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Week 3: Magnetic Fields and Static
Electricity

Part 1:
Magnetic Fields

Background

A magnet moved into the
space nears a second magnet, experiences a force as it enters the magnetic
fieldof the second magnet. The magnetic field model is a conceptual way of
consideringhow two magnets interact with one another. The magnetic
field model does not consider the force that one magnet exerts on another one
through a distance. Instead, it considers the condition of space around
a magnet. The condition of space around a magnet is considered to be changed by
the presence of the magnet. Since this region of space, or field, is produced
by a magnet, it is called a magnetic field. A magnetic field can be
represented by magnetic field lines. By convention, magnetic field lines
are drawn to indicate how the north pole of a tiny imaginary magnet
would point when in various places in the magnetic field. Arrowheads indicate
the direction that the north pole would point, thus defining the direction of
the magnetic field. The strength of the magnetic field is greater where the
lines are closer together and weaker where they are further apart. Magnetic
field lines emerge from a magnet at the north pole and enter the magnet at the
south pole. Magnetic field lines always form closed loops.

Magnetic field strength is defined in terms of the
magnetic force exerted on a test charge of a particular charge and velocity.
The magnetic field is thus represented by vectors (symbol B) which
define the field strength, also called the magnetic induction. The units are:

Since
a coulomb(s) is an amp, this can be written as

which
is called a tesla (T). The tesla is a measure of the strength of
a magnetic field. Near the surface, the earth’s horizontal magnetic field in
some locations is about 2×
10^-5 tesla. A small bar magnet
produces a magnetic field of about 10^-2
tesla, but large, strong magnets can produce magnetic fields of 2 tesla.
Superconducting magnets have magnetic fields as high as 30 tesla. Another
measure of magnetic field strength is called the gauss (G) (1
tesla = 10⁴ gauss).
Thus, the process of demagnetizing something is sometimes referred to as
“degaussing.”

In this experiment you
will investigate the magnetic field around a permanent magnet.

Procedure

1. Tape
a large sheet of paper on a table, with the long edge parallel to the
north-south magnetic direction as determined by a compass.

2. Center
a bar magnet on the paper with its south pole pointing north. Use a sharp
pencil to outline lightly the bar magnet on the paper. Write N and S on the
paper to record the north-seeking and south-seeking poles of the bar magnet.
Place the bar magnet back on its outline if you moved it to write the N and the
S.

3.
Slide a small magnetic compass
across the paper, stopping close to the north-seeking pole of the bar magnet.
Make two dots on the paper, one on either side of the compass and aligned with
the compass needle. See Figure 3.1.

S

Bar magnet

First
dot

N

Compass

Second dot

Figure 3.1

4. Slide
the compass so the south pole of the needle is now directly over the dot that
was at the north pole of the needle. Make a new dot at the north pole end of
the compass, exactly in front of the needle. See figure 3.2.

5. Continuing
the process of moving the compass so the south pole of the needle is over the
most recently-drawn dot, then making another new dot at the north pole of the
needle. Stop when you reach the bar magnet or the edge of the paper.

6. Draw
a smooth curve through the dots, using several arrowheads to show the direction
of the magnetic flux line.

7. Repeat
procedure steps 3 through 6, by starting with the compass in a new location
somewhere around the bar magnet. Repeat the procedures until enough flux lines
are drawn to make a map of the magnetic field.

S

Bar magnet

First
dot

N

Third dot

Second
dot (exactly under south pole of compass).

Figure 3.2

Results

1. In
terms of a force, or torque on a magnetic compass needle, what do the lines
actually represent? Explain.

2. Do the lines
ever cross each other at any point? Explain.

3. Where do the
lines appear to be concentrated the most? What does this mean?

Part 2: Static
Electricity

Background

Charges of static electricity are produced when
two dissimilar materials are rubbed together. Often the charges are small or
leak away rapidly, especially in humid air, but they can lead to annoying
electrical shocks when the air is dry. The charge is produced because electrons
are moved by friction and this can result in a material acquiring an excess of
electrons and becoming a negatively charged body. The material losing electrons
now has a deficiency of electrons and is a positively charged body. All
electric static charges result from such gains or losses of electrons. Once
charged by friction, objects soon return to the neutral state by the movement
of electrons. This happens more quickly in humid air because water vapor
assists with the movement of electrons from charged objects. In this experiment
you will study the behavior of static electricity, hopefully on a day of low
humidity.

Procedure

Part A: Attraction
and Repulsion

1. Rub
a glass rod briskly for several minutes with a piece of nylon or silk. Suspend
the rod from a thread tied to a wooden meterstick as shown in Figure 3.3. Rub a
second glass rod briskly for several minutes with nylon or silk. Bring it near
the suspended rod and record your observations in Data Table 3.1. (If nothing
is observed to happen, repeat the procedure and rub both rods briskly for twice
the time.)

2. Repeat
the procedure with a hard rubber rod that has been briskly rubbed with wool or
fur. Bring a second hard rubber rod that has also been rubbed with wool or fur
near the suspended rubber rod. Record your observations as in procedure step 1.

1 2 3 4 5 6 7 8 9

Thread

Meter stick

Suspended rod

Second rod

Figure 3.3

3. Again
rub the hard rubber rod briskly with wool or fur and suspend it. This time
briskly rub a glass rod with nylon or silk and bring the glass rod near the
suspended rubber rod. Record your observations.

Part B: Charging by
Induction

1. Inflate
two rubber balloons and tie the ends. Attach threads to each balloon and hang
them next to each other from a support. Rub both balloons with fur or wool and
allow them to hang freely. Record your observations in Data Table 3.2.

2. Bring
a glass rod that has been rubbed with nylon or silk near the rubbed balloons.
Record your observations.

2.
Detach one of the balloons by
breaking or cutting the thread. Rub the balloon with fur or wool for several
minutes. Hold the balloon against a wall and slowly release it. Record your
observations.

Results

1. Describe two
different ways that electrical charge can be produced by friction.

2. Move
a hard rubber rod that has been rubbed with wool or fur near a very thin,
steady stream of water from a faucet. Describe, then explain your observations.

3. Was
the purpose of this lab accomplished? Why or why not? (Your answer to this
question should be reasonable and make sense, showing thoughtful analysis and
careful, thorough thinking.)