Download UNIT IV PHYSICS 212 ELECTROMAGNETISM In these activities we

UNIT IV
PHYSICS 212 ELECTROMAGNETISM
In these activities we explore some of the links between electricity and magnetism. We will
also look at the relevant benchmarks published by the American Association for the
Advancement of Science (AAAS). These benchmarks denote what they think every
American child should understand by the end of a given grade. We begin with a short review
of the basic properties of magnets.
I. Magnets and Magnetic Fields
1. Find one or more magnets. Use a magnet to make something move without touching it.
Describe what you moved.
Key idea (Gr K-2 benchmark): Magnets can be used to make some things move without
being touched (AAAS benchmark 4G/P2)
2. Take a couple moments to find examples of objects that the magnet will attract or repel,
plus examples of objects that the magnet does not affect. In each case, predict whether the
outcome of your test before you do it.
Object
Prediction
Result
All the objects that you found that were affected by the magnet are said to be “magnetic.”
Many people expect that any object made of metal will be magnetic. These people are
incorrect. Each of the magnetic objects that you found contain one particular element that is
magnetic. If one knows something about the structure of materials, then one can predict
whether or not an object will be magnetic. Otherwise, one can use a magnet to test whether a
given object contains that element.
Key idea (Gr 3-5 benchmark): (you fill in the blanks) Without touching them, a magnet
pulls on all things made of ___________ and either pushes or pulls on other
______________. (4GE2)
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Actually, the element mentioned in the benchmark (iron) isn’t the only one magnets attract,
but it is the only one that is important in everyday circumstances. It is safe to say that all the
items you found that were attracted to the magnet contained iron. Steel paperclips, for
example, are an alloy [mixture] of iron, carbon, and small quantities of other materials.
Elements other than iron that are attracted by magnets include cobalt and nickel, but these
other elements are quite rare and the attraction is typically weaker than for iron. Materials
such as aluminum and copper are not noticeably affected by magnets like the ones we work
with, but are slightly affected by very strong magnets. For elementary and middle school
purposes, though, it’s simplest just to keep with the simple statement in the benchmark.
3. Spend a little time with a compass and a bar magnet, reviewing how the magnet deflects
the compass. (Recall from Physics 112 that a compass needle is nothing more than a small
magnet that is free to rotate.) You can draw a little sketch below of the deflection of the
compass at various points near the magnet:
N
S
The magnet is able to exert a force (and torque) on the compass needle without touching it.
Our only other example of “force at a distance” in this course has been gravity. In all other
cases, objects have exerted forces on each other only when in physical contact. You should
recall that in our discussion of gravity, we said that one can think of there being a
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“gravitational field” that surrounds the earth. Any object with mass that is in this
gravitational field will feel a gravitational force. In a similar way, we can think of there
being a magnetic field surrounding the magnet, so that another object with magnetic
properties (such as a compass) that is inside the field will feel a magnetic force. The
direction of the magnetic field at a given location is the direction that a compass needle
points when a small compass is placed at the location.
In Physics 112 you did an activity “Magnetism & Thermal Energy” in which you
experimented with combinations of magnets, and saw how magnets that are aligned with
each other combine to make a stronger magnet, while magnets that are assembled randomly
tend to cancel each other out. Something similar happens within every chunk of iron. One
can think of each iron particle as being a small magnet. These little magnets can flip one
direction or another inside the material. If they line up with each other, the result can be a
macroscopic magnet. If they don’t line up, the result is, well, a chunk of iron.
Let’s explore the phenomenon of “magnetic force at a distance” by doing the activity “Up in
the Air” from the program TOPS.
We find in this activity that placing magnetic materials in a region where there is a magnetic
field can alter the field, and even destroy it. Describe what you think happens at the particle
level when you place a magnetic material between the magnet and the paperclip:
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II. Electric currents as sources of magnetic fields
In this section we establish a link between electricity and magnetism. We begin by recalling
and repeating an activity from Physics 112. Wrap a wire several times around a compass to
make a coil. Orient the compass so that the wires overlay the compass in the same direction
as the compass arrow. Then run a current through the wire and note its effect on the
compass. (Do this for only a short time as the wire may get very hot!)
What happens if you switch the direction of current flow?
Key idea: Electric currents exert forces on magnets. (This is part of a benchmark. The full
benchmark will be given below.)
Since we’ve introduced the idea of magnetic fields, we can rewrite the key idea as:
Electric currents produce magnetic fields.
Since an electric current is nothing more than a collection of moving charges, alternative
statements are “Moving electric charges produce magnetic forces,” which is part of a Grade
9-12 benchmark, or “Moving electric charges produce magnetic fields.” You’ve probably
noted by now that the official benchmarks talk about forces instead of fields.
Here is another Gr9-12 benchmark, which provides an application of our principle: “Electric
currents circulating in the Earth’s core give the Earth an extensive magnetic field, which we
detect from the orientation of our compass needles.” (Science For All Americans p. 56)
Truth is, no one has ever directly observed these electric currents. We infer their existence
from the existence of the earth’s magnetic field. But it makes more sense to imagine massive
electric currents deep within the molten portion of the earth than it does to imagine a huge
buried bar magnet!
As a classroom application of this principle that currents generate magnetic fields, we will
use a current-carrying wire to move an object other than a compass. More specifically, we’ll
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try and move a paperclip. To maximize its movability, deform the paperclip into a basic V
shape, but with rather big “ears,” and suspend the two ears of the V on horizontally oriented
straws (you can tape the straws in place, but make sure the paper clip is able to swing). Try
bringing your current-carrying coil near the paperclip and see if you can attract the paperclip.
Note: The force on the paper clip will be small and might be difficult to detect. To get this
to work, you might want to coil about 100 cm wire around the end of a straw, and then point
the straw at the paperclip.
Question: Why does coiling the wire help?
Another application is the electromagnet. Construct an electromagnet by wrapping a very
long coil of wire (e.g., 100 cm) around the entire length of a large iron nail. The nail should
behave as a magnet when a current is run through the wire. When no current flows, the nail
should return to its non-magnetic state. Does your electromagnet attract the paperclip?
Question: If an electromagnet exerts a force on a paperclip, does the paperclip exert a force
on the electromagnet? (Can you detect such a force?)
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Use a compass to check the direction of the magnetic field around your electromagnet. Is
your electromagnet similar to a bar magnet?
Question: Explain, at the particle level, how an electromagnetic works.
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Let’s think about what we’ve accomplished in terms of energy transformations: By making a
paperclip move, we have converted electrical energy into mechanical energy. This is an
energy transformation with numerous applications, and is the same transformation as occurs
in an electric motor. If time permits at the end of the lab period, you can make a simple
motor by following the instructions in TOPS “Hat-pins compass” and “pin motors” (You can
skip steps 6 and 7 of the first activity and steps 2 through 4 of the second).
III. The Effect of Magnetic Fields on Currents
Construct the apparatus shown in the TOPS activities “On-Off Motor.” Don’t worry if it
doesn’t work yet!
The most important thing to note in this activity is that the coil between the two magnets
experiences a force (or torque) when a current flows through the coil. In other words,
magnets exert forces on electric currents. The direction of the force depends on the direction
of the magnetic field and the direction of current flow—and is perpendicular to both. In this
example we have a coil of wire, so different parts of it experience forces in different
directions, and the coil tends to twist into a particular position when the current is flowing.
For a motor, we want the coil to spin, and not just realign. To do this, we need to be able to
switch the directions of the forces, or at least turn the force off briefly so the coil can spin
past the “criticial point” where it tends to get stuck. That is what the thread is for—to turn
off the current at the right point in the rotation. You should adjust your setup until you
succeed in making your motor spin. You must show your prof your spinning motor!
We’ve demonstrated the existence of the force on a current-carrying wire. More generally,
magnetic fields exerts forces on moving charges. Your professor might have a demo for
this. Otherwise, you can just imagine a standard TV set. TV sets work by sending a beam of
electrons toward a screen. The beam is deflected and aimed by using magnetic fields, so that
it sweeps rapidly across the screen. The screen emits light when the electrons hit. You can
read more about this in most textbooks.
Key ideas: Grade 6-8 Benchmark—“Electric currents and magnets can exert a _________
on each other.” (4G/M3).
Let’s recap some of what we know about how electric currents and magnetic fields are
related.
1. electric currents (and moving charges in general) produce magnetic fields
2. magnetic fields exert forces on electric currents (and on moving charges in
general.)
Electric generators
Electric motors allow us to convert electrical energy to mechanical energy. Now we want to
go the other direction, and convert mechanical energy to electrical energy—i.e., to start with
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mechanical motion and end with an electric current. As for the case in which we start with
electric current, we somehow will need to use magnets or magnetic fields.
One way to achieve our goal is to take a wire, or a coil of wire, and physically move it
through a magnetic field. Note that we now have charges (that are inside the wire) moving
through a magnetic field. The result (see #2 above) is a force on the charges in the wire. The
direction of this force is perpedicular to the direction of motion of the wire, and can be along
the wire. This force can start a current flowing! Most electric generators work on this simple
principle of rotating a coil of wire between two magnets.
We could return to our electric motor and run it backwards. If we replace the battery with a
very sensitive current meter, then twirling the coil should cause a small current to flow
through the circuit. Unfortunately the current generated with such household materials is
very small. However, with the right equipment it is not hard to convert mechanical motion to
electrical current or electrical energy. Powerplants do it all the time to produce the
electricity. The lab also has one or more demonstration devices that you can examine.
Question: What is the source of energy for the electric current?
We’ve probably accomplished as much as we need to for elementary or middle school, but
some profs don’t know when to quit and there is one more wrinkle “we” want to explore.
First, a brief aside regarding electric fields.
Gravitational fields surround objects like the earth, and objects with mass that are in
gravitational fields experience gravitational forces.
Magnetic fields surround magnets, and magnetic materials that are in magnetic fields
experience magnetic forces.
Electric fields surround electric charges, and other charges that are in electric fields
experience electric forces.
We have seen that moving charges produce magnetic fields.
The next step is:
Moving magnets produce electric fields.
One can push this just a little deeper, and ask how it is that a point in space can “know” that a
neaby magnet is moving. Magnets have magnetic fields surrounding them, and when a
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magnet moves, the magnetic field surrounding the magnet changes. What happens at a point
away from the magnet is that the changing magnetic field produces an electric field.
In other words:
Changing magnetic fields produce electric fields.
We can demonstrate this principle simply by moving a magnet toward a coil of wire, and
noting that a current is induced in the coil.
Relevant benchmarks: “Moving electric charges produce ___________________ forces
and moving magnets produce ___________________ forces.” (Grades 9-12, 4G/H5)
and
OPTIONAL: Build your own generator. (See the prof if you want some ideas.)
One final wrinkle: If changing magnetic fields produce electric fields, one might wonder if
changing electric fields produce magnetic fields. The answer is an emphatic yes.
Now it’s fun to think about whether one could establish a “feedback loop” in which a
changing electric field produces a changing magnetic field, that produces a changing electric
field, that produces a changing magnetic field, etc. The answer is yes. This feedback loop
can lead to having changing electric and magnetic fields propagate through space as
“electromagnetic radiation.” Examples of electromagnetic radiation include radio waves,
microwaves, infrared radiation, visible light, ultraviolet light, and x-rays. Which of these is
present depends on how rapidly the electric and magnetic fields are changing, but they all
have the same speed through empty space.
Let’s recap the sources of electric and magnetic fields:
Electric fields are produced by electric charges and by changing magnetic fields.
Magnetic fields are produced by magnets, by moving charges, and by changing electric
fields.
And one final benchmark:
“The interplay of electric and magnetic forces is the basis for electric motors, generators,
and many other technologies, including the production of electromagnetic waves.” (Grades
9-12, 4G/H5).