessness.

Feelings of weightlessness are not due to a decrease in forces of gravitation; people do not feel forces of gravity. What you feel is the force of a seat (or other external object) pushing on your body with a force to counteract gravity's downward pull. A 180-pound person at rest in his office chair experiences the seat pushing upwards on his body with a force of 180 pounds. Yet at the top of a pendulum ride, the same 180-pound person will feel less than this normal sensation of weight. At the very top of the pendulum ride, riders begin to fall out of their seats. Since a 180-pound person is no longer in full contact with his seat, the seat is no longer pushing on him with 180 pounds of force. Thus, the rider has a sensation of weighing less than his normal weight.

Why do riders experience high g-forces on pendulum rides?

As riders pass through the bottom of the circular arc, they often experience high g-forces. Once again, these g-forces are not evidence of increasing forces of gravitation, but the result of increases in the amount of force applied by the seat upon their bodies. Understanding this demands a little information about circular motion.

The motion of an object in a circle requires that there be a force directed toward the center of the circle (sometimes called a "centripetal force"). This means that at the bottom of the circular swing, there must be an upward force (since the circle's center is upward). Gravitational forces are always directed downward upon a rider's body; thus, gravitational forces cannot meet this centripetal force requirement. The seat must supply the centripetal force, pushing upwards on the rider with a force greater than gravity's downward pull. For a 180-pound person, the seat might have to supply 360 pounds of upward pull. This is twice the usual amount experienced by our 180-pound rider. For this reason, we would say the rider experiences 2 g's of force (a seat force that is 2 times the gravity force).

Pendulum

Pendulum rides are a little like the swing sets you might remember from your childhood. Swings give you a feeling of flying in a controlled manner. You pump your legs to provide enough force to increase the height of the swing's arc, and enjoy the increased velocity of the downward swing. When you stop pumping, the swing gradually slows and then stops.

What causes the feeling of "weightlessness" on pendulum rides?

Riders often experience near-weightlessness as they approach the top of a pendulum ride. If the ride is the type that makes a complete 360-degree circle, they experience a feeling of complete weightlessness.

Feelings of weightlessness are not due to a decrease in forces of gravitation; people do not feel forces of gravity. What you feel is the force of a seat (or other external object) pushing on your body with a force to counteract gravity's downward pull. A 180-pound person at rest in his office chair experiences the seat pushing upwards on his body with a force of 180 pounds. Yet at the top of a pendulum ride, the same 180-pound person will feel less than this normal sensation of weight. At the very top of the pendulum ride, riders begin to fall out of their seats. Since a 180-pound person is no longer in full contact with his seat, the seat is no longer pushing on him with 180 pounds of force. Thus, the rider has a sensation of weighing less than his normal weight.

Why do riders experience high g-forces on pendulum rides?

As riders pass through the bottom of the circular arc, they often experience high g-forces. Once again, these g-forces are not evidence of increasing forces of gravitation, but the result of increases in the amount of force applied by the seat upon their bodies. Understanding this demands a little information about circular motion.

The motion of an object in a circle requires that there be a force directed toward the center of the circle (sometimes called a "centripetal force"). This means that at the bottom of the circular swing, there must be an upward force (since the circle's center is upward). Gravitational forces are always directed downward upon a rider's body; thus, gravitational forces cannot meet this centripetal force requirement. The seat must supply the centripetal force, pushing upwards on the rider with a force greater than gravity's downward pull. For a 180-pound person, the seat might have to supply 360 pounds of upward pull. This is twice the usual amount experienced by our 180-pound rider. For this reason, we would say the rider experiences 2 g's of force (a seat force that is 2 times the gravity force).

Ride Safety

Going on amusement park rides is one of the safest forms of recreation. According to the International Association of Amusement Park Attractions, you are more likely to be injured when you play sports, ride a horse, or even ride a bicycle. Statistics show the occurrence of death to be approximately one in 250 million riders.

What do other numbers say?
This group's statistics are supported by those of the National Consumer Product Safety Commission. It estimates that more than 270 million people visit amusement parks each year, and that 7,000 people out of those 270 million go to emergency rooms for injuries they receive on amusement park rides--that's only 0.00259 percent of riders.

What causes injuries?
Both of these groups report that the main reason for deaths and injuries on amusement park rides is preventable error. This would include such things as the lack of routine maintenance and the disregard of safety rules by both operators and riders. Almost every ride has a set of safety rules. These usually require that riders meet certain criteria relating to age, height, and weight, or warn them not to ride if they have certain medical conditions.

For example, small children might be barred from some rides because of their low body mass. People with back or neck problems may be at a greater risk of injury on rides that create force on these areas. A ride's designers understand the forces acting on the rider and create the safety rules for his reason.

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Venus Tunay BSPT2

Experiment
Part I: The Focal Length of a Converging Lens
For this part, use the large bi-convex lens, an "object" (lamp), and an imaging screen. See Figure 12.1 for set up.
Assuming the thin lens equation (12.1) is satisfied, we may use the optical bench to measure u and v for several images in order to obtain a value for the focal length of the biconvex lens. The sign convention for equation (12.1) is that v is taken to be positive to the right of the lens and negative to the left.
Procedure
Place the object on the optical bench near one end, and place the screen at the opposite end, for example at 0 cm and 100 cm.
Slide the lens along the bench between object and screen and find all (two or less) positions where the lens produces a sharp image on the screen.
For each position of the lens, record u and v in a table and describe the image using the terms real or virtual, erect or inverted, and magnified or reduced.
Move the screen about 10 cm closer to the object and try to find all images again. Record u and v. and the image attributes.
Repeat, moving the screen closer to the object by 10 cm until there are no more sharp images on the screen. At this point the two images have come together until there is only one. There are no more images for closer separations.
Part II: The Focal Length of a Concave Mirror
For this part use the concave mirror, the lamp, and the imaging screen. The thin lens equation is still obeyed, only u and v are defined as in figure 12.2, where v is taken to be negative behind the mirror.
Figure 12.2
Procedure
Place the mirror at one end of the optical bench as in figure 12.3. It will remain there the whole time. At the opposite end put the lamp.
Figure 12.3
Now slide the screen along the rule and find all the positions where the mirror produces a sharp image on the screen. There may be some difficulty in locating the image because the screen will block the rays from the object. You will have to aim the object slightly to one side (or angle it up) and hold the screen next to the optical bench.
In the table on your lab sheet record the values of u and v and the attributes of the image.
Next move the object 10 cm closer to the mirror, finding again the place(s) where image(s) are formed. Record u and v and the image attributes.
Repeat these observations moving the object each time 10 cm closer to the mirror, until it is no longer possible to form an image on the screen.
Part III: The Compound Microscope
The compound microscope uses two lenses. The one closest to the object is called the objective lens; the one closest to the observer is called the eyepiece.
The objective lens has a small focal length, possibly only a few millimeters. The object is placed slightly beyond the focal point of the objective in such a location as to form an enlarged real image inside the focal point of the eyepiece.
The eyepiece is then usually adjusted to form an enlarged virtual image about 25 cm from the viewer, as 25 cm is the standard value for the "near point" of the eye, or the closest point upon which the eye can focus. By having the image at the near point, its apparent size is maximized. The total magnification M of the microscope is the product of the lateral magnifications of the objective lens m1 and of the eyepiece m2 (i.e., |M| < 1 ="=""> image reduction, if M is positive the image is erect, and if M is negative the image is inverted.),
M = m1 m2. (12.2)
If the eyepiece forms a virtual image 25 cm behind the eyepiece, the image distance v is equal to -25 cm. We then may use the thin lens equation to write the magnification as
, (12.3)
where fe = 20cm is the focal length of the eyepiece.
We will use the above equation for m2 along with a measured value for m1, to investigate the total magnification M of the microscope.

Procedure
Construct a compound microscope on the optical bench as in figure 12.3 by combining the short focal length bi-convex lens with the large bi-convex lens used in the first section.
Figure 12.3
Tape a resistor to the side of the imaging screen and illuminate it with a desk lamp. Adjust the positions of the two lenses so that you obtain the best magnified focused image.
Holding the ruler between your eye and the eyepiece, make a rough measurement of the apparent size of the object.
Keeping the distance between the ruler and the object(and your eye) the same, remove the object from the microscope and estimate its actual size (as in Figure 12.4). The ratio of these sizes gives an estimate of the magnification M for the lens system.

Prelab Homework

The prelab homework must be done at home and handed to the lab TA before you start the lab. Read the instructions for this lab.

Questions

• Determine the locations of the images of the objects( ) drawn below.

• Describe the images using the terms magnified or reduced, erect or inverted and real or virtual.

  

  

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Experiment

Part I: The Focal Length of a Converging Lens

For this part, use the large bi-convex lens, an "object" (lamp), and an imaging screen. See Figure 12.1 for set up.

Figure 12.1

Assuming the thin lens equation (12.1) is satisfied, we may use the optical bench to measure u and v for several images in order to obtain a value for the focal length of the biconvex lens. The sign convention for equation (12.1) is that v is taken to be positive to the right of the lens and negative to the left.

Procedure

• Place the object on the optical bench near one end, and place the screen at the opposite end, for example at 0 cm and 100 cm.

• Slide the lens along the bench between object and screen and find all (two or less) positions where the lens produces a sharp image on the screen.

• For each position of the lens, record u and v in a table and describe the image using the terms real or virtual, erect or inverted, and magnified or reduced.

• Move the screen about 10 cm closer to the object and try to find all images again. Record u and v. and the image attributes.

• Repeat, moving the screen closer to the object by 10 cm until there are no more sharp images on the screen. At this point the two images have come together until there is only one. There are no more images for closer separations.

Part II: The Focal Length of a Concave Mirror

For this part use the concave mirror, the lamp, and the imaging screen. The thin lens equation is still obeyed, only u and v are defined as in figure 12.2, where v is taken to be negative behind the mirror.

Figure 12.2

Procedure

• Place the mirror at one end of the optical bench as in figure 12.3. It will remain there the whole time. At the opposite end put the lamp.

  
  

  Figure 12.3

• Now slide the screen along the rule and find all the positions where the mirror produces a sharp image on the screen. There may be some difficulty in locating the image because the screen will block the rays from the object. You will have to aim the object slightly to one side (or angle it up) and hold the screen next to the optical bench.

• In the table on your lab sheet record the values of u and v and the attributes of the image.

• Next move the object 10 cm closer to the mirror, finding again the place(s) where image(s) are formed. Record u and v and the image attributes.

• Repeat these observations moving the object each time 10 cm closer to the mirror, until it is no longer possible to form an image on the screen.

Part III: The Compound Microscope

The compound microscope uses two lenses. The one closest to the object is called the objective lens; the one closest to the observer is called the eyepiece.

The objective lens has a small focal length, possibly only a few millimeters. The object is placed slightly beyond the focal point of the objective in such a location as to form an enlarged real image inside the focal point of the eyepiece.

The eyepiece is then usually adjusted to form an enlarged virtual image about 25 cm from the viewer, as 25 cm is the standard value for the "near point" of the eye, or the closest point upon which the eye can focus. By having the image at the near point, its apparent size is maximized. The total magnification M of the microscope is the product of the lateral magnifications of the objective lens m1 and of the eyepiece m2 (i.e., |M| < 1 ="=""> image reduction, if M is positive the image is erect, and if M is negative the image is inverted.),

M = m1 m2. (12.2)

If the eyepiece forms a virtual image 25 cm behind the eyepiece, the image distance v is equal to -25 cm. We then may use the thin lens equation to write the magnification as

, (12.3)

where fe = 20cm is the focal length of the eyepiece.

We will use the above equation for m2 along with a measured value for m1, to investigate the total magnification M of the microscope.

Procedure

• Construct a compound microscope on the optical bench as in figure 12.3 by combining the short focal length bi-convex lens with the large bi-convex lens used in the first section.

  

  Figure 12.3

• Tape a resistor to the side of the imaging screen and illuminate it with a desk lamp. Adjust the positions of the two lenses so that you obtain the best magnified focused image.

• Holding the ruler between your eye and the eyepiece, make a rough measurement of the apparent size of the object.

• Keeping the distance between the ruler and the object(and your eye) the same, remove the object from the microscope and estimate its actual size (as in Figure 12.4). The ratio of these sizes gives an estimate of the magnification M for the lens system.

Figure 12.4

Part IV: The Telescope

A. The Astronomical Refracting Telescope

For an astronomical telescope, the objective lens is always a lens with a long focal length. The objective lens produces a real image at its focal point. This image is magnified with the eyepiece. In this case, the eyepiece is usually adjusted so that the virtual image is at infinity, in this case a piece of paper across the room, but it may be adjusted to be closer.

Figure12.5

Because theta and theta1 are small, they can be approximated as tan(theta) and tan(theta1), respectively. If h is the height of the real image I1 formed by the objective lens, then

, (12.3)

, (12.4)

and so,

,

where fo and fe are the focal lengths of the objective and eyepiece lenses, respectively. In addition, fo is the distance from the objective lens to I1 (because the image is at infinity) and fe is the distance from the eyepiece to I1 if the eyepiece is adjusted to give an image at infinity. The angular magnification then is,

(12.5)

. (12.6)

Procedure for Astronomical Telescope

• Construct an astronomical telescope on the optical bench as in figure 12.5 using the small bi-convex lens for the eyepiece and the large bi-convex lens for the objective.

• Attach the lenses to the optical bench such that the distance between the lenses is about equal to the sum of their focal lengths.

• Focus on a distant target using the telescope and estimate its apparent size. A good object to help in your estimation is a ruler or something with a regular pattern of lines such as the window blinds.

• Estimate the size of the object when you look directly at it with the naked eye. Also record what type of image you see (upright or inverted, left-to-right inversion or not).

B. The Terrestrial Telescope (i.e. Opera Glass)

As in the astronomical telescope, the terrestrial telescope utilizes a long focal length lens and a short focal length eyepiece to produce a magnified image of a distant object. The magnification is again given by M = fo/fe. However, the eyepiece is a "meniscus" or concave lens rather than a convex lens, and things are somewhat different for this kind of telescope.

Procedure

• Construct a terrestrial telescope on the optical bench by replacing the eyepiece from the astronomical telescope with a meniscus lens like that shown in figure 12.7. Use the same objective lens as in the astronomical telescope.

  
  
  

  Figure 12.7

• Put the eyepiece at the very end of the optical bench.

• Aim at a distant target and focus the telescope by sliding the objective back and forth until the image is sharply focused.

• In the same way as before, estimate the magnification of the telescope by comparing the image seen in the telescope with the image seen with the naked eye. Is the image erect or inverted, direct or left-to-right inverted?

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Data Analysis

Part I- The Bi-convex Lens

• Using the data from your table, graph u vs. v, on linear paper and draw a smooth curve through the points. From the equation, if u becomes very large, what happens to v? Also if v becomes very large, what happens to u? Compare these results with your graph.

• Draw a graph of 1/u vs. 1/v. The graph should be a line of slope -1 and the y-intercept is 1/f. The graph is of the thin lens equation,

  1/u + 1/v = 1/f (12.1)

• Using the graph in 2) above, estimate the focal length of the lens. Using the data in your table, calculate the focal length f and its uncertainty for each pair of values, u and v. Compute the average focal length for the lens and compare this to the graphical estimate from 2) above. This value will be useful later on in the experiment.

• Finally answer the following:
  • For the bi-convex lens, what is the closest that the image and object can come to each other, and how did you deduce this from the graph?
  • Objects which are closer to the lens than the focal point do not produce real images, so where do the object and image appear on the graph of u vs. v (indicate on your graph from 1))?
  • Describe the images seen using the terms magnified or reduced, erect or inverted, and real or virtual for the two cases below:
    • When the lens is closer to the object that to the image (v>u).
    • When the lens is closer to the image than to the object (u>v).

Part II- The Concave Mirror

• Make a graph of u vs. v and1/u vs. 1/v for the concave mirror. Label the axes and draw smooth curves through the points. Estimate the focal length of the mirror from the graph.

• Using the data in your table, calculate the focal length f and its uncertainty for each pair of values, u and v.

• Compute the average focal length for the mirror and compare this to the graphical estimate.

• Describe the images seen using the terms magnified or reduced, erect or inverted, and real or virtual for the two cases below:
  • when the mirror is closer to the object than to the image (v > u).
  • when the mirror is closer to the image than to the object (u > v).

Part III- The Compound Microscope

• Answer the following questions:
  • Is the image erect or inverted? Is there a left-to-right inversion (i.e.-left and right switched, as in a mirror?)?
  • The quality of the image is probably quite poor. Describe what is wrong with it.

• Given the parameters fe,fo, and u (object distance from first lens), find an expression for M, the magnification of the microscope. Leave the expression in terms of the above quantities.

Parts IVA&B-The Astronomical and Terrestrial Telescopes

• Compare your estimated value of M for the astronomical telescope to the theoretical value M = fo/fe and answer the following questions:
  • Is the image erect or inverted?
  • Is there a left-for-right inversion?

• Answer the above questions for the terrestrial telescope.

• In terms of the image quality, or "ease of use," are there any important differences between the astronomical and terrestrial telescopes?

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* For Your Rumination

The inverted real image

Here is a quick, easy, and perhaps perplexing demonstration you can do at home. There is no doubt that images in the eye are inverted, for this is the result of the crossing of light rays in the optical system of the eye. Why, if the images are upside-down, do we see things the right side up? It is quite possible to read plain print upside-down, and children just beginning to read sometimes choose to do so. A constellation of stars visible in both the northern and southern hemispheres is upside-down in one and not in the other, yet neither observer thinks of himself as anything but the right side up! Just as we "see" the normal, inverted image the right side up, if we produce an image right side up we may expect to "see" it inverted.

Make a pinhole (with a straight pin) in a piece of a file card. Hold the card about seven inches in front of one eye and look at the sky, the ceiling, or a blank wall. Then hold the pin vertically with its head uppermost between the card and your eye. The pinhead is too near to be focused in the ordinary way by the eye, but the rays of light from the hole cause it to cast a shadow on the retina. This shadow is right side up, and so the pinhead is seen upside-down in the hole. If there are several closely spaced holes there will be several shadows, and an inverted pinhead will appear in each hole!

If you have the time, study the inversion of your own image in a mirror. Why does the left side of your head appear on the right, and the right on the left in the image? Try tipping your head to one side and looking in the mirror. Why does your chin not replace your forehead and vice versa in the image? Is the inversion of the images that we see in a mirror merely a fallacy of our own perception? Can you sketch the paths of rays of light into your eye from the parts of your head you observe in a mirror?

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Radioactive decay

For decay rate in a more general context, see Particle decay.

Nuclear physics

Radioactive decay Nuclear fission Nuclear fusion Becquerel · Bethe · Marie Curie · Pierre Curie · Fermi

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a random process on the atomic level, in that it is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable.

The SI unit of radioactive decay (the phenomenon of natural and artificial radioactivity) is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of TBq (terabecquerel) or GBq (gigabecquerel) are commonly used. Another unit of (radio)activity is the curie, Ci, which was originally defined as the activity of one gram of pure radium, isotope Ra-226. At present it is equal (by definition) to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 10¹⁰ Bq. The use of Ci is presently discouraged by SI.

Explanation

The neutrons and protons that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The electrostatic force is also significant, while the weak nuclear force is responsible for beta decay.

The interplay of these forces is simple. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could fall into a lower-energy arrangement (with the extra energy moving elsewhere). One might draw an analogy with a snowfield on a mountain: while friction between the snow crystals can support the snow's weight, the system is inherently unstable with regard to a lower-potential-energy state, and a disturbance may facilitate the path to a greater entropy state (i.e., towards the ground state where heat will be produced, and thus total energy is distributed over a larger number of quantum states). Thus, an avalanche results. The total energy does not change in this process, but because of entropy effects, avalanches only happen in one direction, and the end of this direction, which is dictated by the largest number of chance-mediated ways to distribute available energy, is what we commonly refer to as the "ground state".

Such a collapse (a decay event) requires a specific activation energy. In the case of a snow avalanche, this energy classically comes as a disturbance from outside the system, although such disturbances can be arbitrarily small. In the case of an excited atomic nucleus, the arbitrarily small disturbance comes from quantum vacuum fluctuations. A nucleus (or any excited system in quantum mechanics) is unstable, and can thus spontaneously stabilize to a less-excited system. This process is driven by entropy considerations: the energy does not change, but at the end of the process, the total energy is more diffused in spacial volume. The resulting transformation alters the structure of the nucleus. Such a reaction is thus a nuclear reaction, in contrast to chemical reactions, which also are driven by entropy, but which involve changes in the arrangement of the outer electrons of atoms, rather than their nuclei.

Some nuclear reactions do involve external sources of energy, in the form of collisions with outside particles. However, these are not considered decay. Rather, they are examples of induced nuclear reactions. Nuclear fission and fusion are common types of induced nuclear reactions.

Discovery

Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might be connected with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent minerals on it. All results were negative until he used uranium salts. The result with these compounds was a deep blackening of the plate.

It soon became clear that the blackening of the plate had nothing to do with phosphorescence, because the plate blackened when the mineral was in the dark. Non-phosphorescent salts of uranium and metallic uranium also blackened the plate. Clearly there was a form of radiation that could pass through paper that was causing the plate to blacken.

Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum shielding. Gamma rays can only be reduced by much more substantial barriers, such as a very thick layer of lead.

At first it seemed that the new radiation was similar to the then recently discovered X-rays. Further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula (see below).

As for types of radioactive radiation, it was found that an electric or magnetic field could split such emissions into three types of beams. For lack of better terms, the rays were given the alphabetic names alpha, beta and gamma, still in use today. It was obvious from the direction of electromagnetic forces that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was clear that alpha particles were much more massive than beta particles. Passing alpha particles through a very thin glass window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are helium nuclei. Other experiments showed the similarity between beta radiation and cathode rays; they are both streams of electrons, and between gamma radiation and X-rays, which are both high energy electromagnetic radiation.

Although alpha, beta, and gamma are most common, other types of decay were eventually discovered. Shortly after discovery of the neutron in 1932, it was discovered by Enrico Fermi that certain rare decay reactions yield neutrons as a decay particle. Isolated proton emission was eventually observed in some elements. Shortly after the discovery of the positron in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons (positron emission), analogously to negative electrons. Each of the two types of beta decay acts to move a nucleus toward a ratio of neutrons and protons which has the least energy for the combination. Finally, in a phenomenon called cluster decay, specific combinations of neutrons and protons other than alpha particles were spontaneously emitted from atoms on occasion.

Still other types of radioactive decay were found which emit previously seen particles, but by different mechanisms. An example is internal conversion, which results in electron and sometimes high energy photon emission, even though it involves neither beta nor gamma decay.

The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element polonium and to separate a new element radium from barium. The two elements' chemical similarity would otherwise have made them difficult to distinguish.

The danger classification sign of radioactive materials

The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when the Serbo-Croatian-American electric engineer Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to X-rays. His injuries healed later.

The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927 Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel prize for his findings.

Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and radioactive quackery. Examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood (Curie later died from aplastic anemia assumed due to her work with radium, but later examination of her bones showed that she had been a careful laboratory worker and had a low burden of radium. A more likely cause was her exposure to unshielded X-ray tubes while a volunteer medical worker in WWI). By the 1930s, after a number of cases of bone necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.

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Elaine M. San Andres BSPT 2

Fractional horse-power motor

Demonstration

If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed.

Apparatus and materials

• Electric motor, fractional horse-power
• String, length of
• Power supply, low-voltage, variable
• Demonstration force meter (5 kg / 50 N)
• Retort stand, boss, and clamp

Technical notes

The fractional horse-power motor should operate from approximately 12 volts DC, which is conveniently obtainable from the variable low-voltage supply. The field and armature connections should both be connected, in parallel, to the voltage supply.

It is helpful to use a motor with a removable plate which can be taken off to reveal the commutator and brushes.

Safety

Read our standard health & safety guidance

Procedure

a Show the motor in operation.

b Remove the plate on the end to show the brushes and internal movement. Allow students to look at this so that they can identify the parts. Replace the plate.

c Attach the string to the spindle of the motor. Do this by tying it to a spoke of the pulley wheel and winding it several times round the spindle.

d Attach the other end to the demonstration force meter which is suspended from a retort stand.

e Increase the voltage of the supply gradually. Observe the force with which the motor pulls on the force meter. (Note that the motor should not be in this condition for long, as it is being heated with many watts of electrical power. Raise the voltage carefully to avoid overloading the power supply and the motor.)

Teaching notes

1 If students have built their own model electric motor, it is useful if they can see a commercial motor doing a useful job and find out how it is constructed. In most cases the magnets are actually electromagnets. (Beware: most commercial motors are induction motors rather than moving coil motors!) The armature is likely to be wound in slots in a soft iron block so that it acts as several armatures placed at an angle to each other. This is so that the motor runs more smoothly.

2 An ammeter placed in the armature circuit will show how the current changes when the motor is doing a job such as hauling up a load.