Thermal Fan

Age

Elementary School, Middle School, High School

Format

Stage Show

Materials

   Thermal fan (a small, handheld fan with two broad metal legs)
   Three small cups (each large enough to fit one leg in)
   One large cup (large enough to fit both legs in at once)
   Hot, cold, and room-temperature water
   Heating and cooling source (microwave and ice) 

Safety Precautions

If using a microwave, follow the appliance's instructions carefully. Use microwave-safe containers.Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.Be extremely careful when handling hot water.

Preparation

1. Prepare hot and cold water in advance, if necessary. Fill the large cup with room-temperature water, two of the small cups with hot water, and the third small cup with cold water. Ensure that it will not cool/warm to room temperature before presentation.

2. Prepare and practice a dialog written for your presenters.

Demonstration

Prepare two small cups: one filled with hot water, one with cold. Extreme temperatures are preferable – icewater and boiling water are ideal. Immerse one leg of the thermal fan in cold water, the other in hot water. After at most one minute, the fan blade should begin rotating vigorously. Nudge the blade to start it if necessary.

Also try immersing the legs in two containers with water at the same temperature. The blade will not turn.

What to Say

As you perform the demonstration, point out concepts of the first two laws of thermodynamics and entropy. For advanced students, introduce concepts of the thermoelectric effect.

Sample Script(for two presenters, for high school audience):

Presenter A: Science Theatre just bought a new piece of equipment! It's called a thermal fan.

Presenter B: Great, let's try it out!

Well... it didn't come with any instructions. The advertisement just said it would convert water into wind! It sounded exciting. I guess we just need to stick it in water.

(The presenters insert both legs of the thermal fan into a large cup of room-temperature water. The fan blade should not spin.)

Hmm, I must be doing something wrong.

B: Wait, I think I understand. Water – like everything, all matter – has inherent energy called “thermal” or “internal” energy.

Oh, right, and the internal energy depends on temperature.

B: Right, because temperature is really just a measurement of how fast all the molecules are moving – their kinetic energy.

So hotter water would have more internal energy and might actually be able to power the fan. I bet if we stick these legs in really hot water, the legs will suck up the liquid's internal energy and use it to spin the blades.

B: What we're really doing is converting thermal energy into mechanical energy, not just water into wind.

And we'll also need to conserve energy: the first law of thermodynamics.

B: So every joule of energy we use to power the fan will be taken from the water, and the water's internal energy and temperature will have to drop to conserve energy.

How do we calculate that temperature drop?

B: I think it goes like this:

(Write on blackboard) Ä"U"="cm" Ä"T"=Ä"Q" ???

Ok, so delta U is the change in internal energy, and it equals “c”, the specific heat – a constant that's different for each different material; times “m,” the mass of the water; times “delta T,” the change of the water's temperature... but what is delta Q?

B: It's just the heat flow out of the water. Heat flow is another form of energy.

So what we're REALLY doing is converting the water's internal energy into heat flow, and then the heat flow into mechanical work to power the fan.

B: Right. Well, let's try it!

(The presenters insert each leg of the thermal fan into a small cup of hot water at equal temperatures. The fan blade should not spin.)

It's still not working! What are we doing wrong?

B: Wait – there's another law of thermodynamics.

The second law? Doesn't that just say that you can't build a perpetual motion device?

B: Yes, that's one consequence of the second law. It says that because no engine can be perfectly efficient, over time more and more of its energy will no longer be available to do work and it will eventually stop functioning. The second law also says that heat will only flow from a hotter to a colder body.

Oh, so that's the real reason that the room-temperature water couldn't power the fan – the problem was that it was all at the same temperature, not just that it didn't have enough internal energy.

B: Right, and I think the hot water didn't work for the same reason.

This all has to do with entropy. Entropy is essentially a measurement of how disordered something is. A clean room has less entropy than a messy one. If the location of everything in your room is pretty much random – like the position of molecules in a fluid – then there are thousands of places for your bedsheet to be in the room, only one of which is neatly tucked in on your bed.

B: So the odds of something being neatly ordered are far lower than the odds of it being completely disordered.

Yes, and once something is completely disordered, we say it is at a stable equilibrium. For the molecules in a fluid, this means that no matter how much the molecules move around, they system as a whole can't possibly get more disordered – it has maximum entropy.

B: The most important aspect of entropy is that the second law of thermodynamics says that the only actions that can possibly happen in an isolated molecular system are ones that will increase the entropy of the system. If you want to decrease the entropy, you have to somehow supply the energy to do it.

B: So if we want to get energy from hot water, we have to setup some way to increase the entropy of the system. When we had two cups of hot water, all the hot molecules were already evenly distributed in the system – some in one cup, some in the other. The system started off with maximum entropy.

Hmm, so if we found some way to start the system off with low entropy, it would gradually approach a state of high entropy – a stable equilibrium – and during that time the system would lose some energy that we could use to power the fan.

B: Ah, here's an idea – let's have one cup of hot water and one cup of cold water. The system will start out with low entropy – low disorder – because all of the cold molecules will be neatly separated from the hot ones.

All right, let's try it...

(The presenters insert each leg of the thermal fan into a small cup of water, one hot and one cold. After at most one minute, the fan blade should begin to spin. Nudge the blade to start it if necessary.)

B: It worked! I still don't understand how the thermal energy from the system gets converted into mechanical energy. The box the thermal fan came in mentioned something called the Seebeck effect. Oh, the Seebeck effect! I've heard of that. Let's talk a little bit about electricity. The thermal fan is made out of metal. Metal has the property of electric conduction – it can conduct an electric current.

(The presenters may find it useful to draw diagrams of these concepts on a blackboard.)

B: The current is an electron phenomenon. When we talk about electrons, we usually are interested in how they bind to molecules. In the case of a conductor, they can move around between molecules more or less freely. They're like molecules in a fluid that move around randomly, bouncing off other particles and changing directions constantly.

Right, and a current exists when all the electrons happen to move in the same direction all at once. This doesn't necessarily mean that they all move directly in one direction, it just means that they have a net movement in that direction. They still move almost randomly and bounce around constantly, it's just that the direction they bounce in is slightly biased towards a particular direction.

B: Electric current is what used to power components in electric circuits. For instance, in an incandescent light bulb, we force a large current of electrons through a very skinny tungsten filament. The filament is such a tight squeeze that the electrons bounce into the tungsten atoms more often than in a wide copper electrical wire. Each time an electron bounces into an atom, it losses some of its kinetic energy to the atom – so the atom gains some kinetic energy.

Since temperature is just kinetic energy, when the atoms gain kinetic energy they are heating up.

B: Right. All objects emit light. At room temperature, most of that light is in the infrared. At higher temperatures, objects emit more visible light – it's called blackbody radiation. The lightbulb filament is heated by the electric current until it emits lots of visible light.

This fan is another example of an electric component that runs on current. The current powers an electric motor which spins the fan blade.

B: So where does the current come from? It all has to do with the electrons and entropy. The metal leg we put in the hot water heated up – all the electrons were given a lot of kinetic energy from collisions with the electrons in the water. The electrons in the cold leg cooled down. To reach a stable state of maximum entropy, the kinetic energy from the hot electrons diffused toward the cold electrons, through a chain of bounces and collisions.

That sounds an awful lot like a current.

B: It is – this diffusion essentially creates a current that is used to power the fan.

Ok, that makes sense. So what is the Seebeck effect?

B: Oh, Thomas Seebeck just happens to have been the one to first notice something like this happening. His fan didn't spontaneously turn on, but he noticed that two metals at different temperatures always had a voltage gradient.

Voltage gradient? Meaning that the two metals are at different voltages, different electrical potentials?

B: Right.

The concept of electric potential is exactly like the concept of gravitational potential. A ball ten feet above the ground has lots of gravitational potential energy. When the ball falls, the potential energy is converted to kinetic energy as it falls to the ground, where it has lower potential energy. The potential difference between its initial and final point is what makes it fall.

B: It's the same for electrons, except that its an electrical potential instead of gravitational. The electron current we talked about corresponds to a potential difference between the hot and cold legs of the thermal fan.

So if there's no temperature difference, there's no potential difference, and then we have no current to power the fan.

The presenters should thank the audience and prompt for questions.)