Skip to main content
\(\require{cancel}\)
Physics LibreTexts

6.2: Activities

  • Page ID
    28190
  • Things You Will Need

    Nothing! You will be extracting data from the photos given below.

    Force Between Two Bar Magnets

    Before we look at the data, a look at the apparatus used is helpful:

    apparatus.png

    Two magnets are placed in separate aluminum boxes, and are held apart with opposite poles facing each other. One is fixed to an adjustable platform, which allows for the gradual change in separation of the magnets, while the other is attached to a force sensor that measures the pull of the other magnet and sends that data to the computer.  The separation data is also sent to the computer from a pulley sensor. A string attached to the platform is wrapped around the pulley and attached to a small weight on the other end.  As the platform rises, the weight drops, and the string rotates the pulley, recording the distance displaced by the platform. The force (in newtons) and the separation of the magnets are then recorded on a graph, with samples taken by the computer at short regular time intervals. The combination of points thus recorded forms a graph of force vs. separation. You can see the data-recording process below:

    magnetic_force.gif

    Here is the a blow-up of the data we have acquired from this run:

    F_vs_r.jpg

    Our goal is to confirm the power law we derived using the method described in the Background Material.  

    1. Start by constructing a table of data, using values of \(F\) and \(r\) taken from the graph above.  You will need at least 8 points for this table. Include in the table columns for the computed numbers you will need for your graph.
    2. Make the plot of the data and determine a best fit line.
    3. Determine the power law the force-vs-separation data exhibits from your best fit line, and compare it with your theoretical prediction.

    Force on a Moving Charge

    Next we will check to see if our calculation of the deflection of the electron beam of a CRT is confirmed by the experimental evidence.  As we saw in the Background Material, a great many constants are needed for this computation. It just so happens that they are recorded below:

    • charge of electron: \(e=1.6\times 10^{-19}C\)
    • mass of electron: \(m_e=9.1\times 10^{-31}kg\)
    • radius of the wire coils: \(r=16cm\)
    • separation of magnet from coils, along the axis: \(z=4.0cm\)
    • number of turns in each coil: \(N=100\)
    • voltage drop of CRT accelerating electron beam: \(V=700V\)
    • length of CRT (from electron ejection to screen): \(L=20cm\)

    The electron beam strikes the bottom grid line on the CRT screen when there is no current in the coils. As the current is dialed-up, the dot on the CRT screen moves up (see the pictures below).  The multimeter in the foreground measures the current through the coils in amps, and the grid lines on the CRT screen are separated by \(1.0cm\).

    deflection_1.jpg  deflection_2.jpg

    deflection_3.jpg  deflection_4.jpg

    deflection_5.jpg  deflection_6.jpg

    1. In which direction is the magnetic field that deflects this electron beam (which passes through the centers of the Helmholtz coils, parallel to their common axis)?  Explain.
    2. Create a data table, and use it to plot the data points with the adjustable current as the independent variable and the resulting beam deflection as the dependent variable.
    3. Use your graph to confirm the formula derived in the Background Material.

    Lab Report

    Download, print, and complete this document, then upload your lab report to Canvas. [If you don't have a printer, then two other options are to edit the pdf directly on a computer, or create a facsimile of the lab report format by hand.]

    • Was this article helpful?