How do you present essential scientific principles in an experiential and entertaining way, such as fog in a jar? Below is the physics behind selected museum exhibits
Alluvial container, Galileo
Published in Galileo, Issue 69, May 2004
As every year, this Passover the science festival was held at the Weizmann Institute of Science in Rehovot. It is a festive and fascinating event, which attracts many science enthusiasts and the curious of all ages. The abundance of workshops and the great variety of scientific demonstrations sometimes means that the visitor to the festival is left with half his lust in his hand.
The colorful and spectacular scientific experience was experienced, but many questions remained unanswered, due to the shortness of time and the need to adapt the explanations to the general public. The purpose of this list is to delve deeper into the physics behind some of the demonstrations presented at the festival; This is an attempt to show how essential scientific principles are presented in an experiential and entertaining way. Some of the demonstrations can (and is recommended!) be reproduced by home means.
the easy side
We open with "The Light Side of Weight", a collection of graceful demonstrations dealing with the concepts of weight, gravity and pressure. Not only Indians sleep on a bed of nails; In this workshop, the viewers can also experience this, and discover that actually, sitting on a nail chair hurts less than expected. If the nails are dense enough, the weight of the body is distributed between them and they do not penetrate the flesh. But try to imagine sitting on one nail! This is how we get to know the concept of pressure: pressure is a force per unit area. The force - in this case your weight - is distributed over many nails, so the pressure is not great.
Our feeling of weight is actually caused by the fact that we feel a load on our legs when we stand. The weight is the force that the earth exerts on us - this is the force by which it pins us to the ground. Could there be a situation where the Earth exerts a force on us, but we are determined to be weightless? The "Einstein Funnel" (photo 1) demonstrates this: the ball hanging over the side of the funnel is tied to the center of the funnel with a rubber band; The rubber band is stretched by the weight of the ball.
Will it fit inside it?
How is it possible (Einstein asked, according to the legend) to put the ball into the funnel in one simple movement and without touching the ball? The answer is surprisingly simple: raise the funnel and then let it fall. When the funnel and ball are in free fall together, the ball is effectively "weightless", then the stretching of the rubber band will pull the ball back into the funnel. The ball obviously has mass, but the feeling of weight disappears. Einstein realized that a person in free fall would not feel his own weight.
If you were in an elevator, and the cable suddenly broke, the feeling would be exactly the same as the feeling of a person in an elevator far from any body that might exert a gravitational force (until the moment it hits the ground...). It is sometimes mistaken to think that astronauts in orbit around the Earth feel weightless because they are so far from Earth that they do not feel the effect of its gravity; In fact, the extra distance of the spaceship from the center of the ISS is small in relation to the radius of the ISS, and the astronauts do not feel weight because they and the spacecraft they are in are in free fall under the influence of the gravitational field of the ISS.
Why don't you fall?
Why don't they fall to the ground? Because in the first place they have enough peripheral speed to keep them in orbit (see an explanation of circular motion in the column "Physics from the Basics" in the previous issue). A similar feeling of weightlessness can be achieved - and this is done for astronaut training - if you fly in a plane and let it and everyone in it free fall. It has nothing to do with the height to which the plane rises; The free fall, as long as it lasts, provides an authentic feeling of weightlessness.
A facility located in the science park is the "trampoline", which demonstrates the reduced weight on the surface of the moon (photo 2). On the surface of the moon, due to a different mass and radius than the earth, there prevails a gravitational force equal to one-sixth that of the earth. The trampoline, with its slanted wooden side, represents the surface of the moon; Most of the weight is carried by a cable to which the user is tied, so that his feet are free to walk on the "surface of the moon".
cable load
Hitting the surface of the tree will bring the hitter to a higher height than it would have happened on the ground, and the time until hitting the ground will be longer. The facility was designed so that the load on the legs is equal to one-sixth of the user's weight; If so, what is the load carried by the cable? The immediate tendency is to answer that if the legs carry one-sixth of the weight, then the cable carries the remaining five-sixths; But joining forces is done in a different way. We must take into account the direction of the force, and then we will find that the cable carries about 5.9 sixths of the weight, that is, almost the full weight.
Another space-related demonstration is the inflatable planetarium (image 3). In the space of the planetarium is a rotating drum with a lamp in the center. The drum is pierced so that the light emanating from the holes projects the night sky on the round walls, as they appear on a particularly clear night. The sitters see the stars rise and set, the summer sky and the winter sky, and learn to recognize the North Star and the prominent constellations.
Get to know the Malacca Plain
Some of the groups, such as Cancer, Scorpio, Virgo and Taurus, are on the line outlined by the sun in its movement (from our point of view on earth, of course). These are the "signs", and this line is nothing but the "Flag Plain". Such and other constellations are actually not groups at all; In order for a collection of stars to be called a "group" in some physical sense, there must be an interaction between them, i.e. a gravitational pull.
The "Scorpius group", for example, is a collection of stars at different distances from us, which happen to, due to the perspective derived from our relative position in the galaxy, appear to us to be close to each other. The characterization of these stars as "groups" is similar to seeing shapes in the clouds; People in ancient times looked at the stars, and related stories and mythology to them, but for the most part, the stars in a "constellation" are far from each other and do not influence each other.
What determines the color?
Their brightnesses also differ, and are related to their size, their distance from us and the developmental stage in which they are. This developmental stage also determines the star's color, which is a function of the temperature of its surface. In the nozzles of the drum in the center of the planetarium, a transparent material of different colors is set, so that yellow, red and blue stars can be seen.
If you go outside on a clear night, you will be able to notice these color differences; Note that these color differences originate from the stars themselves, compared to other distortions of the star's color that originate from atmospheric disturbances. When the star appears to flicker and change colors rapidly, this is a disturbance that originates from temperature differences in the atmosphere.
Never follow the sun
The closest star to us is a medium and yellowish star - the sun. The sun is not brighter than other stars but it is closer, so it is dangerous to watch it with an unprotected eye, not least in a telescope that focuses the radiation on the eye. A simple device safely projects the image of the sun into the eyes of festival visitors, allowing them to see sunspots - areas of slightly different temperature on the surface of the sun, which indicate magnetic activity and are suspected of influencing the Earth's climate.
The sun is very active: hydrogen turns into helium all the time in its core (a process called nuclear fusion), and the enormous energy created in this way is pushed out in several ways: conduction - the spread of heat through the material; Radiation – progress of photons; and convection - movement of hot material to the colder areas. The phenomenon of convection can be seen at the weather station.
Hot and cold luck
At the weather station you can see the relative movement of hot liquid and cold liquid, painted in different colors. The hot material rises and spreads, as happens with hot air currents (thermals). Sometimes you can see birds of prey and birds of prey, paragliders and air gliders soaring into the air without moving a wing and without using an engine, when they are aided only by the phenomenon of convection.
We know that it is colder on the summit of Mount Hermon than at its foot. This phenomenon is called the temperature drop - the temperature "falls" with height. The sun heats the surface of the earth, the air near the ground heats up. When the air mass is warm enough, convection occurs, and the hot air rises. The phenomenon of convection is related to the formation of clouds: on a day when the air is sufficiently moist, a cloud forms at the head of a "thermal" - a column of rising air. The water vapor transported in the air reaches a height where the temperature and pressure are suitable for condensation, and then the cloud can be seen.
Mist in a jar
To save festival visitors the tedious journey up the thermals to the base of the cloud, there is a "fog in a jar" demonstration at the weather station. A jar containing a little hot water, and inside it... a rubber glove whose hem is stretched over the rim of the jar. When the glove is worn on the palm of the hand and the gloved hand is pulled out, a partial vacuum is created in the jar: the pressure in the jar drops immediately, and water vapor rises from the hot water and fills the jar with fog - which is actually a kind of cloud.
The demonstrations in Ze'ar - Anfin may create the impression that the weather is a clear and understandable phenomenon. Why, then, is it not possible to predict the weather for more than a few days, and that too with a known degree of inaccuracy? It must be remembered that the weather system is complex, and in fact an example of a chaotic system. Even if we now accurately measure many parameters such as atmospheric pressure, temperature, humidity, etc., we will have difficulty predicting the weather a few days later.
The Butterfly Effect
A chaotic system is characterized by a strong dependence on the initial conditions. It is enough that a small mass of air will move in one direction and not another - and the behavior of the system will be affected in a significant way that cannot be predicted. This phenomenon is known as the "butterfly effect", due to the claim that the flapping of a butterfly's wings in China may eventually cause a storm in Europe.
Those who wish to see with their own eyes an example of a chaotic system will return to the science garden, and observe the chaos pendulum (photo 4). It is a pendulum consisting of several arms whose movements affect each other. If we leave the pendulum at a certain point and film its movements, and then release it from exactly that point and follow its movements again, we will find that these are two sets of movements quite different from each other.
tiny difference
A tiny difference in the initial angle, or a slight oscillation of one of the arms, is enough to significantly affect the behavior of the system. The weather system also behaves this way, and this makes it difficult to accurately predict, even if we have at our disposal a sophisticated system for sampling and collecting data. Air pressure is a fascinating topic that was the basis of several demonstrations at the festival.
Sometimes we tend to ignore the fact that air is a substance, with mass and weight, capable of exerting force. We are constantly under a heavy shell of air that surrounds us from all sides and presses on our body strongly. In order to understand this, let's think about the simple question: why does the air that is one meter above us not come down?
clinging to the air
The reason is that it is supported by the air below it, just as if it were resting on a table. There is pressure between the air above and the air below; The pressure works in all directions: down, up and to the sides. From this it is understood why the pressure at sea level is greater than the pressure near the top of a mountain: the air at sea level carries above it the weight of a thicker layer of air than the air near the top carries.
In space, where there is no air at all - there is no pressure. This is a pressure drop (similar to the temperature drop mentioned earlier) - the pressure decreases as you increase in height. This pressure drop is responsible for the fact that a balloon filled with helium (or alternatively, a balloon filled with hot air) rises. The pressure at the bottom of the balloon is greater than that at the top; The pressure difference creates an upward force. For this, it is necessary for the balloon to have volume: the larger its volume, the more significant the pressure difference will be, as will the upward force.
It's all a matter of pressure
If the balloon is lighter than air (and helium, or hot air, contains less mass per unit volume than the air around them) then its weight is less than the force resulting from the pressure difference. The pressure difference wins, and the balloon rises. The exact same pressure difference would exist around the balloon if it were full of water, but then its weight would be too great, and despite the upward force exerted by the air, the balloon would descend.
In such a situation it is said that the weight (gravitational force) is greater than the force resulting from the pressure difference. For a block of air of any size, the force acting on it under the influence of the air around it is exactly equal to the block's weight. We know this, because in the absence of an external force, the air is at rest (point for thought: why does a rubber balloon filled with air descend, and not float in place?)
How does it work in water?
The same is true of water: water exerts pressure. Since water is denser than air, the pressure it exerts is greater. This is why we feel lighter in water than in air. If the water contains a lot of salts (like in the Dead Sea), you can actually float on its surface. It is interesting to note that fat people float more easily than thin people: although their weight is greater, their volume is also greater, and their average density is lower since fat is not particularly dense.
A ship, for example, will float as long as its average density is lower than the density of water. Although a ship can be made of metal, the hull of the ship is hollow. "Density of the ship" is the weight of the air in it together with the weight of the metal, divided by the volume of the ship. In the same way we see that a deep bowl floats more easily than a flat plate in a sink full of water.
Water up to the sky
The classic demonstration of water pressure can be seen in "Water Water to the Sky", where the system of "Interlocked Vessels" is presented (photo 5). The vessels, which have different shapes, are connected to each other at the bottom. The water level in all vessels is the same. The pressure at the bottom of a column of water is not related at all to the shape of the column, but only to its height.
In the combined vessels, the air outside and the liquid in the vessel are free to move in order to equalize pressures. The result is that the surface of the water is at exactly the same height in all the vessels: if the pressure were greater at any point, a flow would be created to the area with the lower pressure, until an equilibrium is formed. When the vessel is balanced, the water pressure at the bottom of all columns is the same. So is the air pressure at the top of the liquid columns. This is a steady state.
motion effects
So far we have discussed the pressure of air or liquid that is not moving. The movement creates other interesting effects. An air pressure demonstration presented by the "Havida", and another demonstration held at the Science Garden, show Bernoulli's principle: pressure drop as a result of flow. Air pressure originates from the movement of gas (or liquid) particles in all directions. The collisions of these particles with bodies in their environment create the pressure.
When there is an orderly flow, the velocities of the particles are directed in one direction, and their random movement to the sides decreases. Therefore the pressure perpendicular to the flow direction decreases. A nice demonstration of this is this: take two balloons tied with strings, and hold them a few centimeters apart. What will happen if you blow air into the space between the two balloons? Contrary to intuition, the balloons will not recoil from the blow: on the contrary, they will stick together due to the drop in pressure in the area where the air flows.
blow on the balloon
Only if we blow directly on the balloon will we make it recoil. As long as the exhalation is between the balloons, the pressure perpendicular to the exhalation direction will decrease. A similar demonstration is the ball in the hair dryer: an air stream from a hair dryer is directed towards the ceiling. A light ball is placed on the stream. What will happen? (Picture 6) It is expected to see that the ball will bounce upwards and then fall, but it will not: it will remain floating above the hair dryer. Any attempt to deviate from the airflow to the sides will encounter the standing air, which exerts more pressure on the ball than the flowing air. Therefore the ball will stabilize in the center of the current.
This is the principle by which airplanes fly and birds fly: the wing is built so that airflow in the lower part of the wing is slower than in the upper part, and because of this the pressure in the upper part of the wing decreases, and a pressure difference is created that creates an upward force. The pressure difference in this case originates from the air movement, unlike the pressure difference we described earlier, which exists in stagnant air.
About the Weizmann Institute, initiator and host of the festival:
The Weizman Institute of Science, named after Chaim Weizman, is located in Rehovot. In the area of the campus is the Weizman mansion, where the first president of the country, who was a chemist by training, and his wife Vera, who was a doctor, lived. The institute has existed for about seventy years and currently employs about 2500 people. It is a scientific research institute, where the exact sciences are studied, including physics, chemistry, biology, mathematics and environmental sciences, as well as intermediate fields.
In addition to research and teaching (the institute trains graduate science students), the Weizmann Institute has extensive educational activities for the general public, including classes, popular lectures and sessions for schools; And we will mention the visitors' center and the science garden named after Klor, which is a science museum at the center of the festival.
