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Charge and the structure of matter

Page history last edited by Julia Gouvea 8 years, 6 months ago

Course content > Newton's Laws > Kinds of Forces  > Electric Forces

 

Prerequisites

 

The structure of matter

You know from your study of chemistry that matter is made up of atoms that combine into molecules and crystals.  To understand how molecules and crystals form and the properties of macroscopic matter we need to go more deeply into atomic structure.  The key ideas we will need to know are as follows:

  • Atoms are made up of electrons and nuclei;
  • The sizes of atoms are determined by the distribution of the atom's electrons;
  • The nuclei are small but contain almost all of the mass of the atom;
  • The bonding between atoms that builds molecules and crystals arise from the electrical forces between the electrons and nuclei and the sharing of electrons between different atoms.

 

The nuclei in an atom can be treated for most of chemistry and biology as if they were classical "point masses" in the Newtonian sense -- objects that can be modeled as having no structure and no significant size.  The only thing that matters for us is how they couple to (create and feel) long-range forces -- gravity (they have a mass), electric forces (they have a charge), and magnetic forces (they have a magnetic moment).  In fact, we know that they are made up of neutrons and protons, and we even know a lot about the structure of neutrons and protons.  But the structure of the nucleus and the structure of neutrons and protons don't play a role in the behavior of molecules at normal temperatures.  They play no role in understanding biology (except that the substructures explain radioactivity and we can use them as probes of ordinary matter).  

 

The electrons are more complex.  They are much lighter than nuclei (an electron has a mass ~1/2000 that of a neutron or proton) and as a result, spread themselves out over a much larger volume than the nucleus does.  This "as a result" is a throwaway line here. It arises from the quantum nature of matter and from the fact that quantum properties are more important the lighter an object is.  This is not at all obvious and is beyond the scope of what we can discuss here.  A useful way to think about it, is that because it is so light, the electron is moving very fast -- spinning around and filling the space of the atom like the blades of an electric fan. This isn't quite right -- the electron can be in many places at once -- but it isn't too bad as a starting point. 

 

What matters for us here, for both electrons and nuclei, is their electrical character.

 

Charge and the electric force

We have identified three action-at-a-distance forces: gravity, electricity, and magnetism.  Where do they come from? Why do they happen? It's difficult to think about what kind of answer we might give to that question. Why do objects exert forces on each other when they bang into each other? We might answer that objects can't occupy the same space and so they push each other out of the way. But that assumes a particular property of matter -- that they can't occupy the same space. The answer for our a-a-a-d-fs is this:

 

The basic particles of which matter is made, have properties -- mass, charge, and magnetic moment.  Each of these properties both creates and responds to long-range action-at-a-distance forces.

 

That's it.  That's the analog of "objects can't occupy the same space" that has to satisfy us -- for now -- as to why there are normal forces when objects interact.  (Eventually, we will see these macroscopic contact forces as consequences of the electric force and the laws of quantum mechanics.) The result is

 

Every charge exerts action-at-a-distance electric forces on every other charge.  It's this force that holds atoms together. 

 

Our model of matter has all matter made up of electrons and nuclei and the nuclei made up of protons and neutrons.  The charge on the electron is in some sense opposite to the charge on a proton in that an electron and proton at the same place will produce forces on a third charge that cancel. This maps nicely onto positive and negative numbers which cancel when we add them.  We call the charge on a proton positive and the charge on the electron negative.

 

Since most matter is made up of equal numbers of positive and negative charges mostly all electric forces cancel and we don't usually see their effects. But because the forces between charges depend on distance, the distribution of the positions of positive and negative charges can be important.

 

For biological and chemical systems, the fact that electrons are light and can be moved from one atom to another -- or shared between atoms -- plays a critical role. Just to remind you of an important point that you learned in chemistry:

 

A molecule that has more or fewer electron than it has protons has a net charge and is called an ion. Electrons and ions are the primary sources of electrical effects in matter.

 

It is also true that for some molecules, the distribution of charge in the molecule is not uniform.  One side or end might have more negative charge and another might have more positive charge. Since distance matters in electric forces, orienting a lot of such molecules together can have a significant effect.

 

[You can make a macroscopic object like this by taking something made up of such molecules and aligning them. You can do this with wax by putting it in an electric field when it is molten and then freezing it.  The frozen wax bar will then have a built in electric effect -- like having plus charges on one end and minus on the other. It acts like a bar magnet but with electric rather than magnetic forces.  Such an object is called an electret.]

 

We will consider these issues in quantitative detail in the follow ons.

 

Follow-ons

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