Date: Mon Apr 9 12:28:51 2007

Author: Paul Nord

Subject: Re: Visual Electricity Demonstrator


Thanks. No objections to "Magnetism is the pushing or pulling force
that exists when a magnetic material is near."?

Perhaps that's good enough for a 4th grade text. It would be pretty
hard to defend all of the implications of that statement.

The use of the word "near" here is something that has come up a lot
in this curriculum. In principle, the effect is infinite. Shouldn't
we also add, "between magnetic materials." Also below is the
description of resistivity as a "quality" of a material... but that
may be pardonable. At other times there have been superlatives that
aren't completely justified. An example of that would be something
like, "In a DC circuit the charges _always_ flow in one direction."
To which I would say, "The _net_ flow is in one direction. There is
probably a lot of brownian motion going on and probably a good bit of
shuffling around."

Excellent comment on the Earth's ____. I wouldn't have thought to
put electromagnet in there. But it does fit perfectly. However, the
truer statement would be, "Because of Earth's Magnetic Field, a
compass needle points in the direction of the local magnetic field."
There are a number of places where the local geology conspires to
make a compass point east-west.

Am I just expecting too much?


On Apr 9, 2007, at 9:21 AM, William Beaty wrote:

> On Sun, 8 Apr 2007, Paul Nord wrote:
>> Bill, et. al.,
>> Comments on this 4th grade textbook review?
> Hmmmm, that's odd. My last message vanished and wasn't posted.
> It's not
> in the archive. I'll try again.
> Let's see...
> #4 mentions "flow of current." There is no such stuff as "current
> electricity" which flows through wires. Charge is the stuff that
> flows.
> #5 should be "path" not "direction." Charge flow in one direction
> is DC,
> not parallel circuit.
> #6, three different answers are correct.
> #8, I think it's slightly more understandable to say "value of
> current"
> or even "rate of charge flow," rather than "amount of current."
> The word
> "amount" is too commonly used to describe amounts of rice, water, etc.
> Current is a rate.
>> 1. If charged particles in an object are not balanced, the object
>> builds up _____.> Static Electricity
>> 2. A charge in motion is called a(n) _____.> Electric Current
>> 3. > Magnetism is the pushing or pulling force that exists when a
>> magnetic
>> material is near.
>> 4. The quality of > Resistance____ opposes the flow of electric
>> current through a
>> material.
>> 5. Current flows in only one direction in a(n) _____.> Series Circuit
>> 6. Because of Earth's ____> Magnetic Field, a compass needle
>> points in a north-south
>> direction.
>> Magnetism
>> 7. One advantage of a(n) ____> Electromagnet over a natural magnet
>> is that its
>> magnetic field can be turned off.
>> 8. A(n) > Parallel Circuit_____ can handle appliances that use
>> different amounts of
>> electric current.
>> And should I bother correcting his teacher about her description of
>> batteries as supplying "juice"? It's because there is a liquid
>> inside the battery.
>> Paul
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