Module: Electromagnetic Interactions
DCI: PS3.C When two objects interacting through a field change
relative position, the energy stored in the field is changed.
PE: HS-PS3-5 Develop and
use a model of two objects interacting through electric or magnetic fields to
illustrate the forces between objects and the changes in energy of the objects
due to the interaction.
No Clarification Statements
So I was not sure what was meant by the standard. I found this, written by a guy who, apparently, knows his stuff.
Forces are the interactions between two or more objects. Electric and magnetic forces can act over a distance (the objects do not have to be in contact) – one model to explain how these objects can interact without being in contact is by using the idea of electric and magnetic fields. Students do not need to be able to do advanced calculations with fields but should be able to use fields to draw or describe the interactions between objects that are not in contact. Even though more than two objects can interact, students will only be assessed on the interactions between two objects at a time – this is a place to differentiate: more advanced students can work on interactions of multiple objects but this is not needed to prepare all students for the assessment.
Fields
can also be used to explain changes in potential energy between two interacting
objects. For example, two objects with
mass are attracted – this can be exampled using a gravitational field
(different from electric and magnetic, but related – used here as an example
not something students need to know for these
performance expectations). In order to
move objects with mass apart, you would have to do work. This work means the system has more energy
after the objects are moved apart.
However, where is that energy?
Often we talk as if it is associated with one of the objects (ex. “If I
hold this tennis ball up high it has
more gravitational potential energy); however this is not really accurate. If the Earth were not part of the tennis ball
system, there would not be an increase in gravitational potential energy. The energy is due to the interaction between
the Earth and the tennis ball thus it is more complete to think of the energy
as being stored in the field between the two objects. Similarly, if you had two charged objects one
way to describe the interactions between those two objects is by using the idea
of electric fields. If the two objects
have opposite charges, they would be attracted and there would be less energy
associated with the field if the objects are close together. Work would be needed to separate the objects,
meaning more energy is “stored” in the field when the charged objects are
farther apart. However, this is all reversed
if the objects have the same charge (objects would repel, so work would be
needed to move the objects together, lower energy would be associated with the
system when the objects are farther apart – this reversal depending on the
relative charge of the two objects is a potential source of assessment
questions to see how well students understand and can apply ideas versus
memorizing arbitrary rules).
The
other form energy can take is kinetic energy – or energy of motion. This motion could be the motion of objects as
they move through the field to a lower energy or more stable relationship. The motion could also be at the
submicroscopic scale as the movement/vibrations of atoms and molecules. This atomic level movement accounts for
thermal energy. The total amount of
energy must always remain constant, but it can be transferred between different
systems or transformed between different forms (ex. potential energy associated
with the field between two charged objects -> movement of those
objects).
Now all of that is a little much, and focused a lot on gravity, as opposed to magnetic and electric fields, but he goes on to say the following.
Students will need to develop
a definition of forces that is based on the interaction of two objects. This interaction between objects may be
observed as a change in motion or distortion of the shape of the object (cause
and effect).
- Electric and magnetic forces
can be attractive or repulsive: attractive if the two objects have opposite
“charges” (electric) or “poles” (magnetic); repulsive if the two objects are
the same.
- Electrostatic forces and
magnetic forces can cause a change in motion even if the two objects are not in
contact with each other (forces that act over a distance).
- Electric and magnetic fields
can explain how objects can interaction without being in contact.
- Electric and magnetic fields surround charged
objects (electric) and magnetic objects, as other charged/magnetic objects move
through the field, changes in motion are observed AND the field is altered.
- Energy can be associated with
motion (kinetic) or stored (potential).
Since two objects that are interacting could cause a change in the
motion of those two objects, there is potential energy stored in the system. One way to think of this is thinking of the
energy as being stored in the field.
- As objects move through the
field the energy will be transformed between kinetic energy and potential
energy stored in the field.
- All the above ideas/models
have to match our observations/data collected.
If data does not match a model, the model should be revised or
replaced. It is okay to start with
multiple possible models/ideas – as more information is gathered, the various
ideas can be evaluated and revised.
- Though the terms used may be
arbitrary at some level (“positive charge”, “north pole”) the ideas should be
consistent with evidence and allow us to explain observations and make
predictions about future tests.
Possible Question Starters:
An electric field will most likely surround an object that has -
correct answer = charge
If two objects are interacting in a magnetic field, changing the relative positions of the objects will result in which of the following?
correct answer = energy stored in the field
