• Investigate the deflection of an electron beam by a magnetic field.
• Investigate the deflection of an electron beam by an electric field.
• Construct a velocity filter using orthogonal electric and magnetic fields.
• Estimate the specific charge of an electron.
Experiment description: ..
In a Thomson tube electrons pass horizontally through a slit behind the anode and impinge upon a fluorescent screen placed at an angle to the electron beam on which they can be observed. Beyond the slot there is a plate capacitor. The electric field between its two plates deflects the electron beam in a vertical direction. In addition Helmholtz coils can be used to rate a magnetic field in a horizontal direction perpendicular to the motion of the electrons that also deflects them in a vertical direction.
If the direction of motion is perpendicular to a uniform magnetic field, electrons are deflected in a circular path with a centripetal force resulting from the Lorentz-force.
The velocity of the electrons depends on the anode voltage UA. This means that measuring the radius of the path allows the specific charge of an electron to be determined as long as the homogenous magnetic field B and the anode voltage UA are both known.
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