Anyone who has taken high school physics has seen the following assembly. You drop one ball from the left hand side and the ball from the farthest right hand side gets knocked away. This is to illustrate the conservation of momentum and energy.

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I am reminded of this as I read an article on wikipedia on AC current.

(See animation here: http://en.wikipedia.org/wiki/Transmission_line)

enter image description here

In this animation, the electrons are moving on the transmission line just like the motion of the balls on the Newton's Cradle.

Can someone elaborate:

  1. Whether the motion of the electron displayed in the animation is accurate for AC current moving down a transmission line (are electrons really getting pushed back and forth and why)

  2. Principle behind this cyclic motion

  3. Is there any correlation between the motion of the balls on the Newton's Cradle and the motion of the electrons?


1 Answer 1


No, the electrons are not moving like the balls on Newton's cradle. In Newton's cradle, the special thing is that the intermediate balls do not move at all, and even the moving ones don't half of the time. On the other hand, the electrons pictured show a wave movement, where all electrons move continuously.

Now if you look at the Newton cradle microscopically, you'll find that the atoms in it will also make a wave movement, transporting the momentum to the other side. That atom movement is indeed not unlike the electron movement in the transmission line. But that's obviously not what you referred to.

Specifically to your questions:

  1. The motion is accurate. The force that gets the electrons moving is the electric field. In AC, it is actually an electromagnetic wave travelling along the wire. Note that the electrons not only react to the electromagnetic field, but also are sources; the oscillating electrons themselves emit electromagnetic waves which in part cause an effective slowdown of the travelling wave, and in part just get emitted from the wire (the wire effectively acts as antenna).

  2. The principle of the motion is that of waves. Note that water waves are not that different; it's just that they also have a vertical component of motion (that's why you can see them). But note that just like the electrons in the AC wire, the water molecules in a water wave only oscillate. You can see that if something swims on the water: It will not be pushed forward by the wave (unless the waves are steep enough that gravitational pull significantly moves the objects down the wave).

  3. Apart from the fact that both are periodic motions, there's not much they have in common, at least at the macroscopic level. At the microscopic level, the momentum indeed is transported by a wave, in this case a sound wave.

    Note that a slight modification of Newton's cradle can indeed give you a macroscopic wave motion (although at the expense of no longer showing what's actually the point of Newton's cradle): If the balls are not touching each other in their rest position, you'll get a visible wave moving through the chain.

BTW, there are nice animations of Newton's cradle in the Wikipedia article

  • $\begingroup$ "If the balls are not touching each other in their rest position, you'll get a visible wave moving through the chain" Can you support this claim or is it just a guess? $\endgroup$
    – anderstood
    Aug 23, 2016 at 19:11
  • $\begingroup$ @anderstood: It's basic causality: No ball will start moving before it gets hit, and the balls go with finite speed. $\endgroup$
    – celtschk
    Aug 23, 2016 at 20:44
  • $\begingroup$ Ooh I see, I had misunderstood your point. $\endgroup$
    – anderstood
    Aug 23, 2016 at 21:04

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