1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Curve STRAIGHTENING ARGH!

  1. Nov 29, 2004 #1

    Yes, curve straightening....every high school student's favorite thing.... :confused:

    So in my Physics class, our teacher did a demo with the CRT (cathode ray tube) and then he assigned us with the lab exercise. Here are the components of what I'm supposed to include:

    Here's the equation presented when Thomson discovered the charge to mass ratio of an electron:

    v²/Br = q/m ; where: v = velocity (m/s)
    B = magnetic field strength (T)
    q = charge on electron (C)
    m = mass (kg)
    r = radius (m)

    Challenge: Find the equation for q/m in terms of V.

    Okay, this part I got.

    Equations I used:

    qV = 1/2mv² and the other equation used: v²/Br = q/m

    Using the first question I solved for v²:

    qV = 1/2mv² ; v² = 2qV/m

    My next step taken:

    V/Br = q/m ; q²/m² = v²/B²r²

    q²/m² = 2qV/mB²r²

    Here is the equation for q/m in terms of V:

    q/m = 2V/B²r²

    Then we played around with the cathode ray tube, and told us to make a chart. It's for each time he increases the diameter of the circle (beam of electrons), we record the voltage.

    This is another challenge he set for us:

    Write the equation so that q/m is the slope.

    Here's what I did....I'm not sure if I linearized it right (bad grammar....English is not my first language, so I apologize for this).

    So I started with this equation again:

    q/m = 2V/B²r²

    Equation for a straight line is: y = mx + b

    In this case, I don't have a 'b', so it's just y = mx

    My equation (to linearize) I came up with is:

    y = mx

    2V=(q/m)(B²r²) ; where: 2V is the 'y part'
    (q/m) is the 'm part' (slope)
    and (B²r²) is the 'x part'

    After coming up with our equation, he said that make a new table of values. With our new table of values, he said to make a straight-line graph. So my question is (after going through all this mess of typing), is my table going to have a column for 2V and B²r²? As for the graph, is it going to be 2V vs. B²r²? (Where 2V is on the y-axis and B²r² is on the x axis)?
  2. jcsd
  3. Nov 29, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, in a word: yes.

    You've already answered your own question haven't you???

  4. Nov 29, 2004 #3
    I was just wondering if I did everything right....

    That's all....cause it seemed complicated at first....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook