The top plate is negative and the bottom plate is positive. We can adjust the el

Question

The top plate is negative and the bottom
plate is positive. We can adjust the electric field. The electrons enter the
apparatus moving perpendicular to the electric field and with a speed v.
An unknown magnetic field B is also being applied to the system. We are
trying to find out which direction the magnetic field is acting in, and how strong
it is. Symbols X and . denote vector cross and dot products respectively.
                                    - - - - - - - - - - - - - - - -

                                                +++++++++++++++++

23: Which of the following expressions gives the magnetic force FB on a
general particle of charge q and velocity v moving in a magnetic field B?
a) q (v X B)
b) q/(vB)
c) q (v . B)
d) v X B.v
e) B/v
We adjust the electric field so that the electron travels through the apparatus
UNDEFLECTED. The electric field is now Eu.

24: By considering the electric force on the electron, and the magnetic force
on the electron (from question above), which magnetic field direction is
consistent with the electron traveling undeflected?
a) out of the page
b) into the page
c) from bottom to top of page
d) from top to bottom of page
e) from right to left of page
25: What is the strength of the magnetic field, in terms of the electron charge,
e, electron speed, v, electron mass me and electric field strength Eu.
a) (Eu)/v
b) v/Eu
c) Euv
d) (e Eu)/mev
e) (e)(me)(Eu)/v
26: The electrons are replaced by positrons, which we will assume are

identical except with positive charge. What do we do to the electric field to
ensure that the positrons travel through the apparatus undeflected?
a) turn off
b) reverse direction
c) point out of the page
d) point in to the page
e) don

Explanation / Answer

23. a

24. c

25. a

26. b

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