MadSci Network: Physics

Re: Why is the coulomb used (please read below)?

Date: Sat Aug 21 11:58:43 2004
Posted By: Nauzad Tantra, Undergraduate, Production/ Industrial engg., D J Sanghvi college of engg.
Area of science: Physics
ID: 1093015239.Ph

Hi Chris,
A Coulomb is essentially a unit of charge. If a large number of charged
particles are placed on an object, the object is said to posess charge
which is the sum total of the charge on each particle.

Current essentially tells us the speed of flow of charge. That is how fast
the charges are passing through a given conductor.

An analogy of the above could be given as follows:
A charge (coulomb) could be analogous to water in a lake. A lot of small
water droplets get together to form a large body of water. Similarly, a lot
of smaller charged particles combine to give a summation of their charges.

Just as water flows from an outlet to the lake to a lower level, and we can
measure the speed of the water flow (cusecs), charges flow from a charged
body towards an oppositely charged body and we can measure the flow (Ampere).

Getting back to your question, you would now appreciate that charge need
not always be related to the flow of charge and hence, two different units,
(one for measuring the quantum of charge and the other for measuring the
discharge (flow) of charge) are required. 

For example, to calculate the force exerted between a positively charged
body and a negatively charged one, we have Coulombs equation of force. In
this equation current plays no role in determining the result. While in
determining the Current passing through a material at a given Resistance
and Voltage you need not know the amount of charge.

To answer your second question, which is whether electrons and protons are
the same except for the nature of their charges (+,-) the answer would
require a little more inspection.

From only an electrical charge point of view, the above can be said to be
right. However, protons and electrons are very different particles. Their
sizes and other physical properties are very different from each other.
They even interact with other particles differently. For example, you would
find protons and neutrons in the core of the atom, but a neutron and
electron would never be found at the core, or together for that matter.

To make use of your analogy of red and blue balls, one of them could be a
red football and the other a blue marble, both of which weigh the same. But
their sizes, shapes etc are all different.

Hope that answers your questions.

Nauzad Tantra

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