Tuesday, March 20, 2007

Don't teach that right hand rule

Right hand rules in high school physics: is there a point? I'm in the midst of my electromagnetism unit, and there is much to learn, but also much to leave to future courses for students who wish to know more.

The former includes the right hand rule for determing the direction of the magnetic field around a current-carrying wire. This is a tough row to hoe, as it requires students to have a modicum of agility in thinking in three dimensions. My experience tells me that high school students have few opportunities to navigate the geometrical challenges of three-dimensional space. But the state of California wants its physics students to know this, so I teach it. I introduce it with a lab activity.

Among the things to leave to future physics coursework, I would include the current-field-force right hand rule. You know, the one involving the mutual perpendicularity of current, magnetic field, and magnetic force. Students are reeling from the 3-D demands of the first right hand rule. So why add this insult to their injury? What's the payoff? What gift does this knowledge bear that makes the pain of learning it worthwhile? In high school? Even the great state of California deems it non-essential.

Basic electromagnetism is rife with intangible fields and geomagnetic mayhem. There's some counter-intuitive stuff in this material. Can't we save this bit of trivia for another, later course?

Oh, and don't get me started on the left hand rule. (You really should click that link if you want a laugh.)


MV said...

Fleming's right hand rule is used for finding the direction of induced current and is often called the 'dynamo rule'. I have never seen Fleming's name connected with a right hand rule for finding the direction of the magnetic field produced by a current carrying cinductor. The rule for this is usually called 'right hand palm rule' or 'right hand grip rule' or 'right hand thumb rule'.

Dean Baird said...

True enough about Fleming. I think the illustration caption might be off. But more importantly, I think the illustration is intended as a joke! Try to contort your hand as illustrated without breaking bones.

I had not heard the terms "right hand palm rule" or "right hand grip rule." I should probably get out more.

My point with the post is that we high school physics teachers are attracted to the notion of teaching the right hand palm rule when there is really no compelling reason to do so. Leave it for college (or the AP course).

Barb A. said...

What a lot of people leave out when teaching the "right hand rule" or "right hand grip rule" is whether they're talking about pointing the thumb in the direction of conventional current flow or in the direction of electron flow. Many sources omit to state this, and it is driving me nuts. I'm working on editing a textbook for a short course in electicity and electromagnetism.

Things have gotten so contentious around here that I got a much-needed laugh out of the drawing you posted. I'm going to see if I can find someone in my yoga class who can make his hand do that.

Dean Baird said...

I will confess that when I hear the term "current," I think *only* of conventional current. I never teach "electron current."

I think you will find that published textbooks stick with conventional current as well.

I think it's only classroom physics teachers who elect to go maverick with "electron current."

I could be wrong.

(Are you catching my ironic use of "I"? Or is it ironic at all?)

Barb A. said...

Regarding current flow vs. electron flow: you're right that most US textbooks on electricity stick to conventional flow. Electronics are another story: Our electronics instructor swears that it is virtually impossible to teach about electronics without talking about electron flow. He says that if you try to explain a transistor, for example, it gets hopelessly confusing if you use conventional current.

Since he's the one teaching from the book I'm trying to edit, I like to try to respect his wishes. Furthermore, our mutual boss was educated in the UK and it appears that UK sources are more likely to use electron flow, and propose a "left-hand grip" rule (a result of driving on the left?). So, the problem isn't that both of them want to use electron flow but rather that one (the American) wants to use the right hand and the other (the Brit) wants to use the left...
As I understand it (please correct me if I'm wrong), the right thumb should point in the direction of conventional flow, while the left thumb should point in the direction of electron flow. Then the fingers all wrap in the same circular direction, which is logical, because "conventional flow" and "electron flow" are just two ways of describing the same physical phenomenon. Do you agree?

I found a source that I hope will settle the arguments: http://www.army.mod.uk/linkedfiles/royalartillery/units/royal_school_of_artillery/bst_handout_e02.pdf
This is a neatly produced document, part of the British Army's Basic Technology Course, and it lays out several "hand rules" including "grip" and "Fleming's", very clearly, starting on page 4.