Download Magnetism Lesson 2

Magnetism Lesson 2
Magnetic Fields and the Plotting Compass/
Theory of Magnetism
Overview
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What is a magnetic field?
Activity: Plotting the field lines of a magnetic field
The Earth’s Magnetic Field
Theory of Magnetism
What is a magnetic field?
Have you ever dropped a box of pins onto the floor? You will find that using a
magnet to pick the pins up is far more effective than picking them up by hand one
by one. You would have noticed that the pins are all attracted to the magnet.
This effect can be explained by the concept of a magnetic field around the
magnet.
A magnetic field is the region where a magnetic force is exerted on any
magnetic object placed within the influence of the field.
Note: Like gravitational force, magnetic force is a non-contact force. Thus, it
can be exerted from a distance. In a sense, the effect of the magnetic field of a
magnet can be likened to the effect of the earth’s gravitational field.
To show the patterns of a magnetic field around a bar magnet, you can sprinkle
iron filings lightly over the paper and tap the paper gently with a bar magnet
directly below the paper. The iron filings will far into a certain pattern which is a
magnetic field pattern. This pattern can be seen in Figure 21.22 of your
textbook.
BUT the above activity can only show you the pattern of the magnetic field. To
also see the direction of the magnetic field, you need to conduct an experiment
involving a small plotting compass, which is what we will do now.
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005
Activity: Plotting the field lines of a magnetic field
You can refer to page 339 of your text book for instructions on this experiment or
the screen.
Aim:
To plot the field lines of a magnetic field.
Apparatus:
Bar magnet, plotting compass, plain paper.
Procedure:
1. Place the bar magnet at the centre of the piece of paper so that
its North pole faces North and its South Pole faces South.
2. Starting near one pole of the magnet, mark the positions of the
ends, North and South, of the compass needle X and Y using a
pencil.
3. Move the compass until one end is exactly over Y and mark the
new position of the other end with another dot.
4. Repeat the process of marking the dots until you reach the other
pole of the magnet.
5. Connect the series of dots and this will give you the plot of the
field lines of the magnetic field.
Before you start!Precautions:
1. Check that the plotting compass is in good working order.
2. Ensure that there is no strong magnetic field (other than the
Earth’s magnetic field) around the plotting compass as this will
interfere with your results.
The typical field pattern for the case of a bar magnet can be seen in Figure 21.25
of your textbook. This pattern is neglecting the effect of the Earth’s magnetic
field, which will also be explained later.
See applet: Magnetic fields of a bar magnet
The lines you will obtain “coming out of the magnet” represent the direction of the
magnetic field and are thus called magnetic field lines. Your magnetic field
lines should be “coming out” of the North Pole of your magnet and “going into”
the South Pole of your magnet.
These lines are also known as lines of force because if we place any magnetic
materials in the region of these field lines, the materials will experience a
magnetic force directed along these lines.
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005
The line of force is hence defined as the path along which an imaginary “free”
North Pole will move if it is free to do so.
The plotting compass can also be used to plot the combined magnetic field due
to neighbouring magnets. Figures 21.26 to 21.28 of your textbook will show you
the pattern of these combined magnetic fields. When the field lines are close
together, the field is stronger. As you can see, the field lines near the poles of
the magnets are closer together, hence the magnetic field strength at the
poles of the magnet are the strongest.
If you place the like poles of two magnets together, what you will obtain is a
neutral point because the field due to one magnet cancels out the other. Hence,
there are no magnetic field lines at the neutral point. A plotting compass placed
at the neutral point will naturally be able to point in any direction due to the
absence of magnetic field lines. Such a scenario is illustrated in Figure 21.26 of
your textbook.
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005
The Earth’s Magnetic Field
The Earth behaves as if it has a magnetic field with both magnetic North and
South Poles. Which mean to say, we can think of the Earth as having an
imaginary bar magnet inside it as shown in Figure 21.29 of your textbook.
As mentioned in the previous lesson, if you let a magnet hang freely, one end of
the magnet will point towards the Earth’s magnetic North. This end of the
magnet is the North-seeking Pole of the magnet which is in fact the South Pole of
the magnet. (Remember? Unlike poles attract.)
Fun facts!
The earth’s magnetic North lies somewhere in the sea north of Canada
but is shifting slowly over the years. The current theory no is that the
Earth’s magnetic field is probably caused by electric currents circulating
within the core of the Earth. Such currents are thought to be
generated by the convection in the Earth’s liquid core. The energy for
convection is thought to be due to the conversion of nuclear energy
brought about by radioactive processes in the core.
In the simplest terms, Earth can be thought of as a dipole (2-pole) magnet.
Magnetic field lines radiate between Earth's north and south magnetic
poles just as they do between the poles of a bar magnet. Charged
particles become trapped on these field lines (just as the iron filings are
trapped), forming the magnetosphere.
Earth's magnetic field lines are not as symmetrical as those of the bar
magnet. The impact of the solar wind causes the lines facing sunward to
compress, while the field lines facing away from the Sun stream back to
form Earth's magnetotail. The magnetosphere extends into the vacuum
of space from approximately 80 to 60,000 kilometers (50 to 37,280 miles)
on the side toward the Sun, and trails out more than 300,000 kilometers
(186,500 miles) away from the Sun.
See picture: Earth’s magnetic field
As you have seen from the picture, the Earth’s magnetic North is different from
its geographical North. Your magnet or compass is in fact attracted to the
magnetic North of the Earth and not the geographical North.
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005
When doing experiments involving magnets, like in the one you did in the activity
earlier, it would be best if you aligned the magnet to the Earth’s magnetic North.
This is to prevent the Earth’s magnetic North from interfering with the experiment
and altering the pattern of the magnetic field lines.
The inference of the Earth’s magnetic North in the experiment can be likened to if
a strong magnetic field were to be placed near the magnet during the course of
the experiment. The plotting compass would be affected by the other magnetic
field and the results of the experiment will be distorted.
Theory of Magnetism
If you took a piece of magnetized bar and cut it into three smaller pieces, each
piece will become a little magnet with a North and South pole.
Figure 2.1 A magnetised bar
Figure 2.2 An unmagnetised bar
Therefore a magnet can be said to be made of lots of "tiny" magnets all lined up
with their North poles pointing in the same direction. At the ends, the "free" poles
of the "tiny" magnets repel each other and fan out. If you joined the arrows up,
you will get the magnetic field of the magnet.
This explains why the magnet is strongest at its poles, where the magnetic
field lines are located very close together.
See applet: Magnetic fields of a bar magnet
Within the magnet itself, however, the magnet is strongest in the centre, were the
field lines are not splaying out and hence close together.
In Figure 2.2, the “tiny” magnets are all pointing in random directions. The
resulting magnetic effect of all the “tiny” magnets are then cancelled out and the
bar is said to be unmagnetised.
Based on this theory, we can account for the following:
 Storage of magnets using keepers
If we store magnets placing them side by side, the magnets may become
wekaer after some time. This is due to the “free” poles near the ends of the
magnets repelling each other. This will upside the alignment of the “tiny”
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005
magnets. To prevent the weakening of the magnets, we can store bag
magnets in pairs by using two pieces of soft-iron, which are called keepers.
These keepers are placed across the ends of the bar magnets. You can refer
to Figure 21.14 in your textbook for a picture.
 Magnetic saturation
Every magnet has a maximum possible strength. This happens when all the
tiny magnets are aligned in the same direction.
 Demagnetisation of magnets
Demagnetisation is the process of removing magnetism from a magnet.
Some methods of demagnetisaion include heating and hammering. They
cause the atoms of the magnet to vibrate vigorously and hence cause the tiny
magnets to become “disorientated” as in Figure 2.1 and thus demagnetising
the entire magnet. More on the methods of demagnetisation will be explained
in the next lesson.
Bibliography
http://www.windows.ucar.edu/
http://www.physics.brocku.ca/courses/1p23/images/f21005.jpg
http:// www.scifun.ed.ac.uk
Chua Si Hui and Fan Ruimin Janice
Sec. 4L
Dunman High School
2005