A mass spectrometer was used in the discovery of the electron. In the velocity selector, the electric and magnetic fields are set to only allow electrons with a specific velocity to exit the fields. The electrons then enter an area with only a magnetic field, where the electron beam is deflected in a circular shape with a radius of 8.0 mm. In the velocity selector, E = 400.0 V/m and B = 4.7 x 10-4 T. The same value of B exists in the area where the electron beam is deflected.

a) What is the speed of the electrons as they exit the velocity selector?
b) What is the value of e/m of the electron?
c) What is the accelerating voltage in the tube?
d) How does the electron radius change if the accelerating voltage is doubled?

Explanation:

Radius of dee, r = 8 mm = 0.008 m

Electric field, e = 400 V/m

Magnetic field, B = 4.7 x 10^-4 T

mass of electron, m = 9.1 x 10^-31 kg

charge of electron, q = 1.6 x 10^-19 C

(a) Let v is the speed of electrons.

v = 661098.9 = 661099 m/s

(b)

e / m = 1.76 x 10^14 C / kg

(c) Let K be the kinetic energy

K = 0.5 x mv²

K = 0.5 x 9.1 x 10^-31 x 661099 x 661099

K = 1.99 x 10^-19 J

K = 1.24 eV

So, the potential difference is

V = 1.24 V

(d) if the acceleration voltage is doubled

V = 2 x 1.24 = 2.48 V

So, Kinetic energy

K = 2.48 eV

K = 2.48 x 1.6 x 10^-19 = 3.968 x 10^-19 J

Let v is the speed

K = 0.5 x mv²

3.968 x 10^-19 = 0.5 x 9.1 x 10^-31 x v²

v = 933856.5 m/s

Let the new radius is r.

r = 0.0113 m = 1.13 cm

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