Name: _____________________ Class: Physics 214
SSN/ID:   _____________________ Section & Group: ____________
Lab 9 - Wave Period and Magnetic Fields

Objective
Use a Simple Harmonic Oscillator java applet to ascertain the relationship between the spring constant, the displacement, and the period of the mass on the spring. Also, use a magnetic field java applet to map out the magnetic field associated with a magnet and find out the relationship between density of field lines and field strength.

Equipment
See the Java applets below.

Definition
The idea in the first experiment is to find the period of the wave (i.e., the amount of time for one complete wavelength) and to see how the period depends on the amplitude or displacement and also on the spring constant, which represents the stiffness of the spring (and hence, the strength of the restoring force).

In the second experiment, you just have to trace out the magnetic field of a magnet (or electric charges). Recall that a field lets you visualize the distribution of forces and that each line is a line of force, representing both the direction and strength of the force a particle would feel if it were placed at that location. Remember also that the idea of fields was originally introduced as a mathematical abstract that was useful for calculations but, like many such mathematical ideas introduced only to make calculations easier, soon proved to be a truer representation of Nature (e.g., like Copernicus' heliocentric universe).

For more on Simple Harmonic Motion, please see the following links:

Procedure

Part A: Period of a Simple Harmonic Oscillator

URL: http://www.aug.edu/~chmtmc/ntnujava/springWave/springWave.html

  1. Double-click the time (this is the time shown in the Windows tray on the task bar in the lower-right corner of the screen) to show the clock (for your time measurements). Alternatively, you can use your own clock if you have one.
  2. With the left mouse button, click and drag the blue mass down until the y-coordinate is at the -30 position:
  3. Release the mass and then click the → button
  4. Use the clock to note the time for one period
    • Position the clock so you can see it and the applet
    • Wait for the blue line to cross the horizontal axis; this is your start time (Tstart).
    • Note the time for the blue line to again cross the horizontal axis in the same manner and at the same position; this is your end time (Tend).
    • The period is the difference between the two times (Tend - Tstart).
    • Make sure the period was the time for one complete wavelength (λ):
  5. Repeat steps 2-4 for a y-coordinate position of -50
  6. Now change the k value to 1.0 and set the y-coordinate position to -50
  7. Find the period, T, again

Run y-coordinate k-value Timestart [sec] Timeend [sec] Period [sec]
1.   -30 0.5      
2.   -50 0.5      
3.   -50 1.0      

Part B: Magnetic Field of a Bar Magnet

URL: http://home.a-city.de/walter.fendt/phe/mfbar.htm

  1. Use the compass to map out the field lines by moving it around the bar magnet
  2. Make sure you get at least 20 or so lines (you need enough lines to map the entire magnetic field so pick representative lines)
    • Hint: start off with the compass right above the middle of the magnet and just move it in a little circle around the magnet, stopping every few millimeters.
  3. Once you have the field mapped, draw it out in your notebook (make sure you keep track of which way the arrows should go)
  4. What is the magnetic (or electric) field? Where is the density of the lines greatest (and what does that mean)?
  5. What do the lines represent?

The Java Applet:

The magnetic field of a bar magnet can be investigated with a compass needle. The magnetic poles of both bar magnet and compass needle are symbolized by the following colours:

north pole red
south pole green

If you move the magnetic needle with pressed mouse button, the magnetic field line through the center of the compass needle will be drawn with blue colour. The blue arrows mark the direction of the magnetic field which is defined as the direction indicated by the north pole of the compass needle. If you turn the magnet by using the red button, the direction of the field lines will reverse. The left button makes it possible to clear all field lines.

Questions

  1. In Part A, what do you notice about the period in the three runs? Why do you think it was such?
  2. In Part A, did you find that the period had any dependance on the initial displacement (or amplitude)?
  3. In Part B, the highest density is at the corners... can you tell me why? Think of the definition of density (in this case, number of field lines per unit area) and what the area is.

Notes