Thomson's Experiment
Thomson's Experiment
- One of the Scientists that worked with Cathode Ray Tubes was an experimentalist named J. J. Thomson.
- He was the one who discovered that the Cathode Rays were deflected by electric fields.
- After that experiment, he worked on another experiment with Cathode Ray Tubes.
- The aim of the experiment was to measure the Charge to Mass ratio of the Cathode Rays.
- It was set up as follows:
- A CRT was built with two parallel electric plates in one axis
- A magnetic field was produced by current carrying coils in the other axis (perpendicular to the electric plates).
- The Cathode Rays were passed through two thin collimator slits to produce a thin beam rather than a Ray.
- This was aimed at a fluorescent screen to produce a dot.
- The experiment was as follows:
- Stage 1
- The electron gun was initially fired without any electrical input (no E or B).
- This position of the dot was noted on the screen.
- Then both fields were turned on.
- The strengths of the fields were then altered so that it passed through undeflected.
- This means that the dot was at its initial position without the fields on.
- This was to equate the Magnetic force and the Electrical force.
\begin{equation} F_{magnetic} = F_{electric} \end{equation}
(2)
\begin{equation} qvB = qE \end{equation}
(3)
\begin{align} v = \frac{E}{B} \end{align}
- Stage 2
- The Electric field was then turned off.
- This meant that there was now a deflection, but it was only due to the magnetic field.
- Thompson then measured the radius of this deflection.
- The Electric field was then turned off.
\begin{equation} F_{magnetic} = F_{centripetal} \end{equation}
(5)
\begin{align} qvB = \frac{mv^2}{r} \end{align}
(6)
\begin{align} \frac{q}{m} = \frac{v}{Br} \end{align}
Substitute (3) in
(7)\begin{align} \frac{q}{m} = \frac{E}{B^2r} \end{align}
And so the charge to mass ratio could be found.
Implications
- Thomson found that the ratio of the Cathode Ray particles was 1800 times greater than that of a Hydrogen ion
- He then decided that its mass was 1800 times smaller rather than having a charge 1800 times greater through a series of other experiments.
- Which meant that it was a sub-atomic particle
- Since the ray was the same regardless of the materials used for the Cathode, he proved that these Cathode Ray particles were a constituent of every atom.
- Later, these would be known as electrons.
page revision: 4, last edited: 28 Oct 2011 12:07