Charge is carried by electrons moving. The protons are always stationary.
The answer I found online is the protons are stuck in the nucleus so they can't move ("strong nuclear force"). But why can't the whole positively charged atom move?
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3$\begingroup$ Because in solids at least, it is stuck to other atoms. $\endgroup$– Olin LathropCommented Jan 17, 2014 at 21:53
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1$\begingroup$ They can move, but tend not to because they are held in place by other atoms. $\endgroup$– JoldCommented Jan 17, 2014 at 22:00
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$\begingroup$ @OlinLathrop How? Why is it stuck? $\endgroup$– dfgCommented Jan 17, 2014 at 22:00
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$\begingroup$ @elfmotat Why are there held by other atoms? How can positive atoms stick to other positively charged atoms? $\endgroup$– dfgCommented Jan 17, 2014 at 22:00
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$\begingroup$ Related: physics.stackexchange.com/q/60848/2451 $\endgroup$– Qmechanic ♦Commented Jan 18, 2014 at 2:27
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5 Answers
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Atoms in a solid are usually stuck to each other in some sort of a rigid lattice, which gives the solid its structure and shape. This is not the only possible state of affairs: they can also slide past each other, if the temperature is high enough, but then they are likely to do so endlessly, all of them, and the material becomes a liquid. And, indeed, in a liquid you can have both negative and positive charge carriers, where the positive ones are entire atoms with one electron stripped off.
answered Jan 17, 2014 at 22:04
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$\begingroup$ Would be good if you could elaborate on the seemingly paradoxical fact that it is the electrons, the ones which are free to move, which are holding the atoms in place. $\endgroup$ Commented Jan 18, 2014 at 1:48
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$\begingroup$ @ComptonScattering Yup that was a big part of what confused me! Could you add something in about that Emilio? $\endgroup$– dfgCommented Jan 18, 2014 at 2:32
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$\begingroup$ Also if the field is strong enough can it cause to atoms stuck in the rigid lattice to break off and cause the object to essentially "morph"? ("change shape") $\endgroup$– dfgCommented Jan 18, 2014 at 2:33
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Positive charges certainly can move: alpha particle of Helium nucleus in Rutherford_scattering is an example. They can scatter alpha particle: 2 protons (charge 2$e^+$) and 2 neutrons.
Even if in the solid state, like metal, you can still throw the whole thing together (moving in the space). Can you just throw the metals or hit metallic baseball bat?
What you are talking about is the "relative motions" between electrons(e) and nucleus(N) (e respect to e, e respect to N, or N respect to N. It is trivial to see e-e and e-N count more than N-N). Since nucleus are much more heavier(about 2000 times larger or more), you only care much about the "relative motions" of electrons moving in the solids.
answered Jan 17, 2014 at 23:39
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2$\begingroup$ Giving a counter-example, as you did here, is a good approach. Movement of positive ions do constitute a current in many situations. I had batteries in mind as a counter-example myself, but there are limitless possibilities. $\endgroup$ Commented Jan 18, 2014 at 0:50
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1$\begingroup$ Nice answer. Indeed in the solid state, we focus on e-e and e-N motions. $\endgroup$– user36094Commented Jan 27, 2014 at 4:05
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Based on the comments below the question, I see that the question have moved from the movement of positively charged particles to why positive ions stay put even when electrons move.
This should not be pictured as electrons far away from positively charged ions moving willy-nilly while positively charged ions stick together. You need to start at the top, Lets take a good conductor, a metal. In a metallic lattice, the neutral atoms
make bonds due to various dipole type interactions (induced dipole force, london force etc) due to the distance between positive nuclei and negative electrons as well as the constant motion of electrons.
During the lattice is being formed and atoms are coming closer, the electrons become more and more free as the energy levels at different distances start becoming equivalent ( due to energy level overlap of different atom's electrons).
Thus after the lattice is formed, the atoms are trapped amongst each other unable to move while electrons have a highway or huge places in entiee lattice where the energy levels are equal. Electrons being free start moving here and there but as electrons both repel themselves and are attracted by the positive ions which they create after motion do not move very far, say if electron of atom A moves towarsds B, then elecrtron of C may come towards A. So in free state at various times positive ions both do and do not exist.
Upon the application of an electric field all these electrons which we have termed free electrons start moving opposite to the direction of field while performing several collisions with the positive ions. But still in a circuit electrons keep of flowing and at most times an electron is near a positive ion making it neutral.
The ions do try to move but due to absense of space for motion just wind up vibrating, even when magnitude of electric field is increased only more electrons find sufficient energy to leave the atoms thus making an ion of more positive charge.
Now to answer your original question
But why can't the whole positively charged atom move?
They can but when they do the metal/conductor starts losing their shape and either are melted to liquid or ionised into gas. In both these states even though equivalent energy states as in case of metals do not exist, but due to freedom of movement even small electric fields set both the electrons and positively charged ions in motion.
The phenomenon is very very common practical when you dissolve $NaCl$ in water both ions not even electrons ie ${Na}^+$ and ${Cl}^-$ take active participation in the conduction of electricity.
answered Jan 17, 2014 at 22:43
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This is complementary to the other answers.
Matter as we know it consist of atoms, which are usually neutral having the same number of electrons to protons. A spill over field from the distortions of the motions of electrons about the nucleus generates the forces, called Van der Waals forces, that create chemistry in matter in bulk.
Matter has four phases, two of them directly related to the VW forces.
Solid: the VW forces hold the atoms in rigid structures and only the external electrons move within the structure, in metals the motion of electrons is appreciable. By moving from the atom the atom becomes positive and is called an ion.
Liquid: the atoms and molecules have mobility so this does allow to have motion of positive ions and not only a motion of the electrons. See this battery for example.
Gas: in gases ions can move . Once an electron is struck out of the last energy level the atom is left a positive ion and it moves because of the definition of gas.
plasma: is the fourth state of mater where a gas is completely ionized and of course positive ions move.
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Let us consider a superconducting loop floating in the vacuum of space. We bring a magnet at the center of the loop. What happens?
Some electrons start to revolve around the center of the loop, let's say they revolve in the clockwise direction. This happens because every negative charge experiences a force when the magnet moves, this phenomenon is called induction.
And rest of the loop starts to revolve counter-clockwise, because every positive charge experiences a force when the magnet moves.
There is no friction between the two revolving things, so they keep revolving forever.
This is not a weird special case. Positive charges move. A passenger on a boat turns lights on, electrons start to circle around, boat must start to spin around to conserve angular momentum.
