Download Unit 07 Magnetic Fields

Magnetic Fields; Action at a Distance
Physics 100L Exercise 7
Action at a Distance
Magnetic forces, like electrical and gravitational forces, can occur between objects separated in
space with no apparent intervening medium. This so-called action-at-a-distance problem
puzzled physicists for a long time. After all, how does one object know that the other is there?
One of the theories that caught on for handling action at a distance was that of fields.
Fields
The way physicists described action-at-a-distance phenomena was by
imagining that objects created a field about them. For example,
massive objects such as the earth created a gravitational field, and
charged bodies produced an electric field. These fields would then
produce forces on other objects as they traveled by. For electric
fields, this process can be summarized in the following diagrams:
Electric charge
creates field
How does this magnet
attract other objects?
Field acts on
another charge
The important thing to understand is that fields, when they were first invented, was merely a
mathematical tool for understanding how objects interacted with each other. Later, physicists
were to discover that fields really do exist, and that they can carry momentum and energy just as
particles can.
Magnetic Fields
Scientists have known about the effects of magnetism for a long time. When fields were
discovered, the concepts were applied to magnetism in a similar manner to that of electricity,
except for one major difference: there are no such things as magnetic monopoles. Magnetic
“charge,” like electric charge, comes in two possible flavors – instead of positive and negative
we call them “north” and “south.” However, unlike electric charges, magnetic charges never
appear by themselves – they only appear together. If you take a bar magnet and break it in half,
you don’t end up with one north pole and one south pole; rather, you end up with two smaller
magnets! Today, there is some debate over the existence of magnetic monopoles, but none have
yet been observed.
Permanent Magnets
In today’s experiment, we will be examining two ways in which magnetic fields can be created.
This will lead us to the connection between electricity and magnetism that was discovered by
Oersted. The first method for generating magnetic fields is the one most people are familiar with
– permanent magnets. Certain substances, such as iron, can be “magnetized,” that is, their atoms
can be arranged in such a manner as to create a permanent magnetic field. Two examples of
such magnets are the bar magnet and the horseshoe magnet. We will be using a compass to
“map” the fields created by these two types of magnets. This is made possible by the fact that
compasses always align themselves along a magnetic field – since the Earth creates a magnetic
field whose poles are in the north and south, a compass can be used to tell direction on the
surface of the planet. We will use the compasses to tell the direction of the magnetic fields
created by our permanent magnets. Before you make your maps, determine which end of your
compass is north by holding it away from any magnets. The end of the compass arrow that is
pointing to the geographic north (either the silver end or the blue end) is pointing in the direction
of the magnetic field. Using this knowledge you can now determine the direction of the fields of
the permanent magnets. Follow the instructions on your worksheet to map the magnetic fields.
Magnetic field lines always go from a north pole to a south pole. Is this the case in your
drawings?
Electric Current and Magnetic Fields
The other way to create magnetic fields is to use electric current. This was an important
discovery in physics, as it was the first link between the similar phenomena of electricity and
magnetism. Later, physicists were to find that electricity and magnetism were actually two
different manifestations of the same phenomenon – the electromagnetic field. The original
discovery made by Oersted is that an electric current produces a magnetic field in the area
around it.
The magnetic field produced by an electric current is slightly
different from that of a permanent magnet. Instead of starting at a
north pole and ending at a south pole, the field lines simply curl
around the wire in a circle. The direction of the field can be found
from the following right hand rule: take your right hand and point
your thumb in the direction of the current. Your other four fingers
now curl in the same direction as the magnetic field.
The magnetic field of an
electric current
Instead of mapping the field lines, in this part of the experiment we are going to use several
compasses to determine what the field looks like and how far from the current it extends. Follow
the instructions on your worksheet and fill in the drawings on the right side of the sheet. Does
the field obey the right-hand-rule? From your drawings, can you determine how far the field
extends from the wire? How do you know this?