Use the following values for mass and charge: an electron has mass me = 9.11×10-

Question

Use the following values for mass and charge: an electron has mass me = 9.11×10-31 kg and charge ­e, a proton has mass mp = 1.67×10-27 kg and charge +e, an alpha particle has mass malpha = 6.65×10-27 kg and charge +2e, where e = 1.60×10-19 C. An electron is released from rest in a vacuum between two flat, parallel metal plates that are 7.0 cm apart and are maintained at a constant electric potential difference of 610 Volts.
A) If the electron is released at the negative plate, what is its speed just before it strikes the positive plate?
B) What is the kinetic energy of the electron in units of eV?

Explanation / Answer

part (b) is easiest, do it first:

since the potential between the plates is 610 V, the kinetic energy of the electron is 610 eV.

part (a): (610 eV) * (1.602 * 10-19 J/eV) = 9.77 * 10-17 J is the electron's KE in joules.

This is equal to (1/2)mv2    ;solving for v, v = (2K/m)1/2

v = [(2 * 9.77 * 10-17 J)/(9.11 * 10-31 kg)]1/2 = 1.46 * 107 m/s

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