A2 Physics, electron beams, f =bil, hall probes

1. Electron beams

2. Your Learning
Write an explanation of how a motor works
Including
Magnetic fields (of the wire and magnet)
Force on a wire F = BIL
(what affect the speed of the motor)
Flemings left hand rule
(how does this help you know which way it is going to go)

3. How much force does a wire feel?
Force = B-field x Current x length of wire
F = BIl
The Force is greatest when perpendicular to the
magnetic field and zero when parallel
F = BIL sin θ
θ = 90o
(sin θ = 1),
θ = 0o
(sin θ = 0),
B = F/IL
Force (Newtons)
B- field (Teslas)
Current (Amps)
Length (metres)

4. What we are going to do today
• What happens to charged particles in a
magnetic field?
• Why does a force act on a wire in a
magnetic field when a current flows along
the wire?
• What equation can we use to find the
force on a moving charge?

5. The Deflection tube…a basic CRO

6. Thermionic emission
When energy is given to a
wire it heats up but…..
…it can also give off
electrons

7. This is the basis of the electron
gun...
• Electrons ‘boil off’ here They are accelerated here
There is a current here…. And a vacuum here
And a large
accelerating
+Voltage V here

8. Applying a magnetic field
circle if v
perpendicular
to B
spiral in a
dissipative
medium
helix if component of
v parallel to B
F
v
B

9. Researching deflecting electron beams
20th Century Technology Exhibition
Nowadays most new televisions and computer monitors use LCD
displays, but not so long ago they all used picture tubes. In fact,
picture tubes were the main use of electron beams.
Vanita and Dave are planning a museum exhibition of technology from
the last century. They will provide a straight forward explanation to
help people to understand how each exhibit worked. The electron
beams section of the exhibition has:
a television picture tube
an oscilloscope
an x-ray tube.
Your task is to write a straight forward, concise explanation of each
section, and include pictures that will develop peoples understanding
of the uses of deflecting electron beams.

10. Hall Probes
• As electrons move through a piece of n-type semiconductor through
which a magnetic field passes, then the electrons experience a
force (Fleming's left hand rule) which moves them to one side of the
semiconductor slab. An electric field builds up giving a force in the
opposite direction. (and creating a measurable potential difference).
Electron
Electron
+ + + + + +
B- field