Download VII. Electricity Topics Of the four fundamental forces, the most

VII. Electricity
Topics
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Of the four fundamental forces, the most important (and strongest) in our everyday lives
is the electrostatic force. This is the mutual repulsion and attraction of subatomic
particles. We feel this force whenever we attempt to put one solid object through another.
Most macroscopic objects (like a table) are electrically neutral, but only because huge
numbers of positive and negative electric charges are in balance.
Electric forces bind electrons to protons to form atoms, bind atoms together to form
molecules, and bind molecules together to form solids and liquids.
In this unit, we also introduce the abstract concepts of vector and scalar fields, which are
important for further study in this domain.
Questions
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How does matter hold itself together? Why can’t I pass my hand through a table?
What is electric charge? Is it something an object “obtains” or is it more fundamental?
Why do mathematical models work so well in describing the physics of electricity?
How can I produce a large electric spark? To what uses can I put this spark?
How is electricity stored?
Knowledge and skills
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By the end of this unit, students will be able to
o Calculate the number of fundamental charges in a macroscopic charge.
o Determine the charge state of an electroscope based on the charging process and
the position of the electroscope leaves.
o Reason about how static charges are generated when two materials are brought in
contact. Understand and comment on a triboelectric sequence.
o Determine the net charge of a collection of charged objects.
o Describe the processes of charge by induction, polarization, and conduction.
o Calculate two-dimensional vector forces using Coulomb’s Law for a set of three
arbitrarily placed charged objects.
o Sketch the electric field of a small collection of positively and negatively charged
objects.
o Calculate values of the two-dimensional electric field in the presence of a set of
three arbitrarily placed charged objects.
o Reason about the strength of the electric field based on the density of field lines.
o Relate electric potential to electric potential energy, and make calculations based
on this relationship. Also, solve problems using potential and kinetic energy based
on charged particles moving through simple electric potentials.
o Relate electric potential to electric field. Be able to reason about equipotential
diagrams … in particular, be able to locate regions in an equipotential diagram
where the electric field is strongest or weakest, and be able to determine the
direction of the electric field from this diagram.
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Likewise, be able to sketch an equipotential diagram if given an electric field
pattern.
o Calculate simple (one-dimensional) electric field values from electric potential
functions, and vice versa, using algebra.
o Determine the electric charge of two leaves in an electroscope from the angle at
which they are hanging and the masses of the leaves.
o Calculate the capacitance of a parallel-plate capacitor given its geometry and
dielectric. Make educated guesses about the storage capacity of a capacitor given
its geometry.
By the end of this unit, students will understand that
o Like mass, electric charge is a property of fundamental particles, and cannot be
created or destroyed.
o The fundamental unit of electric charge can be determined through Millikan’s oil
drop experiment.
o Neutrally charged objects can experience an attractive electric force when
polarized by a nearby charged object.
o The storage capacity of a capacitor depends only on its geometry and the
additional capacity provided by a polarizable (dielectric) substance.
o A useful but limited analogy can be made between electric potential and height.
o Seen from an energy perspective, differences in electric potential are the cause of
the motion of charged particles.
o Seen from a force perspective, electric fields cause the acceleration of charged
particles.
o The electric field, acting as a “mediator” of force, gets rid of the messy “action at
a distance” problem Newton associated with the inverse-square law of gravity.
o Electric fields obey the “principle of superposition” which means the electric
fields surrounding individual particles can be added together like vectors to create
the electric field of the aggregate.
o Inside conductors (such as metals), the electric field must equal zero. Thus
conductors are equipotentials, and can act as “cages” that block externally applied
electric fields.
o Electric potential, like electric potential energy, is only important in differential or
relative terms. That is, you could add a constant to the total amount of electric
potential possessed by any or all objects in the universe and the subsequent
behavior would be identical to what we see now.
o Electric current is a measure of the net amount of charge which passes by some
location in a certain amount of tim
By the end of this unit, students will be familiar with the following vocabulary words
o charge, positive, neutral, negative, fundamental charge, electric force, inversesquare law, vector field, scalar field, superposition, charge by induction, charge
by conduction, charge by polarization, capacitor, dielectric, permittivity of free
space, dielectric constant, parallel-plate capacitor, Millikan oil drop experiment,
current
By the end of this unit, students will be familiar with the relevant equations from the
course textbook concerning electric forces, fields and potential
Performance tasks
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Students will make conclusions about the triboelectric sequence by measuring the electric
charge transfer between two objects using an electroscope.
Students will conduct experiments that directly demonstrate the ability of a neutrally
charged object to experience an attractive electric force via polarization. Such
experiments might involve small paper fragments, a comb, rabbit fur, water, etc.
Students will charge and discharge a capacitor through an ammeter and light bulb,
demonstrating the relationship between current, charge, and time.