LAB Xi: ATTRACTIVE AND
REPULSIVE FORCES
Problem: What is
the relationship between the distance between two magnets and
the force between them?
Background and Inquiry: Like charges repel unlike charges attract. You have probably heard that
statement before. The same is true of magnetic fields. Take two
of the same type of magnets
and see if you can find out which poles are similar (both south
or both north). Investigate all four magnets. Move the similar
magnets around seeing if you can locate the forces. See if you
can actually feel the attractive and repulsive forces of the magnets.
Which magnets are more powerful, the round ones or the rectangular
ones. What happens when you put two magnets together? Will you
get a stronger force or weaker force, or will the forces remain
the same? Take two similar magnets and hold the opposite poles
apart. Start from a distance of about 5 inches and slowly bring
the opposite poles together. At what distance do you start to
feel a force of attraction? Bring them slowly closer together
trying to identify how the forces change with distance. The relationship
between force between the two magnets is the same relationship
as for electrostatically charged spheres and also for the force
between two masses (gravitation). This is a very important relationship
that shows up in many laws of physics.
Newtons 2nd Law states
F=ma to find the force we need to multiply by mass x acceleration. In
the cgs (metric s;ystem) the acceleration is 980 cm/ses/sec.
Questions:
1) What are the two poles of a magnet called?
2) What is the shape of a magnet field?
3) Where is the strongest field of a magnet?
4) The relationship between force and distance in magnets is also similar
the relationship found in several other systems. Name two of them.
Materials: 2 clamps with magnets attached, balance
with magnets attached, 4 assorted magnets, ruler, tape.
Procedure.
1) A round magnet and rectangular magnet should already be taped to the
balance. (One on each side) Attached to each test tube clamp you
will find a round magnet..
2) Attach one of the clamps (round magnet first) to the microscope stage
so that it can be moved up and down. Adjust the poles so that
repulsive forces are acting.
3) Before you begin to record your data measure the starting point of the
scale (zero reference or value with no forces acting). Record
this value as your starting point. Record this value in table
I.
4) Move the round magnet so the two poles repel over each other and a distance
of 10 cm. Apart. Use the microscope balance to adjust the height.
Measure the force (mass). Subtract the value from the initial
value.
5) Move the magnets closer. Measuring the force at each of the distances
shown in Table I. You will have to change the position of the
clamp several times.
Repeat steps 2 5 measuring the following:
Table II. Attractive force of round magnets.
Table III. Repulsive force of rectangular magnets (replace the round
magnets with rectangular magnets)
Table IV. Attractive force of rectangular magnets.
Results:
Table I. Repulsive Force for round
magnets
TABLE NEEDS CLARIFICATION
Starting (initial) mass value = _____________
Distance (cm.) Mass
Change
in Mass (due to force, gms.)
Force (dynes)
Mass initial
mass Mass x 980
10 cm.
8 cm.
6 cm.
4 cm.
2 cm.
1 cm.
0.5 cm.
Table II. Attractive force for round magnets
Starting (initial) mass value = _____________
Distance (cm.) Mass
Change
in Mass (due to force, gms.)
Force (dynes)
(Starting
initial)
10 cm.
8 cm.
6 cm.
4 cm.
2 cm.
1 cm.
0.5 cm. This
may not be possible to measure. Try and find a method to collect
this data.
Table III. Repulsive force for rectangular magnets
Starting (initial) mass value = _____________
Distance (cm.) Mass
Change
in Mass (due to force, gms.)
Force (dynes)
(Starting
initial)
10 cm.
8 cm.
6 cm.
4 cm.
2 cm.
1 cm.
0.5 cm.
Table IV. Attractive force for rectangular magnets.
Starting (initial) mass value = _____________
Distance (cm.) Mass
Change
in Mass (due to force, gms.)
Force (dynes)
(Starting
initial)
10 cm.
8 cm.
6 cm.
4 cm.
2 cm.
1 cm.
0.5 cm.
Discussion:
1) What are the independent and dependent variables?
2) How are the variables changing with relationship to each other?
3) What happens to the dependent variable when the independent variable
increases? decreases?
4) How does the relationship shown in this experiment compare with other
relationships you have so far seen?
5) How does the equation for this relationship compare with those of other
equations you have studied?
Applications:
1) Magnet fields are presently being investigated in use of high speed
trains (train that would be able to travel over 300 mph. Explain
how a train would be designed using magnets.
