The Magnetic Fields of the Planets

The Earth’s magnetic field results from the movement of molten metal in the Earth’s core. The outer core actually. It’s mostly molten iron, which conducts electricity, and as it convects up and down, like boiling water in a pot, the moving electrical charges create the Earth’s magnetic field. Its a bit like a dynamo.

core-convection-b — The internal structure of the Earth. Movement in the liquid metal outer core (green arrows) generates the earth's magnetic field.

What drives the convection of the outer core? The heat released from the freezing of the liquid metal to the solid inner core. The inner core is ever expanding, and the outer core is getting smaller and smaller. Ultimately, when the entire outer core freezes the Earth’s magnetic field should disappear. But we’ve got some hundreds of millions of years left so we don’t have to worry quite yet.

The Other Planets

The question came up: Do Mars and the other planets have magnetic fields?

Astronomynotes has compiled a table of Planet Atmospheres and Magnetic Fields that shows that of the inner planets — Mercury, Venus, Earth and Mars — only the Earth has a significant magnetic field; Mercury does have its own field but it has less than 1% the strength of the Earth’s.

At present, Mars does not have a magnetic field, but it does have remnant magnetism imprinted on its rocks, indicating that it used to have one in the past. It’s internal dynamo died away long ago. Interestingly, the pattern of Mars’ remnant magnetism indicates that it’s interior was once molten enough that the surface had tectonic plates just like the Earth.

release_1999-56_field — Comparison of Earth's healthy magnetic field and the local, remnant magnetism of Mars. Image via NASA.

On the other hand, the outer planets have much larger magnetic fields; Jupiter’s is almost 20 times larger than Earth’s. The gas giants’ magnetic fields are also generated by fluid motion in their interiors (Stevenson, 1983 (pdf)). It’s likely, however, that in some of these bigger planets, at least, the electrically conductive fluid is not liquid metal like in the Earth’s core, but either liquid hydrogen, or a water solution with dissolved electrolytes.

jupmag5na — Jupiter's strong magnetic field interacts dramatically with its moons, inducing magnetic fields in some. Image via NASA.

Drawings of Jupiter

jupiter-tev — Étienne Trouvelot's drawing of the planet Jupiter from 1880 (via the New York Public Library), combined with an image of the planet from the Cassini spacecraft taken in 2000 (NASA/JPL/Space Science Institute)

The New York Public Library’s website hosts a remarkable collection of Étienne Léopold Trouvelot‘s astronomical drawings by that date back to 19th century.

The beauty and detail of these illustrations are a remarkable testament to the intersection of art and science.

Building a Simple Electric Motor

This is a really simple electric motor that only requires some wire, a battery, and a magnet. Simon Quellen Field has a wonderfully detailed description of how to build the motor, and some elegant tips on how you can make the motor run faster.

My middle-schoolers quite enjoyed building one of these, and I’m planning on having my high-school physics students also try it; only a couple of them claim to have done it before. It should be a good way to tie together electricity and magnetism.

(Evil Mad Scientist has an even simpler motor, but, given that the risk that their homopolar motor is quite capable of launching a drywall nail across the room, I think I’d suggest not trying that one without extremely close supervision.)

Although it’s a bit trickier, another great way of demonstrating electromagnetic induction is to build a simple alternating current generator that runs a small light bulb.

Bill Beaty’s website explains how to build the generator in excellent detail.

The best part of building the generator is that you can actually feel the extra energy it takes to light the bulb, as you spin the magnets.

One Ring

Tolkien recites the One Ring.

What Makes for an Effective School?

Dobbie and Fryer (2011) investigate the key things that make for an effective school. Effectiveness is based on test scores, which is a significant caveat, but most of their results seem reasonable.

Frequent feedback for teachers about their teaching from classroom visits,
Longer teacher hours, (10+ hours per week)

Middle school teachers at better schools worked over 10 hours a week more than lower performing schools.
Interestingly, salary had no discernible effect. It seems that the teachers did not even get paid more for putting in the extra hours. The willingness to put in these extra hours without extra pay implies a different philosophy and culture among the teachers of the “more effective” schools.

Data driven instruction – more effective schools “adjust tutoring groups, assign remediation, modify instruction, or create individualized student goals,” based on frequent feedback from interim assessments.
Feedback to parents – better schools have more frequent communication with students’ parents
High-dosage tutoring – The better performing schools were found to be more likely to offer tutoring where, “the typical group is six or fewer students and those groups meet four or more times per week”,
Increased instructional time – about 8% more hours per year
A relentless focus on academic achievement

This was assessed with a survey of principals. Those who put, “a relentless focus on academic goals and having students meet them” and “very high expectations for student behavior and discipline” as her top two priorities (in either order) scored higher on this assessment of the rigor of school culture.
I have serious reservations about this result. If the key focus of the school is on doing well on tests (as their “academic goals”) they should do better on the tests. This is certainly a good way to score better on standardized tests, but it has serious, negative implications when it comes to creating intrinsically motivated students.

These results come from comparing charter schools in New York City.

Sandra Cunningham has a rather cursory summary in The Atlantic, but her post’s comments section has some very interesting perspectives.

The Rules of Comma Use

Dr. H. gave us a quick refresher on how to use commas this afternoon. Of course, like a true academic, he started with the caveat that, like any other rules of language, the way we use commas has changed over time, and is constantly evolving. Punctuation was initially designed to help the verbal reader, but has developed into a tool to help clarify syntax.

The Rules

From my notes on Dr. H’s presentation: use commas to:

Separate things in a list.
- eg: The flag for Equatorial Guinea is blue, green, white, and red.
- One potential issue with these commas is the last one, the serial comma, which I tend to find very useful in separating ideas, particularly in complex sentences.
To separate “sentences” (i.e. clauses)
- Bob talks in class ________ he’s not always on task.
To indicate “interrupts”
- These are my favorites, because you can use them like parentheses to isolate clauses and clarify the meaning. I have been accused, by someone who shall remain unnamed, of overusing them.
- e.g.: The boy, bothered by the noise in the room, wandered down to his quiet place.
- e.g.: Mary considered her possibilities and, after a period of reflection, came to the conclusion that doing something was in order.
DO NOT USE commas between the subject and verb of a sentence (unless absolutely necessary).

A key thing to remember though, particularly with things like the serial comma, is that you need to be consistent. The syntax of language may evolve over time, but usually not within a single document.