A very simple demonstration is to attach a one kilogram mass to a spring scale and lower the mass into an aquarium of water. The weight under water is considerably less than the weight in air.

We have a much more elaborate apparatus with two large produce-type grocery scales, one over the other. The lower one holds a pan of water, and the upper one weighs a mass in air or in the water. You can measure the dimensions of the mass, compute the volume it displaces, weigh it in air and in water, and note how the weight of the pan of water changes when the mass is lowered into it.

1. Magdeburg hemispheres are evacuated in class and the strongest student challenged to pull them apart.
2. A can with a few cc's of water is heated over a flame. When the can is filled with steam, it is removed from the flame and sealed tightly. Over a period of several minutes atmospheric pressure crushes the can as the steam condenses.
3. A bottle of water has openings at the bottom and top closed with corks. When the bottom cork is removed no water flows out; it is held in by atmospheric pressure. But if the top cork is now removed, the water will come flowing out.

   

4. A stick is placed on the lecture table overhanging the floor. Newspaper is laid on the lecture table over the stick. A sharp blow breaks the stick.

Several short demonstrations nicely illustrate Bernoulli's principle.

1. Support a ping-pong ball or a bloop ball on an air stream from the building supply or a hand-held vacuum.
2. Pick up a 3 X 5 card by blowing through a thimble at it.
3. Blow up a Bernouli bag with one breath
4. A Venturi tube sucks the air out and collapses a bottle.
5. Blow between two pieces of suspended cardboard and they come together.
6. Blow the roof off a house by directing an air stream from a vacuum cleaner over it.
7. Demonstrate the breakdown of Bernoulli.

In a container of water sealed with a rubber diaphragm top is a smaller floating container partially filled with water and with a small hole in the bottom. When the rubber diaphragm is depressed, the air in the smaller container is compressed, increasing the volume of water in the smaller container and reducing its buoyancy. Thus, the smaller container sinks. Its level can be controlled by the finger pressure on the diaphragm.

Water is added to a connecting set of tubes with progressively smaller bores. Capillary action raises the water progressively higher in the smaller bores.

A bell jar with several demonstrations is evacuated in class:

1. A balloon with a tiny amount of air inflates.
2. A metal ball of air is balanced with a mass (shown below). As the bell jar is evacuated, the buoyant force decreases and the ball falls raising the mass.
3. A short mercury column with a small air bubble falls.
4. A container of water boils at room temperature. The water temperature as measured by a thermometer falls rapidly.

A container has four immiscible fluids floating, one above the other - mercury, carbon tetracloride, water, and naptha. You can lower in small cubes - balsa, hardwood, plastic, and aluminum - to float at each interface.

Demo 1
A manometer with a rubber diaphragm-covered probe and an aquarium filled with water and a marked level of depth are provided. The pressure probe is inserted under the water to the marked level of depth and faced up, down, and sideways. The pressure reading is the same for all directions.

Demo 2
Pressure Syringe (aka Pascal's Demonstrater) Fill the syringe with water, push the plunger, and watch the water shoot out of every hole equally.
 

Corn starch and water are mixed in a plate to produce a goopy mixture that can be scooped up and dribbled with a spoon. Inviting the students to look closer, the professor suddenly slams his or her fist down on the mixture, causing the students to jerk back - but the mixture doesn't splatter; it momentarily becomes rock hard when acted on by a large force. Another type of behavior is demonstrated by dragging a spoon rapidly through the mixture to cause a ripping action.

Also available is "silly putty" which is hard and elastic under large forces but flows under small forces. Mold it into a ball and bounce it, and then leave on the lecture table; in ten or twenty minutes it will flow out under gravity.

A set of tubes of different shapes are connected to a common source of water. When filled, the water reaches the same level in all the tubes.

This is a demonstration that pressure depends on depth only and not on the shape of the vessel. The reservoir on the right is adjusted for the same level of fluid in each "vase", and the gauge reads the corresponding pressure.

With a right-angle plywood trough covered with sandpaper, you can put a tremendous curve on a ping-pong ball, even curving upwards from a horizontal throw.

Suggested by Prof. D. Pursey from Iowa State, who is shown above in action.

A can has openings one quarter of the way down, one half of the way down, and three quarters of the way down. When filled, water flows out the openings. From which hole will the stream impact furthest from the bottom of the can?

Bottles of water and glycerin have similar beads in them which sink at different rates according to the viscosity of the fluid.

The Fire Syringe demonstrates an adiabatic process. The autoignition temperature of cotton is 407^oC (765^oF).

A thin piece of pyrolytic graphite cuts through an ice cube. This material, of layered graphene, has excellent electron and thermal conductivity along the layers, but poor conductivity through the layers. This made it good heat shielding material for re-entry vehicles. It's diamagnetic properties are also anisotropic, with a very strong response to fields perpendicular to the layers.

Identical small balls are attached by wax to the ends of six rods of different metals, radiating from a common metal center. The center of the rods is heated with a flame, and the small balls drop off at different times as the heat is conducted out by each different rod to melt the wax holding the ball.

Drinking Bird - The bird dips its beak into a beaker of water "taking a drink" and rotates back up. Cooling by evaporation from its beak draws up a colored liquid (methylene chloride) through a tube in its body overbalancing the bird so it takes another drink. This action repeats indefinitely.

Love Meter - Hold this device in your hand and heat from your hand boils methylene chloride causing it to flow through an intricate series of tubes.

See also Galileo's Air Thermometer [1]

1. Ring and ball - when both objects are at room temperature, the ball will not pass through the ring. But then the ring is heated by a flame, the ball will pass through it.

   

2. Bimetallic strip - bends when heated as the two metals have different coefficients of expansion.

   

How can you walk across hot coals?

Firewalking has been going on all over the world for thousands of years with written records going back to 1200 BC. Eastern Orthodox Christians in Bulgaria firewalk during popular religious feasts. So do Japanese Taoists, Buddhists, Indian Fakirs, the !Kung Bushmen, Polynesians, etc. Some claim that firewalking is an example of mind over matter, or a test of the protective power of faith.

It is true that the temperature of the coals is over 1000 degrees F (535 degrees C), and that human flesh burns at much lower temperatures, but temperature isn't the only part of the relevant physics. It wasn't until the 1770's that Joseph Black figured out the relation between thermal energy and temperature. (He later discovered Carbon Dioxide.) Different substances have different heat capacities. Water is the standard. It takes 4.18 Joules to raise the temperature of 1 cc of water 1 degree C. Our feet are mostly water. The coals have a much lower heat capacity than water. That means that the same amount of energy flowing away from the coals will lower their temperature much more than that same energy flowing to the feet will raise the foot's temperature. If the foot stays in contact with the coals, energy will keep flowing until they both reach the same temperature. However, this takes time, and how much depends on the heat conductivity. There are good heat conductors, like water, and poor conductors/heat insulators, such as ash. The feet cool down the local area of the coals they touch, and it takes time for energy to flow from the rest of the fire to the cool spot. You can sometimes see dull orange footprints in the coals right after someone walks. Water is a good heat conductor and energy transferred to the foot is rapidly conducted away from the contact points so the temperature doesn't rise to the burning point. Temperature, heat capacity, and thermal conductivity are all important in this demonstration.

A more familiar experience which involves the same physics is baking brownies in the oven set to 450 degrees F. Everything in the oven is 450 degrees, but you don't fear putting your hand in the oven air. The air has a very low heat capacity meaning it stores very little thermal energy. Air is also a heat insulator. Your hand (mostly water) cools the air locally and heats up very little. If you stick your finger in the brownie, you might get burned. It is mostly water like your hand and has a pretty good heat conductivity. Thermal energy will flow to your finger raising its temperature quickly. The metal pan is another matter. It has a high heat capacity and a high conductivity. Touch it without a potholder and you might instantly burn your fingers.

What the physics tells you, is that if you walk fast and don't stay in contact with the coals very long, you won't get badly burned. If you believe in mind over matter or the protective power of faith, then time shouldn't matter. This could be a life threatening delusion.

Even knowing the theory, firewalking is still dangerous in practice. There is a lot of energy in a glowing firepit at 1000 degrees F. Second degree burns in the form of blisters are common and more severe burns requiring a trip to the hospital have occurred. Sometimes a hot coal will stick to the foot causing a burn. There can also be hot spots in the fire, pieces of metal, or even pockets of hot steam locked up in the wood. Falling down in coals can be fatal. We will take care to make our walk as safe as possible.

Watch a video explanation of firewalking [2]. The video includes comments from Bernie Leikind, who got his start firewalking at UCLA.

One last note: Just because some aspects of firewalking and heat are "just physics", don't try to copy any fire stunt you might see. There have been many fire performers throughout history who used trickery to amaze audiences, and if you tried to duplicate their trick you would be severely injured. For instance, some performers scooped boiling lead into a ladle, and then poured it into their mouth. Shortly after, they spit out a chunk of cold lead with their teeth impressions in it. However, all was not as it seemed. The ladle had a hollow handle with mercury inside. Instead of scooping molten lead, mercury from the handle filled the ladle. Instead of pouring molten lead in their mouth, the mercury just went back into the handle. The cold lead with the teeth impressions had been hidden in the mouth beforehand.

In order to get liquid nitrogen we need a little extra notice. Some experiments are:

1. Flowers can be frozen brittle or fruit frozen solid so they will shatter when dropped. Another good demonstration is to bounce a racquetball and then use liquid nitrogen to freeze it and shatter it.
2. Balloons of air can be completely deflated and thrown on the floor to reinflate. If helium-filled balloons are used, their volume will be reduced to about one quarter. They are now denser than air and will drop to the floor, rising to the ceiling after a few minutes as they warm up.
3. You can plunge your hand into the liquid nitrogen for an instant. The Leidenfrost effect (vapor barrier) protects your hand.
4. A piece of the new high temperature superconductors will float a small magnet demonstrating the Meissner effect after cooling in LN₂. We also have a highly pinning superconductor which will float over a magnet track, or even under the magnet track
5. Liquid nitrogen can be shown to freeze when the air is pumped out of a glass chamber as depicted in the video below.
6. The temperature coefficient of resistance can be investigated with a copper wirewound resistor and a carbon resistor. A small circuit with a battery, lightbulb and resistor is set up and then the resistor is put into a cup of liquid nitrogen. The change light brightness indicates the sign of the temperature dependence. The temperature dependence of a semiconductor can be investigated by dipping a large green LED into liquid nitrogen and noting the color change.
7. Ice Cream Recipe [3]

A flask of air connects at the bottom to a column of water. When the flask is heated by your hand, the air expands and the water falls. When it is cooled by evaporating alcohol, the air contracts and the water rises. Why doesn't this make a good thermometer? (It also responds to barometric pressure.)

What does the energy locked in 1 gram of sugar, 16 kilojoules, look like? Let's oxidize a gram and see.

This is an example of an exothermic reaction. The white powder is Potassium Chlorate. When melted it becomes a source of reactive oxygen. A typical gummy bear is 1 gram of sugar. Glucose + Oxygen ⇒ Carbon Dioxide + Water + energy

C₆H₁₂O₆ + 6 O₂ ⇒ 6 CO₂ + 6 H₂O + 16 kilojoules per gram of sugar

In cellular respiration sugars are oxidized and the resulting energy is stored in molecules of ATP. Each gram of sugar produces about 3.75 kilocalories or 16 kilojoules of food energy. The average human body utilizes about the same energy as a 100 watt light bulb. 100 watts x 24 hours x 3600 seconds/hour = 8,640 kilojoules per day. (Starvation is considered to be less than 1800 kilocalories or 6,830 kilojoules per day.) If we got all our energy from sugar and starch, we would need about 540 grams/day which is just over 1 pound per day.

If we could power our bodies with electricity from the utility it would take 2.4 kilowatt hours to operate at 100 watts for a day. At the rate of about 10 cents per kilowatt hour, the energy would cost 24 cents.

For more details on cellular respiration from the Khan Academy:

A set of five balls of different metals is heated in boiling water. The balls are then dropped onto a thin paraffin slab. They melt their way through at rates depending on their heat capacity (which depends on both the specific heat capacity of the metal and its mass).

Dissimilar metal wires with two junctions are hooked to a galvanometer. Heating one of the junctions with your fingers produces a substantial reading.

A steel ball bouncing on a polished steel mirror illustrates a highly elastic case; the ball will bounce for a considerable time.

With another piece of apparatus balls of different materials can be bounced on steel. Some (glass, steel) are highly elastic, and some (wood, lead) are highly inelastic. The bounce height can be measured approximately with the apparatus.

Nine basic crystal lattice models [4] are available including face-centered and body-centered lattices [5] and Bravais lattices [6].

See Imiscible Fluids [7]. Four immiscible fluids float one above the other - mercury, carbon tetrachloride, water, and naphtha. Small masses - aluminum, plastic, hardwood, and balsa - lowered into the fluids float at the four interfaces.

A face centered

and body centered lattice are available.

Bravais demonstrated that there are fourteen different point lattices, shown below in a sketch and photograph.

The following basic crystal lattices are available for demonstrations.

A large Pyrex flask of water is heated to boiling; then the flame is removed and the flask sealed off. When water is poured over the flask, cooling it, the steam in the flask condenses reducing the vapor pressure, and the water boils at the lower temperature.

Smoke is drawn into a small chamber and examined under a microscope to observe Brownian motion.

Another demonstration for Brownian motion is an apparatus which gets placed on the overhead projector. Balls of different size can be used to show the random motion of molecules. As the speed on the agitator is turned up, the balls move faster.

Lastly, an ephysics applet [8] for Brownian motion is available on the web.

See Effects in a Vacuum [9]. Water boils at room temperature as the air above it is evacuated. Its temperature as measured by a thermometer drops rapidly. The more elaborate demonstration in which water is frozen in this way requires the addition of a beaker of concentrated sulfuric acid to the vacuum chamber to further reduce the vapor pressure.

A thermoelectric device which runs a propeller has two legs. The device will run if one leg is in a hot cup of water and the other in a cold cup of water, but not if both legs are in the same cup of water. (The hot and cold water can be mixed for the second step to show that the ability to do work has been lost.)

The "Thermobile" will spin (by shape-memory retention) if one end is dipped in hot water. Air at room temperature serves as the other reservoir (also see Laws of Thermodynamics [10]).

A spinner rolls indefinitely up and down a double ramp. Is this perpetual motion in violation of the First Law?

A Hilsch tube device connected to the room air source separates the air stream into hot and cold blowing streams. Does this violate the Second Law?

The "Thermobile" runs when dipped into hot water. Is this a heat engine with only one temperature reservoir?

The thermoelectric converter can also be run between ice water and liquid nitrogen demonstrating that there is still plenty of heat energy in ice water.

Professor Izzy Rudnick's 17-minute film on effects in liquid helium dramatically shows superfluity, fountains, super leaks, etc. and discusses the thermodynamic functions and properties of liquid helium.

A motor-driven molecular motion model which rattles various sized ball bearings around wildly fits on an overhead projector. The model also contains a somewhat larger object which is jiggled by impacts from the balls to demonstrate Brownian motion.

With this apparatus, a ball is bounced on air. From the period of oscillation the ratio C_p/C_v can be calculated. Instructions and method of calculation are available.