Page 439 - Physics Coursebook 2015 (A level)

BOX 27.2: The deflection tube
cathode
–+
+V
electron
beam
B into paper in this region
Vca
anode
Figure 27.9 The path of an electron beam in a deflection tube.
A deflection tube (Figure 27.9) is designed to show a beam of electrons passing through a combination of electric and magnetic fields. By adjusting the strengths of the electric and magnetic fields, you can balance the two forces on the electrons, and the beam will remain horizontal. The magnetic field is provided by two coils, called Helmholtz coils (Figure 27.10), which give a very uniform field in the space between them.
Figure 27.10 A pair of Helmholtz coils is used to give a uniform magnetic field.
where E is the electric field strength between the parallel plates with a p.d of V. The speed v of the electrons is simply related to the strengths of the two fields. That is:
0V
The electric field strength is given by: If the electron beam remains straight, it follows E = V
427
that the electric and magnetic forces on each electron d
must have the same magnitude and act in opposite directions.Therefore: v= V
electric force = magnetic force Bd (upward) (downward)
eE=Bev
Electric and magnetic fields
Now we will consider what happens when an electron
beam passes through an electric field and a magnetic field + at the same time.
Velocity selection
Balancing the effects of electric and magnetic fields is also used in a device called a velocity selector. This is used in devices such as mass spectrometers where it is desired to produce a beam of charged particles all moving with the same velocity. The construction of a velocity selector is shown in Figure 27.11.
The apparatus is very similar to the deflection tube in Figure 27.9. Two parallel plates are situated in an evacuated chamber. They provide a uniform electric field of strength E.
v = BE
therefore:
Chapter 27: Charged particles
+
undeflected ions
+
S
–Q BEv
––
–
magnetic field into plane of paper
Figure 27.11 A velocity selector – only particles with the correct combination of charge, mass and velocity will emerge through the slit S.