Shadows and rays on a screen

Shadows and rays on a screen

Demonstration

Shadows form on a screen when objects interrupt the light from a lamp.

Apparatus and materials

• Compact light source (quartz iodine lamp) or 100 W 12 V lamp

• L.T. variable voltage supply (12 V 8 A)

• Retort stand and boss

• White screen or blank wall

• Card with slot

Technical notes

If you don't have a compact light source use a 48 W 12 V lamp.
Safety

Be aware that compact light sources using tungsten-halogen lamps without filters are significant sources of UV. Ensure that no-one can look directly at the lamps.

Procedure

a Set up the compact light source in front of the screen or wall, preferably with the room darkened.

b Put various obstacles between the source and the screen so that shadows are seen. A card with a slot will give straight shadows on the wall. The shadows will be sharpest when the edges of the card and the slot are parallel to the lamp filament.

c Move the light source closer to the screen so that a wide beam of illumination falls across it.



d Bring up a card with a slit in it as illustrated to show how a 'ray' of light can be made.

Teaching notes

1 The demonstration in b shows what happens when a lamp throws light onto an object and a shadow falls onto a screen.

2 You can vary the relative distances between the lamp and object, and the screen and object, to show the umbra and penumbra in the shadow. You might want to mention eclipses.

3 The narrow slit placed in the beam in d shows how a ray of light can be produced. The ray of light appears to travel in a straight line.

Kinematic Equations and Problem-Solving

The Kinematic Equations

The variety of representations which we have investigated includes verbal representations, pictorial representations, numerical representations, and graphical representations (position-time graphs and velocity-time graphs). We will investigate the use of equations to describe and represent the motion of objects. These equations are known as kinematic equations.

There are a variety of quantities associated with the motion of objects - displacement (and distance), velocity (and speed), acceleration, and time. Knowledge of each of these quantities provides descriptive information about an object's motion. For example, if a car is known to move with a constant velocity of 22.0 m/s, North for 12.0 seconds for a northward displacement of 264 meters, then the motion of the car is fully described. And if a second car is known to accelerate from a rest position with an eastward acceleration of 3.0 m/s2 for a time of 8.0 seconds, providing a final velocity of 24 m/s, East and an eastward displacement of 96 meters, then the motion of this car is fully described. These two statements provide a complete description of the motion of an object. However, such completeness is not always known. It is often the case that only a few parameters of an object's motion are known, while the rest are unknown. For example as you approach the stoplight, you might know that your car has a velocity of 22 m/s, East and is capable of a skidding acceleration of 8.0 m/s2, West. However you do not know the displacement which your car would experience if you were to slam on your brakes and skid to a stop; and you do not know the time required to skid to a stop. In such an instance as this, the unknown parameters can be determined using physics principles and mathematical equations (the kinematic equations).

The kinematic equations are a set of four equations which can be utilized to predict unknown information about an object's motion if other information is known. The equations can be utilized for any motion which can be described as being either a constant velocity motion (an acceleration of 0 m/s/s) or a constant acceleration motion. They can never be used over any time period during which the acceleration is changing. Each of the kinematic equations include four variables. If the values of three of the four variables are known, then the value of the fourth variable can be calculated. In this manner, the kinematic equations provide a useful means of predicting information about an object's motion if other information is known. For example, if the acceleration value and the initial and final velocity values of a skidding car is known, then the displacement of the car and the time can be predicted using the kinematic equations. Lesson 6 of this unit will focus upon the use of the kinematic equations to predict the numerical values of unknown quantities for an object's motion.

The four kinematic equations which describe an object's motion are:



There are a variety of symbols used in the above equations. Each symbol has its own specific meaning. The symbol d stands for the displacement of the object. The symbol t stands for the time for which the object moved. The symbol a stands for the acceleration of the object. And the symbol v stands for the velocity of the object; a subscript of i after the v (as in vi) indicates that the velocity value is the initial velocity value and a subscript of f (as in vf) indicates that the velocity value is the final velocity value.

Each of these four equations appropriately describe the mathematical relationship between the parameters of an object's motion. As such, they can be used to predict unknown information about an object's motion if other information is known.

Lightning

Lightning

Perhaps the most known and powerful displays of electrostatics in nature is a lightning storm. Lightning storms are inescapable from humankind's attention. They are never invited, never planned and never gone unnoticed. The rage of a lightning strike will wake a person in the middle of the night. They send children rushing into parent's bedrooms, crying for assurance that everything will be safe. The fury of a lightning strike is capable of interrupting midday conversations and activities. They're the frequent cause of canceled ball games and golf outings. Children and adults alike crowd around windows to watch the lightning displays in the sky, standing in awe of the power of static discharges. Indeed, a lightning storm is the most powerful display of electrostatics in nature.

In this part of Lesson, we will ponder two questions:

• What is the cause and mechanism associated with lightning strikes?
• How do lightning rods serve to protect buildings from the devastating affects of a lightning strike?

Static Charge Buildup in the Clouds

The scientific community has long pondered the cause of lightning strikes. Even today, it is the subject of a good deal of scientific research and theorizing. The details of how a cloud becomes statically charged is not completely understood (as of this writing). Nonetheless there are several theories which make a good deal of sense and which demonstrate many concepts previously discussed in this unit of The Physics Classroom.

The precursor of any lightning strike is the polarization of positive and negative charges within a storm cloud. The tops of the storm clouds are known to acquire an excess of positive charge and the bottom of the storm clouds acquire an excess of negative charge. Two mechanisms seem important to the polarization process. One mechanism involves a separation of charge by a process which bears resemblance to frictional charging. Clouds are known to contain countless millions of suspended water droplets and ice particles moving and whirling about in turbulent fashion. Additional water from the ground evaporates, rises upward and forms clusters of droplets as it approaches a cloud. This upwardly rising moisture collides with water droplets within the clouds. In the collisions, electrons are ripped off the rising droplets, causing a separation of negative electrons from a positively charged water droplet or a cluster of droplets.

The second mechanism which contributes to the polarization of a storm cloud involves a freezing process. Rising moisture encounters cooler temperatures at higher altitudes. These cooler temperatures cause the cluster of water droplets to undergo freezing. The frozen particles tend to cluster more tightly together and form the central regions of the cluster of droplets. The frozen portion of the cluster of rising moisture becomes negatively charged and the outer droplets acquire a positive charge. Air currents within the clouds can rip the outer portions off the clusters and carry them upward toward the top of the clouds. The frozen portion of the droplets with their negative charge tend to gravitate towards the bottom of the storm clouds. Thus, the clouds become further polarized.

These two mechanisms are believed to be the primary causes of the polarization of storm clouds. In the end, a storm cloud becomes polarized with positive charges carried to the upper portions of the clouds and negative portions gravitating towards the bottom of the clouds. The polarization of the clouds has an equally important affect on the surface of the Earth. The cloud's electric field stretches through the space surrounding it and induces movement of electrons upon Earth. Electrons on Earth's outer surface are repelled by the negatively charged cloud's bottom surface. This creates an opposite charge on the Earth's surface. Buildings, trees and even people can experience a buildup of static charge as electrons are repelled by the cloud's bottom. With the cloud polarized into opposites and with a positive charge induced upon Earth's surface, the stage is set for Act 2 in the drama of a lightning strike.

The Mechanics of a Lightning Strike

The Mechanics of a Lightning Strike

As the static charge buildup in a storm cloud increases, the electric field surrounding the cloud becomes stronger. Normally, the air surrounding a cloud would be a good enough insulator to prevent a discharge of electrons to Earth. Yet, the strong electric fields surrounding a cloud are capable of ionizing the surrounding air and making it more conductive. The ionization involves the shredding of electrons from the outer shells of gas molecules. The gas molecules which compose air are thus turned into a soup of positive ions and free electrons. The insulating air is transformed into a conductive plasma. The ability of a storm cloud's electric fields to transform air into a conductor makes charge transfer (in the form of a lightning bolt) from the cloud to the ground (or even to other clouds) possible.

A lightning bolt begins with the development of a step leader. Excess electrons on the bottom of the cloud begin a journey through the conducting air to the ground at speeds up to 60 miles per second. These electrons follow zigzag paths towards the ground, branching at various locations. The variables which affect the details of the actual pathway are not well known. It is believed that the presence of impurities or dust particles in various parts of the air might create regions between clouds and earth which are more conductive than other regions. As the step leader grows, it might be illuminated by the purplish glow which is characteristic of ionized air molecules. Nonetheless, the step leader is not the actual lightning strike, it merely provides the roadway between cloud and Earth along which the lightning bolt will eventually travel.

As the electrons of the step leader approach the Earth, there is an additional repulsion of electrons downward from Earth's surface. The quantity of positive charge residing on the Earth's surface becomes even greater. This charge begins to migrate upward through buildings, trees and people into the air. This upward rising positive charge - known as a streamer - approaches the step leader in the air above the surface of the Earth. The streamer might meet the leader at an altitude equivalent to the length of a football field. Once contact is made between the streamer and the leader, a complete conducting pathway is mapped out and the lightning begins. The contact point between ground charge and cloud charge rapidly ascends upward at speeds as high as 50 000 miles per second. As many as a billion trillion electrons can transverse this path in less than a millisecond. This initial strike is followed by several secondary strikes or charge surges in rapid succession. These secondary surges are spaced apart so closely in time that may appear as a single strike. The enormous and rapid flow of charge along this pathway between the cloud and Earth heats the surrounding air, causing it to expand violently. The expansion of the air creates a shockwave which we observe as thunder.

Lightning Rods and Other Protective Measures

Tall buildings, farm houses and other structures susceptible to lightning strikes are often equipped with lightning rods. The attachment of a grounded lightning rod to a building is a protective measure which is taken to protect the building in the event of a lightning strike. The concept of a lightning rod was originally developed by Ben Franklin. Franklin proposed that lightning rods should consist of a pointed metal pole which extends upward above the building which it is intended to protect. Franklin suggested that a lightning rod protects a building by one of two methods. First, the rod serves to prevent a charged cloud from releasing a bolt of lightning. And second, the lightning rod serves to safely divert the lightning to the ground in event that the cloud does discharge its lightning via a bolt. Franklin's theories on the operation of lightning rods have endured for a couple of centuries. And not until the most recent decades have scientific studies provided evidence to confirm the manner in which they operate to protect buildings from lightning damage.

The first of Franklin's two proposed theories is often referred to as the lightning dissipation theory. According to the theory, the use of a lightning rod on a building protects the building by preventing the lightning strike. The idea is based upon the principle that the electric field strength is great around a pointed object. The intense electric fields surrounding a pointed object serve to ionize the surrounding air, thus enhancing its conductive ability. The dissipative theory states that as a storm cloud approaches, there is a conductive pathway established between the statically charged cloud and the lightning rod. According to the theory, static charges gradually migrate along this pathway to the ground, thus reducing the likelihood of a sudden and explosive discharge. Proponents of the lightning dissipation theory argue that the primary role of a lightning rod is to discharge the cloud over a longer length of time, thus preventing the excessive charge buildup which is characteristic of a lightning strike.

The second of Franklin's proposed theories on the operation of the lightning rod is the basis of the lightning diversion theory. The lightning diversion theory states that a lighting rod protects a building by providing a conductive pathway of the charge to the Earth. A lightning rod is typically attached by a thick copper cable to a grounding rod which is buried in the Earth below. The sudden discharge from the cloud would be drawn towards the elevated lightning rod but safely directed to the Earth, thus preventing damage from occurring to the building. The lightning rod and the attached cable and ground pole provide a low resistance pathway from the region above the building to the ground below. By diverting the charge through the lightning protection system, the building is spared of the damage associated with a large quantity of electric charge passing through it.

Lightning researchers are now generally convinced that the lightning dissipation theory provides an inaccurate model of how lightning rods work. It is indeed true that the tip of a lightning rod is capable of ionizing the surrounding air and making it more conductive. However, this affect only extends for a few meters above the tip of the lightning rod. A few meters of enhanced conductivity above the tip of the rod is not capable of discharging a large cloud which stretches over several kilometers of distance. Unfortunately, there are currently no scientifically verified methods of lightning prevention. Furthermore, recent field studies have further shown that the tip of the lightning rod does not need to be sharply pointed as Ben Franklin suggested. Blunt-tipped lightning rods have been found to be more receptive to lightning strikes and thus provide a more likely path of discharge of a charged cloud. When installing a lightning rod on a building as a lightning protection measure, it is imperative that the rod be elevated above the building and connected by a low resistance wire to the ground.

The Electric Field Concept

The Electric Field Concept

As children grow, they become very accustomed to contact forces; but an action-at-a-distance force upon first observation is quite surprising. Seeing two charged balloons repel from a distance or two magnets attract from a distance raises the eyebrow of a child and maybe even causes a chuckle or a "wow." Indeed, an action-at-a-distance or non-contact force is quite unusual. Football players don't run down the field and encounter collision forces from five yards apart. The rear-end collision at a stop sign is not characterized by repulsive forces which act upon the colliding cars at a spatial separation of 10 meters. And (with the exception modern WWF wrestling matches) the fist of one fighter does not act from 12 inches away to cause the forehead of a second fighter to be knocked backwards. Contact forces are quite usual and customary to us. Explaining a contact force which we all feel and experience on a daily basis is not difficult. Non-contact forces require a more difficult explanation. After all, how can one balloon reach across space and pull a second balloon towards it or push it away? The best explanation to this question involves the introduction of the concept of electric field.

Action-at-a-distance forces are sometimes referred to as field forces. The concept of a field force is utilized by scientists to explain this rather unusual force phenomenon that occurs in the absence of physical contact. While all masses attract when held some distance apart, charges can either repel or attract when held some distance apart. An alternative to describing this action-at-a-distance affect is to simply suggest that there is something rather strange about the space surrounding a charged object. Any other charged object that is in that space feels the affect of the charge. A charged object creates an electric field - an alteration of the space in the region which surrounds it. Other charges in that field would feel the unusual alteration of the space. Whether a charged object enters that space or not, the electric field exists. Space is altered by the presence of a charged object. Other objects in that space experience the strange and mysterious qualities of the space.

The strangeness of the space surrounding a charged object is often experienced first hand by the use of a Van de Graaff generator. A Van de Graaff generator is a large conducting sphere which acquires a charge as electrons are scuffed off of a rotating belt as it moves past sharp elongated prongs inside the sphere. The buildup of static charge on the Van de Graaff generator is much greater than that on a balloon rubbed with animal fur or an aluminum plate charged by induction. On a dry day, the buildup of charge becomes so great that it can exert influences on charged balloons held some distance away. If you were to walk near a Van de Graaff generator and hold out your hand, you might even notice the hairs on your hand standing up. And if you were to slowly walk near a Van de Graaff generator, your eyebrows might begin to feel quite staticy. The Van de Graaff generator, like any charged object, alters the space surrounding it. Other charged objects entering the space feel the strangeness of that space. Electric forces are exerted upon those charged objects when they enter that space. The Van de Graaff generator is said to create an electric field in the space surrounding it.

Grounding - the Removal of a Charge

Grounding - the Removal of a Charge

Grounding is the process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size. When a charged object is grounded, the excess charge is balanced by the transfer of electrons between the charged object and a ground. A ground is simply an object which serves as a seemingly infinite reservoir of electrons; the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object. In this last section of Lesson 2, the process of grounding will be discussed.

To begin our discussion of grounding, we will consider the grounding of a negatively charged electroscope. Any negatively charged object has an excess of electrons. If it is to have its charge removed, then it will have to lose its excess electrons. Once the excess electrons are removed from the object, there will be equal numbers of protons and electrons within the object and it will have a balance of charge. To remove the excess of electrons from a negatively charged electroscope, the electroscope will have to be connected by a conducting pathway to another object which is capable of receiving those electrons. The other object is the ground. In typical electrostatic experiments and demonstrations, this is simply done by touching the electroscope with one's hand. Upon contact, the excess electrons leave the electroscope and enter the person who touches it. These excess electrons subsequently spread about the surface of the person.

This process of grounding works because excess electrons find each other repulsive. As is always the case, repulsive affects between like-charged electrons forces them to look for a means of spatially separating themselves from each other. This spatial separation is achieved by moving to a larger object that allows a greater surface area over which to spread. Because of the relative size of a person compared to a typical electroscope, the excess electrons (nearly all of them) are capable of reducing the repulsive forces by moving into the person (i.e., the ground). Grounding is simply another example of charge sharing between two objects. The extent to which an object is willing to share excess charge is proportional to its size. So an effective ground is simply an object with significant enough size to share the overwhelming majority of excess charge.

The grounding of a negatively charged electroscope. Electrons were transferred from the electroscope to the ground. But what if the electroscope is positively charged? How does electron transfer allow an object with an excess of protons to become neutralized? To explore these questions, we will consider the grounding of a positively charged electroscope. A positively charged electroscope must gain electrons in order to acquire an equal number of protons and electrons. By gaining electrons from the ground, the electroscope will have a balance of charge and therefore be neutral. Thus, the grounding of a positively charged electroscope involves the transfer of electrons from the ground into the electroscope. This process works because excess positive charge on the electroscope attracts electrons from the ground (in this case, a person). While this may disrupt any balance of charge present on the person, the significantly larger size of the person allows for the excess charge to distance itself further from each other. As in the case of grounding a negatively charged electroscope, the grounding of a positively charged electroscope involves charge sharing. The excess positive charge is shared between the electroscope and the ground. And once again, the extent to which an object is willing to share excess charge is proportional to its size. The person is an effective ground because it has enough size to share the overwhelming majority of excess positive charge.

Lemon Power

Lemon Power

The lemon is a small tree (Citrus X limon, often given as C. limon) originally native to Asia, and is also the name of the tree's oval yellow fruit. The fruit is used for culinary and nonculinary purposes throughout the world – primarily for its juice, though the pulp and rind (zest) are also used, mainly in cooking baking . lemon juice is about 5% to 6% (approximately 0.3 Molar) citric acid which gives lemons a sour taste, and a pH of 2 to 3. This makes lemon juice an inexpensive, readily available acid for use in educational science experiments. Because of the sour flavor, many lemon-flavored drinks and candies are available, including lemonade and lemon heads

What to do with a lemon beside making lemonade

1. 18-gauge copper wire (smaller gauge will work too, but 18-gauge is stiffer)
2. Wire clippers
3. Steel paper clip (Some people find that a 2-inch strip of zinc works better)
4. Sheet of coarse sandpaper
5. Lemon
6. Help from an older friend or an adult

Have your older friend or an adult strip 2 inches of insulation off the copper wire. Clip the 2 inches of bare wire with the clippers.
Straighten out the paper clip and cut about 2 inches of the straightened steel wire, or use a 2-inch piece or strip of zinc.
Use sandpaper to smooth any rough spots on the ends of the wire and paper clip or piece of zinc.
Squeeze the lemon gently with your hands. But don't rupture the lemon's skin. Rolling it on a table with a little pressure works great.

















Push the pieces of the paper clip and the wire into the lemon so they are as close together as you can get them without touching.













Moisten your tongue with saliva. Touch the tip of your wet tongue to the free ends of the two wires.

You should be able to feel a slight tingle on the tip of your tongue and taste something metallic.

Nuclear Chain Reaction

Nuclear Chain Reaction

A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g. ²³⁵U) or the fusion of light isotopes (e.g. ²H and ³H). The nuclear chain reaction is unique since it releases several million times more energy per reaction than any chemical reaction.

1. Bunch of dominos
2. Ruler
3. Flat table that doesn't shake

Arrange the dominos in the pattern shown.
On another section of the table, arrange two straight lines of dominos.








Knock over the single domino in front on the first pattern. Watch what happens.
Now, knock over the first domino in one of the two straight lines.







Take the ruler and hold it anywhere between the dominos lined up in the second straight line. Knock over the first domino and watch what happens. Not all the dominos fell over.

Electricity:Open and short Circuits

Electricity:Open and short Circuits

Introduction to Electrical Circuits: Electrical circuits are the combination's of different electrical appliances connected in a particular manner. Electrical circuit is the symbolic representation of the circuit, which helps us to define all the parameter..

Lantern battery - DO NOT USE ANYTHING HIGHER THAN A NINE-VOLT BATTERY Small light bulb/lamp or small motor Wire to connect battery and lamp terminal (bare wire, not plastics or rubber covered) Wire clippers
Cut three pieces of wire. Connect the wires from the battery terminals to the lamp terminals - lamp or motor will light up or run.
	Take third piece of bare wire and drop across the two bare wires leading between the terminals - notice what happens. The lamp or motor should go out or stop.
Take the third wire that was laying across the other two wires. Take the wire clippers and cut one of the wires leading from the battery to one of the lamp terminals. The lamp or motor should also go out or stop.

Solar Hot Dog Cooker

Solar Hot Dog Cooker

A reflective hot dog cooker can be built from a cardboard box, tin foil, and poster board. Sunlight hits the reflective surface and focuses on the hot dog held in the center. Students can work in
pairs or individually if there are enough materials.

1. A cardboard box
2. tin foil
3. posterboard

Select a long narrow box; the longer the box the more heat collection is possible. Choose a focal length between 5" and 10" and design a parabolic curve as seen in the picture. One template could be used for all the cookers. Trace the curve on the open end of the box so that it is centered and straight.

Cut out the curve with a utility knife. Stress the importance of being exact. Measure and cut a piece of posterboard that will fix flush against the opening to the box. Attach this with tape beginning at the center and working toward to edges.
Cover the curve with white glue and apply aluminum foil shiny side out. Start in the middle and smooth toward the edges. Try not to wrinkle or fold the foil; you want it as smooth as possible.
Use two scraps of cardboard taped to each side as supports. Using the sun or a projector light, test the focal point. There should be a bright spot where light is concentrated; mark this spot and punch a hole for the skewer. Use a section of a coat hanger from which the paint has been removed for a skewer.
Enjoy your hot dog!

Greenhouse Effect

Greenhouse Effect

Recreating the Greenhouse Effect

The Earth's climate has changed many times in the past. Subtropical forests have spread from the south into more temperate (or milder, cooler climates) areas. Millions of years later, ice sheets spread from the north covering much of the northern United States, Europe and Asia with great glaciers. Today, nearly all scientists believe human beings are changing the climate. How can that be?

Over the past few centuries, people have been burning more amounts of fuels such as wood, coal, oil, natural gas and gasoline. The gases formed by the burning, such as carbon dioxide, are building up in the atmosphere. They act like greenhouse glass. The result, experts believe, is that the Earth heating up and undergoing global warming. How can you show the greenhouse effect?

1. Two identical glass jars
2. 4 cups cold water
3. 10 ice cubes
4. One clear plastic bag
5. Thermometer

Take two identical glass jars each containing 2 cups of cold water.
Add 5 ice cubes to each jar.
Wrap one in a plastic bag (this is the greenhouse glass).
Leave both jars in the sun for one hour.
Measure the temperature of the water in each jar.