Question

These are the questions about "Charge-to-mass ratio of the electron"

1. If you bought an unmarked end of a bar magnet near the beam of electrons, explain how you can tell which magnetic pole it was, based upon the deflection direction of the beam. Draw a diagragm to explain your answer.

2. Find the velocity of the electron under one set of operating conditions you used.

3. What characteristic of the electron beam assures us that the electrons all have the same mass? Note: This characteristic forms the basis for a mass spectrometer.

4. Explain on the basis of their own created magnetic fields why a group of electrons all moving in the same direction stay together.

Answer 1

1. if a beam of electrons is disturbed by a magnetic pole from the right ( as seen from the direction of the screen for the electrons)

if the electrons move up, the pole we used is north pole, and if the electron beam is deflected downwards , the pole we used was south ( from left hand rule)

2. let the magnetic field be B

then for charge q, mass m

qvB = mv^2/r

hence

v = rqB/m

where m is mass of electron, v is its speed, r is radius of revolutioni inside the magnetic field, B is magnetic field strength

3. the electron beam has very little spread around its mean wavelength, meaning all the electrons have same waveelngth, and hence same momentum and energy and hence the same mass

4. a moving electron creates a magnetic filed wich makes the already progressing way of this electron as the preferred direction of motion and hence making electrons stay together