Mood Brightener: ...more from Stay Homas. (Confination XIII)

 

Comet Neowise:

If you haven't seen it already make a point of getting out there between 10 and 11 pm. Here is a EarthSky description of where it is in the sky. It is perhaps easier to watch where the sun goes down and 45 min or so after sunset look to the right (more north) from where the sun went down and gently up into the sky from the horizon (20-30 degrees if that helps). Binoculars will be a big help since it is pretty faint. Part of the difficulty is that the most sensitive part of your eye is not where you're looking. That's where the good color vision is. As you scan the sky you will catch it just off the center of your vision with the tail going upwards. It is quite beautiful and it's path is driven by the gravitational field of the sun and tail is pushed outwards by the sunlight. The comet is actually travelling towards it's tail at this point in spite of what we want to believe.

COVID News: What Does This Mean?

Here are a couple of news articles from the last couple of days (Article 1, Article 2) that are totally relevant to our discussion of scientific models. The gist of the recent analyses that are made possible by better tracking data is that a small fraction of infected folks (maybe 10% to 20%) may be responsible for 80% of the cases. It appears that 70% of folks who are infected pass it on to nobody. When a superspreader happens to be in a large social group they seem to infect many many people. In combination with social distancing the rest are unlikely to pass it on. The question for this class is how does this change the model? Does that mean the science before was bad? Do we know this for sure at this point? Gotta talk about this!

Fields: Sources

Fields don't exist on their own. They have sources that create them. The question is how to identify the source. The most effective strategy is to imagine what would have to go away for the effect to go away. What would have to go away to make gravity on this planet go away? What has to go away to make the magnetic field go away? What about the special fields that carry cell phone signals?

This is one way in which quantum fields are clearly a different beast. They have no source. In some sense they are the source which is very confusing.

Fields: Fading Away

The next question is how far do these field things extend? Think about radios for a moment. They definitely fade out as you get further from the transmitter for the station. On the other hand sometimes a better radio can 'bring in' a station from further away. Doesn't that suggest the field is still going? What is the limit? How far away is the farthest thing that we have built that communicates with us? (hint: it's out past Pluto!)

Most small sources act like traditional light bulbs. The light heads out in all directions spreading out evenly across and expanding beach ball. Fields from small (point) sources often act in this way and fade away in proportion to 1/r². This means that if you are twice as far away the field or light is 1/4 as bright (1/2²⁾. Even flood lights dim this same way but they are much brighter because they send their light in a more limited direction. This is the simplest fading process.

Sources that we use for communication (antennae) are actually a combination of a (+) source and a (-) source called a dipole. As a result these fields fade away much more rapidly in proportion to 1/r³. This is one reason we need so many cell towers. There are some other factors at play as well that have to do with the size and power of the tranmission antenna which have to get smaller as we move from 3G to 4G to 5G. As a result the antennae have to be closer together as we go up the chain. In our daily lives this fading of dipole fields is probably the one we most commonly experience.

There are a few sources which fade away more slowly (1/r) but they have limited use and power. Important to note they do exist though.

Magnetism:

Magnets create magnetic fields (makes sense I suppose). Because magnets have a north and south end they are dipoles which means they fade away how? Remember the picture of the earth's magnetic field? Looks something like this:

 

This is a classic dipole field. We have all experienced magnets but making magnets using electricity is something we're less familiar with. If you make a loop of wire and run current through it you get a field that looks identical to that of the earth. As it turns out all magnetic fields arise from current loops though some are atomically small.

Once we realize that currents produce magnetic fields it now becomes possible to image placing wires in the floor of factories or in roadways for robots to follow. As long as the robot has something like a compass to detect the magnetic field we can use magnetic fields to trigger sensors just like the ones we have explored with our robot.

Here's a Puzzle:

If currents create all magnetic fields (including magnets) where is the current that makes the magnetic field of the earth? What do you know about the inside of the earth? There is still a lot we don't know about this actually so I can only share our most current model which will surely change.

If there are currents then it seems like things must be moving inside the earth. This is one of several reasons we believe the core of the earth is 'liquid' in some sense. What might it mean that the moon has no magnetic field? Jupyter does but Mars only has a weak field? Can you apply this model to these different settings?

Finally, we are aware from data gathered off the coast of Oregon and other places around the world that the earth's magnetic field goes away and comes back reversed fairly regularly. This is the ultimate puzzle. How can the currents in the core of the eath change like that? We have few ideas at this point but there are some important consequences if this is a slow process.

Assignment Breadcrumb Reading: Bb Quiz

Fading Away:

How do various fields die away as we get further from the source? You will be choosing multiple answers from a list.

No Additional HW for this last Unit!

Looking Ahead:

Final Week: Work on Take Home Analysis (Part A) and Robot Demo (Part B)