Charge! (The details.)

Intro to electricity

Electricity I - Charge!

Charge

- as fundamental to electricity (& magnetism) as mass is to mechanics

Charge is a concept used to quantatively related "particles" to other particles, in terms of how they affect each other - do they attract or repel?  If so, with what force?

Charge is represented by letter Q.

The basic idea - likes charges repel (- and -, or + and +) and opposite charges attract (+ and -).

Charge is measured in units called coulombs (C).  A coulomb is a huge amount of charge, but a typical particle has a tiny amount of charge:

- the charge of a proton is 1.6 x 10^-19 C.  Similarly, the charge of an electron is the same number, but negative, by definition (-1.6 x 10^-19 C).  The negative sign distinguishes particles from each other, in terms of whether or not they will attract or repel.  The actual sign is arbitrarily chosen.

The charge of a neutron is 0 C, or neutral.


But what IS charge?

Charge is a bit difficult to define.  It is property of particles that describes how particles interact with other particles. 

In general, the terms are negative and positive, with differing amounts of each, quantified as some multiple of the fundamental charge value (e):  e = 1.6 x 10^-19 C.  This number was determined through an historically important experiment, the Millikan Oil Drop experiment, in which the charge of individual electrons was determined by finding the charge on charged oil droplets (through thousands of experiments).  It was tedious, to say the least.

A charge of 1.6 x 10^-19 C hard to visualize, since a coulomb (c) is a huge amount of charge.  One coulomb, for example, is the charge due to:

1 coulomb = charge due to 6.3 x 10^18 protons

I got that number by taking 1 coulomb and dividing it by the charge of one proton:  1/(1.6 x 10^-19 C).

A typical cloud prior to lightning may have a few hundred coulombs of charge - that's an enormous amount of excess charge.

If the charge is negative (-), the excess particles are electrons.

If the charge is positive (+), the excess particles are protons - however, we can NOT easily move protons.  That usually takes a particle accelerator.  In general, things are charged positively by REMOVING electrons, leaving a net charge of positive.  And things can be charged negatively by adding electrons (from somewhere else).  Particles are never created (or destroyed) - only moved around.

Other things to remember:

Neutral matter contains an equal number of protons and electrons.

The nucleus of any atom contains protons and (usually) neutrons (which carry no charge).  The number of protons in the nucleus is called the atomic number, and it defines the element (H = 1, He = 2, Li = 3).

Electrons "travel" around the nucleus in "orbitals."  See chemistry for details.  The bulk of the atom is empty space.

Like types of charge repel.  Opposite types of charge attract.  That's the fundamental rule of electrostatics.

The proton is around 2000 times the mass of the electron and makes up (with the neutrons) the bulk of the atom.  This mass difference also explains why the electron orbits the proton, and not the other way around.

Protons in the nucleus of an atom should, one would imagine, repel each other greatly.  As it happens, the nucleus of an atom is held together by the strong nuclear force (particles which are spring-like, called gluons, keep it together).  

COULOMB'S LAW

How particles interact with each other is governed by a physical relationship called Coulomb's Law:

F = k Q1 Q2 / d^2

Or, the force (of attraction or repulsion) is given by a physical constant times the product of the charges, divided by their distance of separation squared.  The proportionality constant (k) is used to make the units work out to measurable force amounts.  The constant k is defined as (approximately) 9 billion.

k = 9 x 10^9

The units are N m^2 / C^2, which makes the force come out in newtons.

This is an INVERSE SQUARE law, meaning that the force varies as the inverse of the distance squared.  As distance between particles gets bigger, the force gets significantly smaller.

- if the distance between the bodies is doubled, the force becomes 1/4 of its original value
- if the distance is tripled, the force becomes 1/9 the original amount
- etc.

Inverse square laws are common in physics - gravity has the most well-known one.

They look like this, graphically:

Particles!
The "big 3" particles you've probably heard of are:

proton
neutron
electron

However, only 1 of these (the electron) is "fundamental".  The others are made of fundamental particles called "quarks""

proton = 2 "up quarks" + 1 "down quark"
neutron = 2 "down quarks" + 1 "up quark"

There are actually 6 types of quarks:  up, down, charm, strange, top, & bottom.  The names mean nothing in particular.

Many particles exist, but few are fundamental - incapable of being broken up further (so far as we know).

In addition, "force-carrying" particles called "bosons" exist -- photons, gluons, W and Z particles.

See the Standard Model of Particles and Interactions for more details.

http://www.pha.jhu.edu/~dfehling/particle.gif

or here:

http://electron6.phys.utk.edu/phys250/modules/module%206/standard_model.htm