The Left-Hand Rule?

Everyone who’s taken physics at some point knows about the Right Hand Rule. There are two versions. If you have a wire carrying an electric current, you point your thumb in the direction of the current and your fingers “curl” in the direction of the magnetic field.

The other version is that if you have two out of the following three — current, magnetic field, and a force — the right hand rule can help you find the direction of the third one that gets induced from the other two:

Well, there’s one really annoying thing about this rule: it requires the current to be moving positive charges! Well, positive charges do move in electric circuits.

A typical example: for every 100,000 negative electrons that move one direction in a circuit, there is one positively charged iron nucleus that moves in the opposite direction. But the reason we use the convention that positive charges move is, hundreds of years ago, there was no way to tell which charge was moving!

Ben Franklin guessed. He had two possibilities, positive and negative. And he guessed wrong. But somehow, hundreds of years later, we’re still using his incorrect guess. It drives electricians nuts, because they’ve switched over to using negative electrical currents, but physicists haven’t. Is there a way to fix this?

It isn’t hard, all you have to do is use your left hand instead of your right! Same deal, too. Thumb=current, index finger=magnetic field, and middle finger=force. Well, physicists are slow to change their conventions, so perhaps xkcd can help effect change with their comic today:

Really, isn’t physics confusing enough without having to deliberately correct for a mistake we know we’re making? Or is it that hard to change an educational convention?

Comments

  1. #1 eNeMeE
    April 10, 2009

    That would be so awesome. Then I wouldn’t have to put down my pen to check it!

  2. #2 Brian
    April 10, 2009

    No, it’s not that hard. You just need to … build a time machine, I guess.

  3. #3 Sili
    April 10, 2009

    We chemists (I’m cheating and calling myself one) were pretty damn lucky that Fischer guessed better than Franklin when he elucidated the relative structures of the sugars. (He guessed at the absolute configuation, not the relative, of course.)

    Didn’t actually know the convention was due to Franklin. Learn something new every day.

  4. #4 NewEnglandBob
    April 10, 2009

    Does the left hand know what the right hand is doing?

  5. #5 Andrew Sarnat
    April 10, 2009

    I don’t see the big deal about how current is defined. Bio-electric phenomena such as nerve impulses depend on currents of anions such as Na+ and K+. Semiconductor devices use currents of electron ‘holes’ — effectively positive charges. There’s a long list of electrical phenomena where current is mediated — partly or entirely — by positive charge carriers. Why the electron-centric view?!

  6. #6 Sean
    April 10, 2009

    The reason the right hand rule is here to stay is that the the the convention is bigger than just the electric force law. Right handed coordinate systems are used by convention in pretty much all of physics laws that contain cross products in three dimensions (not just electrical ones) and also many mathematical theorems. It would be tough to change the rule for just one equation amongst hundreds — there’s too much inertia to change it at this point. All apologies to the electricians!

  7. #7 Andrew
    April 11, 2009

    I’m with Ethan on this one. I’m an undergrad currently in general physics and the whole positive to negative direction of currents confused the hell out of me for like two weeks. It just seems counter intuitive to me!

  8. #8 Sophos
    April 11, 2009

    Right hand rule or left hand rule? In Malaysia, we use neither. We use the right hand grip rule and Fleming’s left hand rule for motors and Fleming’s right hand rule for generators. Haha
    I have to admit, his mistake confused me at at point, trying to figure out if electrons were flowing in one direction, what is flowing the other direction to call current? One of my friend even suggested that it’s the empty spaces between electrons =X

  9. #9 carmen
    April 12, 2009

    electrons just want to have fun.

  10. #10 Super Torresmo
    April 13, 2009

    “It drives electricians nuts, because they’ve switched over to using negative electrical currents”
    Really?? I’m a Electrical Engineer, but we use positive electrical currents. Witch electricians are you talking about?

    Well, anyway, there is another type of positive current you forgot to mention, the ionic current…

  11. #11 Ethan Siegel
    April 13, 2009

    Torresmo,

    Actual electricians, not electrical engineers. You know, the people who wire buildings up, who pull copper wire through pipe and install receptacles and control panels. My wife was one for a few years, and that’s how I know.

    The thing people don’t realize is that if you leave a circuit on all day, the amount that a typical electron moves is on the order of a foot.

  12. #12 Neil A Benson
    April 13, 2009

    Am I from a different universe, or is my memory impaired?

    Decades ago I was taught the left-hand-rule and electron motion. When I got to college physics, sophmore year, we were told that scientists had decided to use plus motion and the right-hand-rule.

    Neil

  13. #13 darion
    May 12, 2009

    helped very much thanks

  14. #14 andrew
    February 22, 2010

    starts with a bang???? how could 2 little specks in the middle of space start every thing that we see today??? so wat you are telling me is that if i empty out every thing in my garadge and wait 100 years ok not long what about 1000000 years ok so if what yall are saying is if i wait that long a brand new car will apear in my garadge?????
    THE BIG BANG IS FALS!!!!!!!!!!!

  15. #15 Tom Norman
    January 10, 2011

    Franklin probably chose to say current leaves the positive terminal of a battery and enters the negative one for reasons that are purely mathematical. It’s easier to deal with positive numbers than with negative numbers when you are calculating what’s happening in a circuit, and much more confusing when you have to deal with double negatives. His choice makes the math simpler.

  16. #16 Alex
    March 13, 2014

    Alex was here (your favorite student)

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