Problem 20.14 Part A What is the velocity of a beam of electrons that goes undef
ID: 1441835 • Letter: P
Question
Problem 20.14
Part A
What is the velocity of a beam of electrons that goes undeflected when moving perpendicular to an electric and magnetic fields. E and B are also perpendicular to each other and have magnitudes 3500 V/m and 5.1×103 T , respectively.
Express your answer to two significant figures and include the appropriate units.
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Part B
What is the radius of the electron orbit if the electric field is turned off?
Express your answer to two significant figures and include the appropriate units.
m
m/s
r =m
Explanation / Answer
(a)
Condition for no deflection:
Force due to electric field = Force due to magnetic field
q E = B q v
v = E / B
= 3500 / 5.1x10^-3
= 6.9x10^5 m/s
(b)
Centripetal force = Force due to magnetic field
mv^2 / r = Bqv
mv / r = Bq
r = mv / Bq
= 9.1x10^-31 * 6.86x10^5 / ( 5.1x10^-3 * 1.6x10^-19 )
= 7.7x10^-4 m
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