Fellow activities for Science and the World class



Fellow activities for Science and the World class.

In general, I helped with virtually every part of the course: planning, purchasing materials, and helping out in class. I also suggested and helped plan the final project, which was the independent design of something that used mechanical and electrical principles we had discussed in class, such as a house, plane, car, or etc. Deadlines were made for handing in the initial plan and the final projects were supposed to be presented at the end of the term. Unfortunately this part didn’t go so well as time ran short and the teacher didn’t get a lot of student energy invested on the projects. Next time, it would be better to more clearly define what the projects should be about, have more evaluation throughout the year, and give some kind of rubric out to describe the kind of work we expect.

Lecture/Activity

- Magnetism

(My first lecture/activity that I planned and implemented myself)

Materials:

Coated (vinyl insulation) copper wire

Insulated copper wire

Uncoated copper wire

Insulated Nickel wire

Uncoated Nickel wire

3-inch iron nails

3 inch galvanized nails

2 inch nails

2 inch screws

D cell alkaline batteries

Bar type permanent magnets with marked north and south poles

Sheet metal screws or paper clips

Using the inquiry method (trying to, anyway…) I talked with the students about magnetism. I had prepared material previously on electrical conduction and generators and was a little unprepared to lecture on magnetism. What I did know was what related to power generation and so I elected to work towards a group activity where students tested various types of wire and cores together with D cell batteries to generate the most powerful magnet. During the discussion before the activity, I asked what the students thought magnets were. I wrote up comments on the board and the discussion led to poles. I tried to make my board drawings match both what the students were saying and what I was working towards. Ultimately, the class discussion covered some basics about magnetic fields and North and South poles, as well as the nature of attraction of opposite poles.

I then presented to the class the materials I had gathered and asked them to form 3 groups. Because of time limitations, I decided to explain the role of the wire, nails, and batteries and how to wire them together to make a magnet. Then, I gave each group 10 minutes to build a magnet and compare the force (measured by how many sheet metal screws could be lifted) to the permanent magnet. I then gave them each 10 minutes to test different materials to construct the most powerful magnet. I told them all that the three groups would compare their magnets in a little competition at the end of the class. During this rebuilding, I asked them if they noticed anything else about their magnets. The students told me that the magnets were getting hot. I told them to be careful therefore to not leave the batteries connected for long periods of time as the heat represented the energy from the battery and the batteries were going to run dead (potentially hurting their performance in the competition). Basically, the students should finish winding the magnets before attaching the battery to test it and run brief tests.

After the competition, which was pretty lively, the class ended. This established the interest needed for the rest of the magnetism material (more detail given by the teacher along with homework). Also, since this was the motor-building background material, the engagement I witnessed here was important for that lesson as well.

- Using electromagnets to do work (Electric motors)

Materials

Toy Motor kit (available from Sargent-Welch)

Wire cutters

Sandpaper

Vegetable oil

Small regular screwdriver

At the start of the class, the teacher and I handed out the toy motor kits to students and also the directions included with the kits. The students built the motors in pairs. After one class period, most of the class had finished building motors that looked something like what was shown in the instructions. However, none of them worked. I noticed that the wiring instructions were not clear, so since it was a Friday, I took one motor home over the weekend and got it to work using what I knew about field coils and how energy must flow through the armature. I then determined that the directions given with the kits were poorly explained. So, I completely rewrote the instructions with detailed diagrams and step-by-step instructions.

During the next class period, I showed the class my working motor and let them alter their own motors to resemble mine. Troubleshooting proved to be very useful for my own understanding, so I helped the groups to each ask themselves why their own motor wasn’t working and how changing the wires and orientation of different parts of the motor really affected the operation. For example, when the commutator was not aligned properly, the motor would stick in one orientation, even when spun. When students played around with the commutator and determined it should be perpendicular in relation to the armature, the motor spun freely. After a total of 3 periods, including the first one before I had rewritten the instructions, the students all had working motors and took them home to show them off. Ultimately, I never had to give my updated instructions. Because I had figured out where the instructions they had was lacking, I simply pointed those weak areas out and asked the students to figure out for themselves how the instructions could be improved. So, in a future lesson like this, I would ask the students to help me work out what was wrong with the directions as I think it forces them to grasp the phenomena better.

Later on, the students were evaluated by a quiz on this material. In addition, the information they had learned was the basis for future lessons on power generation and distribution.

- Electromotive Force and Power Generation (the relationship of electrical and magnetic fields)

Background material found at:

-

- The “New Way Things Work” electronic curriculum

-

Materials

Demonstration-sized motor/generator (from Jason St. John at BU)(also available at Sargent- Welch)

Demonstration-sized Galvanometer (from Jason St. John at BU)

6 Volt Lantern Battery

This demonstration was designed to take place after students had spent about a week building and altering a toy motor and had learned about the different parts of an electric motor in class. Students had yet to hear anything about power generation, so I decided to transition into generators by making use of the relationship between electrical and magnetic fields.

During prior discussion of their motors, the students had learned that the electrical current in the field coil creates a magnetic field, which generates rotation in the armature. The armature moves because the coil in the armature generates poles that are attracted or repelled by different parts of the field coil. As the armature coil flips due to the power channeled through the commutator, the poles at the tips of the armature coil move towards the opposite poles in the field coil. Basically, the whole armature coil rotates 180 degrees as the constant poles of the field coil attract the new poles established in the armature. So, this process allows for continuous rotation, since the commutator flips the poles each time armature rotates 180 degrees.

So, after reminding them of their motors, I turned the class attention towards the demonstration model motor I had. When I hooked it up to the battery and gently started to spin it to overcome friction, the motor ran on its own. I asked the students to gather around and name some of the parts of the motor. I wanted to do this because the commutator, field coil, and armature were arranged differently from their toy motors, and I wanted them to be familiar with where everything was.

Then, I asked the students to help me hook up the wires differently. We hooked up the galvanometer to the motor and disconnected the battery. Then, I asked one of the students to spin the armature by hand. The needle on the galvanometer jumped, demonstrating that power was flowing out from the motor. Several students doubted that the motor was actually generating power or that what the galvanometer was measuring was actually power, so I tried to light up a light bulb using the wires. Unfortunately the light bulb didn’t light. So, I tried to hook up the galvanometer to the battery and the needle jumped. At this point, most of the students believed that the motor was generating energy. (However the light bulb was what I was hoping would work).

I turned the attention of the class back to the board and explained motional emf. Letting the students provide the lecture material (ex: I asked what happened to the galvanometer needle when the student spun the armature) we discussed how the electrical force detected by the galvanometer must have come from the motor. By spinning the armature, armature coil was spun next to the field coil. The moving coils generated a magnetic field, which then generated an electrical field in the wires of the coils. Basically, motion generated a magnetic field, which in turn generated electricity that was measurable and could theoretically do work. This is the reverse of a motor, which takes electricity to generate a magnetic field, which then generates motion. The interesting thing I wanted the students to learn was that motors could be used as generators and vice versa, due to the relationship between electricity and magnetism.

Students were evaluated on this material by being asked to fill out a worksheet (the teacher wrote one up based on my class lecture) the following period and were later tested on the material.

Lecture (1.5 periods)

- Power Generators

Background:



The teacher also covered transformers before hand.

For this lecture, I strictly used the board. I started with the end of a period, as the teacher was finishing up explaining transformers. I intended to finish up the following day.

I gathered material from the website and also called upon what I remembered from my own experiences as an HVAC technician and from what I have read about in the newspapers. I explained the different kinds of power generators that are used for practical applications, such as gasoline powered generators for home use, larger generators for hospitals such as Harvard Medical School. I also asked them to explain the features of the generator that Chelsea High School has for emergency backup power and got some interaction with the students. Mostly, however, this was a lecture. I also discussed natural gas power, solar power, nuclear power, geothermal power, and coal power; all of which indirectly turn a steam turbine that is connected to the armature or field coil, either of which can rotate. Also, hydroelectric (like the generator in Niagara that serves this region) and wind power, which each using flowing water or wind to power rotation of the coil.

One of the most important things I wanted to describe was how all generators that will be producing power for standard 220V/110V electricity operate on the same principle as the motor/generator that we had used in class. I took the opportunity here to describe the basics of alternating current, which is the type of power we get from the wall outlets. The nature of alternating current is explained in the website I used for background. I related it by talking about how the potential increases in the armature until armature is parallel to the field coil magnetic field and then decreases to 0 as the armature rotates to a perpendicular position relative to the magnetic field. As the armature rotates another half turn, it is parallel again, but the potential is now the exact opposite of the maximum voltage when it was parallel 180 degrees before. So, as the armature turns, the potential increases to a maximum positive voltage, decreases to 0, decreases further to a maximum negative voltage equal in magnitude to the maximum positive voltage earlier produced, and then again increase to 0. In short, the energy produced takes the form of a sine wave when measured by an oscilloscope. This is why the current produced is called alternating current, or AC. To make the example easier to understand, I made an analogy with a rod spinning in a magnetic field where the rod is fixed in the middle so that it rotates like the armature.

During the above explanation, I tried to ask students to lead me along by asking certain questions, such as:

Why does electricity alternate?

In what position should a rotating rod be to generate the greatest voltage?

Where do the electrons go when the rod is perpendicular or parallel to the magnetic field?

The basic idea is that the electrons never leave the rod, the simply move towards the magnetic field along the length of the rod. As the rod moves to a parallel orientation relative to the field, the electrons will gather at the very tip, and therefore have the greatest possible potential. This is because the end packed with electrons will now be extremely negative and the other end will be extremely positive. When the rod is perpendicular, the electrons will now be evenly distributed on the rod, since all points of the rod are now equally close to the magnetic poles. There will be no electrical potential generated in this case.

I mentioned also here that the direct current found in our cars could also be generated easily from AC by special devices in the power plug called diodes. These cancel out the negative potential found in AC power, allowing electrons to only flow in one direction. DC devices that can adapt AC power include cell phone chargers and laptops, among other things. Commutators in certain adapters and generators can do the same thing by only making contact with the circuit part of the time, so the negative potential isn’t passed on.

Distribution Grid

The next topic I wanted to cover was the Distribution Grid. I described here the different parts of the system leading from power generators to our houses. I noted that the electricity produced by generators must first be increased in order to avoid loss over long distances. Students were able to deduce here that friction, a basic scientific principle covered earlier in the course, was responsible for energy loss and that this energy was probably given off as heat. After taking some guesses as to how high the voltage was in these lines, the highest guess being 10,000 volts, I revealed that these lines have electricity between 155,000 and 765,000 volts. This electricity is converted by huge transformers at each power plant to these high voltages from less than 10,000 volts that is produced by the generator. This power can travel as much as 300 miles before it is distributed or passes through more transformers for further travels.

Once the high voltage power reaches the area where it is distributed, it enters a small substation.

Here, transformers reduce voltage to less than 10,000 volts where the power is split off into different directions for local delivery. In neighborhoods, the lines carrying this power are set at 7,200 volts by transformers at the distribution bus, which send power along the electric poles.

Power is then tapped at houses, where it runs through another transformer to bring the line down to 120 volts for the house. Two lines, each with 120 volts that is out of phase, make the maximum voltage in each house 240 volts. This power is what is found at the outlets (either 240 or 120 V).

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