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.