By : Yousuf Ahmed Mohamed
G12
Introduction to Quantum
Physics
Early Atomic Physics
What is Quantum Physics
Quantum Physics is a collection of laws
which explain observations of the tiny
building blocks of all matter.
The world of the quantum must be able to
explain the classical world that we live in.
To understand the quantum world we
need to understand one of the major
building blocks ---- the atom
History of Atomic Structure
The model of atomic structure has
changed as observations have altered our
perceptions
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Democrictus
Dalton
Thomson
Rutherford
………. (The model is not complete)
Democritus
Atoms (Greek for indivisible) are the
smallest unit of matter
Atoms share all of the properties of the
macroscopic object
Atoms are the smallest pieces of matter
which still act as the material from which
they come from
John Dalton
First truly scientific theory of the atom
(results discovered through experiments
with marsh gases)
Proof of early Greek model --- the atom is
indivisible but with no internal
structure
Properties of matter come
from the properties of the atom
Rutherford
1909 – Rutherford performs an experiment
in which alpha particles (He nucleus) are
fired towards a thin foil of gold
Rutherford
Experimental observations indicated that
the majority of the alpha particles passed
straight through, with few being deflected
at small angles and even fewer retro
reflecting from the gold foil
Rutherford
Observations indicate that the atom is
mostly empty space with a dense, central,
positively-charged structure at its center
The electrons (discovered by Thomson)
must therefore exist outside of this central
nucleus …. Orbiting around the nucleus as
planets do the Sun.
Enter Niels Bohr
Bohr succeeded in solving the problem
with the classical model by uniting two
disparate ideas : Planck’s quanta and the
hydrogen emission spectra
Max Planck
Observed the temperatures of cannons as
they were bored out
The colour of the emitted
radiation is related to the
temperature of the cannon
The expected peak intensity
follow the Rayleigh-Jean
law
Rayleigh-Jean Law
Bohr’s theory
Bohr’s theory was the first step in the Quantum
revolution
Postulate of Stationary States : the Hydrogen
atom can exist, without radiating energy, in any
one of a discrete set of orbits of fixed energy
Frequency Postulate : the Hydrogen atom can
emit or absorb a quantity of energy only when
the electron changes from one stationary state
into another. This amount can be calculated by
E=hf
Question
How does the concept of the quantization
of energy circumvent the problem with the
classical model?
The Atom so far ….
A central nucleus (of positive charge) is
surrounded by negatively charged
particles called electrons
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Electrons can only orbit in fixed distances
from the nucleus because they can only gain /
lose a quanta of energy
This prevents the electron from “falling into” the
nucleus
Momentum and position
Imagine that we have a small sub atomic
particle that we want to observe and
record all possible data for.
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We start to attempt to measure the
momentum of the object
To measure the velocity (and therefore the
momentum) we need to set up a set of timing
gates