17-05-2012, 04:25 PM
Microcontroller Programming The Microchip PIC
[attachment=22290]
The Atom
Until the end of the nineteenth century it was assumed that matter was composed of
small, indivisible particles called atoms. The work of J.J. Thompson, Daniel
Rutheford, and Neils Bohr proved that atoms were complex structures that contained
both positive and negative particles. The positive ones were called protons and the
negative ones electrons.
Isotopes and Ions
But nature is not always consistent with such neat models. Whereas in a neutral atom,
the number of protons in the atomic nucleus exactly matches the number of electrons,
the number of protons need not match the number of neutrons. For example, most hydrogen
atoms have a single proton, but no neutrons, while a small percentage have one
neutron, and an even smaller one have two neutrons. In this sense, atoms of an element
that contains different number of neutrons are isotopes of the element; for example
water (H2O) containing hydrogen atoms with two neutrons (deuterium) is called
"heavy water."
Static Electricity
Free electrons can travel through matter or remain at rest on a surface. When electrons
are at rest, the surface is said to have a static electrical charge that can be positive
or negative. When electrons are moving in a stream-like manner we call this
movement an electrical current. Electrons can be removed from a surface by means of
friction, heat, light, or a chemical reaction. In this case the surface becomes positively
charged.
[attachment=22290]
The Atom
Until the end of the nineteenth century it was assumed that matter was composed of
small, indivisible particles called atoms. The work of J.J. Thompson, Daniel
Rutheford, and Neils Bohr proved that atoms were complex structures that contained
both positive and negative particles. The positive ones were called protons and the
negative ones electrons.
Isotopes and Ions
But nature is not always consistent with such neat models. Whereas in a neutral atom,
the number of protons in the atomic nucleus exactly matches the number of electrons,
the number of protons need not match the number of neutrons. For example, most hydrogen
atoms have a single proton, but no neutrons, while a small percentage have one
neutron, and an even smaller one have two neutrons. In this sense, atoms of an element
that contains different number of neutrons are isotopes of the element; for example
water (H2O) containing hydrogen atoms with two neutrons (deuterium) is called
"heavy water."
Static Electricity
Free electrons can travel through matter or remain at rest on a surface. When electrons
are at rest, the surface is said to have a static electrical charge that can be positive
or negative. When electrons are moving in a stream-like manner we call this
movement an electrical current. Electrons can be removed from a surface by means of
friction, heat, light, or a chemical reaction. In this case the surface becomes positively
charged.