28.0 - Introduction
Humankind has long been familiar with magnets, objects possessing “north” and
“south” poles that can attract or repel certain other objects. The ancient Greeks
understood their properties, and the word “magnet” itself likely originates from the
Greek region of Magnesia, where naturally occurring magnets are found. Early
navigators learned to steer their ships with the aid of magnetic devices that were the
forerunners of today’s compasses. The natural world also takes advantage of
magnetism. A notable example of this is Aquaspirillum magnetotacticum, a
bacterium that synthesizes tiny magnets that help it determine which way to move.
We are a little worried about these creatures: Since the Earth’s magnetic field
changes its orientation every several hundred thousand years or so, the
microorganisms may find themselves unintentionally heading away from dinner one
day.
Much more complex creatures, namely scientists, employ magnets as well. In the
16 th and 17th centuries, they learned to create their own magnets and used them to
study the Earth’s magnetic field. In the 19th century, scientists began the crucial
work of piecing together a more complete picture of the relationship between
magnetic fields and electric currents.
Despite centuries of practical use, magnets and their fields still pose mysteries. For
instance, scientists cannot definitively establish the cause of the Earth’s magnetic
field, and they are only able to speculate about why its direction periodically
changes. In addition, physicists continue their quest for the magnetic monopole: a
magnet with just one pole. As you proceed in your studies in this chapter, remember
that you are in the good company of other fine minds who have found the workings
of magnets to be an area of continuing fascination.
You can begin your exploration of magnetic fields by launching the simulations to
the right that show the effect of a magnetic field on the motion of a charged particle.
These two simulations are the same except for the initial viewing angle, the angle at
which you view what is occurring in the simulation. In both, you control the initial
velocity of a positively charged particle that moves in a magnetic field, represented
by magnetic field lines. In the illustration for Interactive 1, you see that the magnetic
field points straight down the screen and the initial velocity vector points to the right,
perpendicular to the magnetic field. As the particle moves you will see, represented
as a purple vector, the force exerted on it by the magnetic field.
Clicking on Interactive 2 launches the same simulation but with the viewing angle rotated 90°. Here, the magnetic field points directly toward
you, and you are seeing the heads of the field lines. With either simulation, you can change the viewing angle by using the slider provided, and
see either of these points of view. In the simulations you will also see a magnetic field meter that is there principally to help you understand the
changing perspective as you manipulate the viewing angle slider.
Launch the upper simulation and conduct some experiments. Does the moving particle travel along a straight line or a curve? To answer this
question, you will need to change the viewing angle, in the process seeing why a three-dimensional view of a charged particle moving through
a magnetic field is so useful.
As you study the path, answer two more questions. First, is the particle’s speed changing? Second, is it accelerating? For the second question,
recall that acceleration measures the change in velocity, which is a vector.
You can also consider the relationship of the directions of the various vectors you see. What is the relationship between the force and velocity
vectors? Are they parallel or perpendicular? What is the angle between the force vector and the magnetic field? Again, the viewing angle slider
proves a useful tool. You can only change the direction of the initial velocity when the viewing angle is set to the far right so that the magnetic
field is pointing straight down the screen; this makes it easy to see the angle between the velocity vector and the magnetic field vector.
As another experiment, set the velocity to zero. Does the magnetic field exert a force on the particle? If the magnetic field exerts a force, the
particle will accelerate. How does this compare to what would occur if the stationary charged particle were in an electric field? (Note: We ignore
other forces, notably gravity and air resistance, in these simulations.)
We have asked a lot of questions above. Answer as many as you can now, and prepare to explore magnetism in depth in this chapter.