Please tell us more about your custom Hand Generators.

Motors and Generators

These are DC gear motors I bought for about $8 apiece on the surplus market and are no longer available from my source. I am experimenting with other motors to compare their behavior with these, which seem to be working quite nicely. The gear motors came from All Electronics. The aluminum crank handles came from Reid Supply, they arrived with solid hubs and I drilled the ¼” blind holes in them. I then used two-ton epoxy to fasten the handles on the shafts. A vice or a clamp is helpful to encourage the shafts to mush all the way down into the epoxy. The motors came with four wires, two of which went to either a tachometer or a clutch of some sort. I cut the extra wires and soldered them together with the motor wires to extend the wires’ reach. Shrink tubing helps support the connections. Heavy alligator clips from Radio Shack completed the assembly. If I were to rebuild these, I’d give each motor at least two feet of wire and stagger the clips so they would be less prone to shorting themselves. Clips with plastic caps on the handles are a luxury to be considered. Avoid hooded clips, they will restrict your possibilities for connections.

At the motor station, I typically ask a student to ‘crank me out some electricity’ on a motor that is not attached to anything. Questioned, she will report that it is easy. I then short circuit the motor by touching the clips together. Suddenly the cranking is not so easy any more. I explain that now we have a circuit, and electricity (electrons) is (are) flowing through the wires. When there is no circuit, we are not moving electrons, not doing work, and indeed we all know that it is easier not to do work than it is to do work.

Next, I connect two motors together and let the student explore what this does. Often I find this is too early to try to explain that a motor and a generator are interchangeable depending on where the energy comes from and how it is being changed. A motor takes in electrical energy and changes it into motion; a generator takes mechanical motion and changes it into electricity. I also show them that if a student cranks a generator and I grab the spinning handle on a motor, the person cranking can feel me holding the motor. This is why we pay for electricity, and should give us renewed respect for batteries and power plants. At this point I step back and let them experiment.

Students will connect motors in series circuits and in parallel circuits, they will discover on their own that they can get shocks, and they will find that the motors can ‘break dance.’

I try to show them that no matter how fast they crank one motor, they can’t get another to go that fast. That would defy the Second Law of Thermodynamics; entropy always increases. (Friction being as relentless as death and taxes.) If they have a series circuit, though, they can add their voltages together to increase the speed of a third or fourth motor. This does not work in a parallel circuit with each of the motors connected to each of two common nodes. In the parallel circuit, the voltages don’t add, although I think the currents do, suggesting that the driven motor(s) might have more torque. Voltage governs speed in these motors.

Students typically don’t realize they can make the motors arm wrestle, and that is fun to show them, taking this opportunity to show them that switching wires can change the direction of a motor’s rotation. They will also discover that there are differences of effect depending on which way they crank and whether they have parallel or series circuits.

By now the time is usually used up, but if students are motivated, we explore series-parallel arrangements. Connect ‘generators’ in series to a set of parallel ‘motors’ for best effect. Interested students will discover that they only get shocks if they are touching the metal while they are disconnecting clips and someone is cranking. Show them that if a series circuit is broken anywhere else, they don’t get a shock. Show them that the shock is current and not static electricity. The shock doesn’t happen when we are just holding two clips. Connecting them together diverts any current through the wires, but disconnecting them results in a sudden mild pulse of electricity.

Inside each motor is a coil of copper wire. When current flows through this coil, it sets up a magnetic field. When the current stops as, for instance, the circuit is broken, this magnetic field collapses, sending a sudden but short-lived spike of electricity through the wire.

The hand generators are an experiment, and I see them as being in destructive testing. I am amazed at how well they have lasted, the only repairs being two handles needing reattachment to their shafts. The clip leads are holding up well, the gear boxes seem to be doing well, and after numerous drops to the floor from table height and plenty of time break dancing, the motors seem to be durable. I feel very lucky to have found these particular motors at a great price, the plastic cases are a plus as they bang around on tables.

Geology questions from a class after our Rocks! program.

How old is our meteorite?

The meteorite fell to earth from interplanetary space. That is, it probably originated inside our solar system. Because it is an iron-nickel meteorite it probably was part once of a larger body, a planet or other large object that was broken up in a collision with another large object.  Most of these collisions would have taken place in the first one or two billion years of our solar system which is about four and a half billion years old. That would make our meteorite probably about three billion years old, give or take a billion. That is really, really, really old, give or take a really.

How heavy are the rocks?

We have a big piece of petrified wood that doesn't travel on the van because it weighs about sixty pounds. Most of our rocks are a pound or two, the geodes and crystals weigh only fractions of a pound. We have tried to collect rocks that are big enough to be interesting to look at or are interesting enough that it doesn't matter if they are small.

How old is the pumice rock?

How old is the obsidian?

Both of these rocks were produced by explosions of the Valles Caldera volcano in the mountains behind Los Alamos and White Rock. I don't know if they were produced at the same time or during the same cycle of eruptions, but the last eruption was between one and two million years ago so that is as young as they can possibly be.

What is our newest rock?

Our newest rock is the concrete core sample which was cut out of a sidewalk sometime in the last twenty years. Concrete is a man-made rock, and I think this core was from a sidewalk somewhere up "on the hill" so it can't be more than fifty years old. There weren't sidewalks here more than about fifty years ago. Sometimes cores are cut to see what condition the concrete is in below the visible surface, other times they are cut to make a place to put up a new sign in the sidewalk.

The other young rocks are also man-made. Slag from the San Pedro Mine near Golden, NM is left over from a gold mining operation in the early part of the twentieth century, and we have one small piece of Trinitite in a plastic globe. The trinitite actually has a birthday, as it was formed when the sand below the first atomic explosion melted on July 6th, 1945.

How old is our oldest rock?

After the meteorite, which I discussed above, our oldest rock is a piece of quartzite from the Brazos Cliffs area near Tierra Amarilla, NM. The rock that forms these cliffs is the oldest surface rock in the state, and is about 1.7 billion years old. That is way before dinosaurs, in fact, it is possible a dinosaur may have tripped on our rock or turned it over looking for goodies to eat!

How many rocks do we have at the museum?

Please don't make me count them! In our rock museum, I think we probably use about forty rocks. We probably have three times that many either stored or in the sample bags. When we think of a rock that we would like to have for the program, we often go outdoors and look for it. It helps to know where to look.

How many pieces of pumice do we have?

We only travel with one piece of pumice, but you have to admit it is a good one! We have some small pieces, but pumice is so soft that when we take them around, they crunch against other rocks that are harder and they get ground into powder. So we usually leave them home.

What are our favorite and least favorite rocks?

I can only speak for myself, but all of the rocks in our program are favorites. My own personal least favorite rock is a boulder in a canyon near Taos that rolled one day when I stepped on it and I broke my leg.

My very best favorites are probably the meteorite and the dinosaur stomach rocks, but I am proud of collecting a very heavy piece of fossil pond scum that rarely visits classrooms because it is so heavy. Each of the rocks in the program has an interesting story, and most rocks you see around have good stories too. Sometimes it is hard to choose one rock story over another.

These are great questions, and as a teacher I can tell you that good questions are one of the nicest things you can give a teacher. Thanks for asking, and keep being good scientists!

Several questions from a class we visited.

What is it like working in your museum?

          The Bradbury Science Museum is a very fun place to work. There are interesting people to work with, and we spend a lot of our time teaching each other interesting things.  The museum is full of fascinating objects and information, and visitors come here from all over the world. The museum is part of one of the top science laboratories in the world, and we get to hear about amazing things the scientists are doing here.

          We teachers do research, schedule visits, give presentations, and plan future activities and projects. Sometimes we have to catch up on our paperwork. We stay pretty busy. Several times each week, when we are lucky, we even get to go out of the museum in the Science On Wheels van to visit students in their classrooms as we did at your school!

Do you enjoy working with science activities with kids?

          There are two of us who are "museum educators." We started out as teachers, and we love working with kids. We also all agree that science is not only very important to learn about, but that it is really cool stuff. We try hard to make our programs as fun and as interesting as we can, and it is all for you students. The big payoff in a job like this is when we get to actually see kids learning from and enjoying what we do.

After a demonstration comparing falling paper and falling books during which we placed the paper on top of the book.

I just got your question asking what would happen if the paper was not entirely on top of the book when we dropped them.

This is a great question, and I am happy to say that I don't know.

Now that you have identified the problem, I suggest you design some experiments, write some hypotheses, follow your procedures, record your observations, and report your results. I would like to see what you discover.

Other questions: Does it matter how big the book is compared to the paper? Does it matter how heavy the book is? Would it work with tissue paper? What happens if you use cardboard in place of either the paper or the book, or both? Would it work if you crumpled up the paper? I don't remember if we tried putting the paper below the book in your class. What if you hold the paper a little bit above the book when you drop them?

One of the best things about science is that the more we find out, the more there is to wonder about.