Transcript 1-Introduction

Electricity and Magnetism Workshop: Introduction
Inquiry-based instruction
My motivations:
Faculty in engineering teach a set of facts and teach students to solve
certain problems. This is perfect for job placement, but does not
prepare students for lifelong learning.
Lifelong learning can be driven by necessity, but its more fun when it
is based on inquiry. We WANT to learn more.
I seek to learn from you through dialog.
My method:
I will approach early science from a historical context because early
scientists ‘inquired.’
Electricity and Magnetism Workshop: Introduction
What do we know now about E & M?
Are electricity and magnetism invisible?
Unlike most fields of physics that deal with light and matter, we can not see
electrical charges themselves.
We cannot tell by observation that high voltage lines are charged.
We cannot tell ahead of time that touching a metal doorknob will shock us.
But the effects of electricity are all around us. We can feel them.
We know now that
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Atoms are composed of positively charged nuclei with orbiting electrons.
Electricity and magnetism are related.
Light can be considered both a wave and a particle (the photon).
All energy (including electrical energy) is quantized.
It is important to know that current is defined as the flow of positive charges,
although it was later found that in metals, it is the negative charges (the
electrons) that move. Electrons traveling in circuits always flow from the
negative terminal of the power source to the positive terminal. Think about it
like this: since electrons have a negative charge they will flow towards a
positive charge. Opposites attract.
Electricity and Magnetism Workshop: Introduction
The electrical construction of the atom
What do we know now?
- All matter is constructed of electrical charges.
- The atom is constructed of negatively charged
electrons orbiting a positively charged nucleus.
The idea of the atom was first devised by Democritus in 530 B.C.
In 1808, an English school teacher and scientist named John Dalton proposed the
modern atomic theory. Modern atomic theory simply states the following:
 Every element is made of atoms
 All atoms of any element are the same
 Atoms of different elements are different in size and in chemical and
physical properties.
 Atoms of different elements can combine to form compounds
 In chemical reactions, atoms are not made, destroyed, or changed (this was
before nuclear power) because there is Conservation of mass.
Electricity and Magnetism Workshop: Introduction
The electrical construction of the atom
In 1898, Joseph John Thomson postulated that the atom was constructed of a
positively charged nucleus with electrons around it.
In 1910-1911, Rutherford performed his ‘famous’ gold foil experiment.
Ernest Rutherford
1871-1937
Apparatus :
What are alpha particles? :
http://www.chemsoc.org/timeline/pages/1911.html
http://www.epa.gov/radiation/understand/alpha.htm
Electricity and Magnetism Workshop: Introduction
Rutherford’s Experiment
Firstly, alpha particles are a type of ionizing radiation ejected by the nuclei of
certain unstable atoms. They have two protons and two neutrons giving
them a charge of 2+ (identical to the nucleus of the helium atom). It is a
relatively heavy, high-energy particle with a velocity in air that is roughly
one-twentieth the speed of light. It is virtually impenetrable which makes it
ideal for this experiment because we want these particles to be deflected by
the components of the atoms making up the gold foil. Rutherford
hypothesized that a beam of alpha particles would pass through thin
sections of foil mainly undeflected and some would be slightly scattered by
electrons. Studying the scattering patterns would tell something about the
distribution of electrons in atoms.
This was not the case. He observed instead that the majority of the alpha
particles passed through the foil undeflected, some were slightly deflected,
a few underwent serious deflections, and others bounced back in the
direction from which they came.
Rutherford explained this saying the atom’s mass was not uniformly
distributed as he previously thought and that most of the atom’s mass and
all of its positive charge were instead centered in a very small region, the
nucleus. The rest of the atom was empty space with units of negative
charge outside the nucleus.
Electricity and Magnetism Workshop: Introduction
The electrical construction of the atom
Rutherford found that the effective radius of the gold
nucleus is
Rn = 1.5 × 10-14 m
While the radius of the gold atom is
Ra = 1.5 × 10-10 m
Thus, the mass of the nucleus occupies 1 part in 1012 by
volume!
Do you suppose this is where the expression “I can see
right through you!” comes from?
http://micro.magnet.fsu.edu/electromag/java/rutherford/index.html