Re: electric charge

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

 

On Tue, 25 Dec 2001 22:47:06 -0500 Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU> writes:
> The comments of this thread prompted me to sketch an
> introduction to electricity. Is it acceptable? I plan to
> distribute it as a handout to students, after performing
> standard demonstrations with rods and pith balls.
> Ludwik Kowalski

 

***Most of us will agree that it is VERY MUCH easier to

find errors in a written passage than to actually write

such a passage without any errors. But as long as you asked,

Ludwik, here are some possible errors that come to mind

after a quick reading of your passage....
 
> Mass m is the mechanical attribute of an object.

 

*** Mass is not the mechanical attribute of an object.

There are other mechanical attributes such as size,

shape, and density. 

 

>It determines how the object accelerates (F=m*a) and

*** It partially determines how an object accelerates .....

 

> how it is attracted by another mass (F=G*m*M/d^2).

 

*** and how moves in an electric or gravitational field.

> The first electric phenomenon discovered was mutual
> attraction and mutual repulsion of light objects under
> the influence of something which was not mass or
> magnet.

 

*** How do you know that this was the "first"

 

> That something was named charge. A glass
> rod robbed with silk, for example, acquires a property
> of repelling another glass rod robbed with silk.

 

*** Whether or not the glass was robbed or purchased

is irrelevant here.

 

> That property, named charge, was initially modeled as a fluid.
> An object containing that fluid was said to be electrified.

*** What was the date and circumstance of the initial modelling

of the property called charge?

 

> Likewise, a plastic rod robbed with wool repels
> another plastic rod robbed with wool. But an
> electrified glass and an electrified plastic attract,
> rather than repel, each other.

 

*** The attraction (or repulsion) of a charged piece of

glass and a charged plastic rod is very tricky. It depends

on the type of plastic, its temperature, and the amount of

rubbing that is done.  

 

 

> This, and many other,
> observation, and lead to a realization that there are
> two kinds of electric fluids, positive and negative.

 

*** In addition to the obvious errors in punctuation and

sentence structure, I wonder if static charges should be called

"electric fluids" at this point.

 

> The term charge used to be interpreted as the "amount of
> electric fluids" or "amount of electricity" which an
> object can acquire or lose.
>
> A modern interpretation is based on the realization
> that submicroscopic particles, protons and electrons,
> are permanently charged with positive and negative
> electricity. A macroscopic object is charged when the
> number of electrons and the number of protons are not
> identical. An excess of protons results in a net positive
> charge while an excess of electrons results in a net
> negative charge.

 

*** Doesn't the above statement conflict with the idea

that the quantity of protons remains fixed within an atom

of glass or rubber.  Isn't a negative charge associated

with a surplus of electons surrounding the nucleus  and

a positive charge associated with a deficiency of such

electrons?

  

> The net charge, like the total mass,
> becomes an attribute of an object. It is an attribute
> responsible for forces between electrified objects.

 

*** But isn't it true that the the total mass of an

object is much less likely to change than its net charge.

> Two similar charges (both positive or both negative)
> always repel but two dissimilar charges (positive and
> negative) always attract. This was the first qualitative
> observation about electric forces.

 

*** Is it the "charges" that repel or attract or is it the

"charged objects" that do the repelling and attracting?
>
> It turns out that the magnitude of an electric force
> between two charges (q1 and q2) is proportional to
> the product q1*q2 and inversely proportional to the
> square of the distance (d^2) between their centers. This
> observation, made by Coulomb, is known of Coulomb’s
> law. It can be written as:
>
>             F = k*q1*q2 / d^2
>
> where k is the proportionality constant. The value of
> that constant can be chosen arbitrarily in order to
> define a unit of electric charge. For the purpose of this
> introduction the unit of electric charge, one coulomb, C,
> we will defined by declaring that k=1,000,000,000.

 

*** Iszn't it important to state the units (newton-meters

squared over coulombs squared) in the definition of k here? 

 

> This is equivalent to saying that the electric charge is one
> coulomb if it attracts or repels an identical charge with
> a force of one billion newtons when the distance between
> the centers of two charges is one meter. One coulomb is
> a very large charge; charges produced on robbed rods
> and plates are usually expressed in microcoulombs or in
> nanocoulombs. Ignoring sign differences we can say
> that the charge of one electron and the charge of one
> proton are identical (1.6*10^-19 C).
>
> The so-called "official" SI definition of the unit of charge
> is conceptually different from the one presented above.
> But in practical terms it is not at all different. In SI the
> ampere, A, is the first unit;

 

*** It would be better to say that the ampere is a defined

in the SI system as a "fundamental" unit rather than

a "first" unit. The terms fundamental and first are not really

synonomous. 

 

> all other electrical units are
> defined in terms of kg, m, s and A. The unit of charge,
> coulomb, C, is defined as A*s. In our sequence C is the
> first electric unit and A will be defined as C/s. Other
> nuances associated with electrical and magnetic SI units
> will be discussed later. Note that F in Coulomb’s law is
> positive when two charges repel (q1 and q2 have the
> same sign) and negative when they attract (signs of q1
> and q2 are different).

 

I hope that my "***" comments are helpful for your purpose.

 

Herb Gottlieb from New York City

The home of the infamous ground zero