Download Magnetic Fields

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Force Fields
When in a deli shop, one is always interested in the price per pound of
the items so that one can determine the price for a given amount of an
item. In the same manner, if you knew the electric force per unit
charge at points in space due to a charge configuration or the
magnetic force per unit pole or moving charge, then one can easily
calculate the electric force or magnetic force an object would
experience at different locations.
The electric force per unit charge is a vector quantity known as the
electric field (E). By determining the electric force on a test charge at
different points around a charge configuration, the electric field can be
“mapped,” or graphically represented by lines of force. The lines of
force are used to visualize the magnitude and direction of an electric
field. If a positive test charge is released in the vicinity of a stationary
positive charge, it will move along the line of force in a direction away
from the source charge.
Since a free charge moves in an electric field by the action of the
electric force, then work (Work = F d) is done by the field in moving
the charge from one point to another. The work per unit charge (W/q o)
in moving the charge between two points in an electric field is called
the potential difference V between the two points. If a charge is
moved from A to B along a path at right angles of perpendicular to the
line of force, no work is done since there is no force component along
the path. Thus the potential difference is zero, and the potential at A =
potential at B. The potential is then constant along paths perpendicular
to the field lines. These paths are equipotentials (in three dimensions,
it is an equipotential surface).
The electric field will be mapped experimentally by determining the
equipotential lines using the galvanometer as the detector. When no
current flows between two probe points (zero deflection on the
galvanometer) there is no potential difference between the points (V
= 0), and the points are on an equipotential.
Similarly, the magnetic force per unit pole is a vector quantity known
as the magnetic field (B). The direction of the force at a particular
location is that of the force experienced by a north magnetic pole.
The magnetic field is mapped using the north pole of the magnetic
needle of a compass. The torque on the compass needle causes the
needle to line up with the field and the north pole of the compass
points in the direction of the field. If the compass is moves in the
direction indicated by the north pole, the path of the compass traces
out the field line.
In this experiment, you will be investigating the concept of the fields.
Electric and magnetic fields will be mapped around different
configurations.
STUDENT OUTCOMES
Through this experiment, students will be able to:
- Describe the concept of a force field
- Explain lines of force and its physical interpretations
- Distinguish between lines of force and equipotentials and their
relationships with work
MATERIALS
Magnetic Field:
Two bar magnets
One horse shoe
compass
paper
Electric Field
Mapping Board and probe
Connecting wires
Power supply
Conducting plates
galvanometer
paper
PRELIMINARY QUESTIONS
1. What is an electric field and what does it tell you?.
2. What are “ lines of forces”?
3. What are equipotentials and how are they experimentally
determined? What is their relationship to the electric field lines?
4. What is a magnetic field, how is it defined, and what does it tell
you?
PROCEDURE
Part I: Magnetic Field
Place the magnets for each arrangement (below) on a piece of paper.
Draw an outline of each arrangement on the paper and label the poles.
Using the compass, trace out on the paper the magnetic field lines as
smooth curves. The compass needle will align with the magnetic field.
So to trace out the field lines, first place the compass near the N pole
of the magnet, then put a dot on the paper on both poles of the
compass needle. (The front end will be the N pole of the compass
needle and the back end will be the S end.) Then move the compass,
so that the S pole of the compass needle will coincide with N dot
drawn earlier. Place another dot on the new N end. Continue moving
the compass until the line ends on the S pole of the magnets on the
paper. (See diagram below.) Draw enough field lines so that the
pattern can be clearly seen.
Draw the directions of the magnetic field lines (N to S).