Transcript Cathode Ray Oscilloscope

Cathode Ray Oscilloscope
Introduction
• Thermionic Emission
• Cathode Rays
• Deflection of an electron beam
• Cathode Ray Oscilloscope
• Uses of the CRO
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Thermionic Emission
There is a certain
minimum
threshold energy
which the
electron must
have to escape.
The negative charges, in the form of
electrons, escape from the filament
when it’s hot because they have
enough energy to get free from the
metal surface.
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Cathode Rays
Beams of electrons moving at high speed = Cathode
Rays
Electrons accelerated toward anode – pass through
the hole – miss the cross – the screen to fluoresce –
cast a shadow of the cross
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Deflection of an electron beam
The deflection of an electron beam can be caused by
1. A magnetic field
Force on electron
2. An electric field
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Cathode Ray Oscilloscope
The CRO has three main parts i.e.
1. Electron Gun
2. Deflecting System
3. Florescent Screen
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Electron Gun
• Electrons are produced by thermionic
emission. Essentially a cathode(negative
electrode) is heated and electrons boil off the
surface to be attracted by a series of anodes
(positive electrodes).
The anodes accelerate the electrons and
collimate them into a narrow beam.
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Deflection System
• Consist of two
pairs of parallel
plates : X-Plates
and Y-Plates
• To display a waveform, a repetitive reversing
voltage is applied to the X-plates.
This causes the electron beam to be slowly
repelled from the left-hand plate and attracted
towards the right-hand plate. On the CRO
screen this translates as an illuminated dot
moving from left to right.
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• The voltage is then reversed and increased
rapidly. The effect is to move the dot very
quickly right to left (fly-back).
The applied voltage is called the time-base.
The curve has the general shape of a 'sawtooth'.
• The p.d. applied to the Y-plates is the signal to
be examined. With the p.d. across the Xplates, a sine waves is displayed
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Display
• The display screen is coated on the inside with
a very thin layer of a phosphor called
cadmium sulphide. This fluoresces (gives
out green light) when electrons impact its
surface.
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Uses of CRO
A CRO can be used for
1. Measuring Potential Differences
2. Displaying waveforms
3. Measuring time interval & Frequency
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Frequency signal
Potential Difference Signal
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X-Rays
X-Rays are produced
when high-speed
electrons are stopped
by matter
In an X-ray tube,
electrons from a hot
filament are
accelerated across a
vacuum to the anode
X-rays
The anode is a copper block with a ‘target’ of a high-melting-point
metal such as tungsten on which the electrons are focused by the
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electric field between anode and concave cathode
X-Rays
X-Ray Properties
1.
2.
3.
4.
5.
6.
Penetrate matter up to 1 mm of lead
Not deflected by electric or magnetic field
Ionize a gas
Affect a photographic film
Cause fluorescence
Give interference and diffraction effects
X-RAYs
X-rays can travel through soft substances but not hard, dense
ones. They travel through soft tissue, but not hard bone.
X-rays are used in hospital to make shadow pictures of parts
of the body.
X-rays are used at airport to check what may be hidden in the
luggage.
Photoelectric Effect
In the photoelectric effect, electrons are
emitted from matter (metals and non-metallic) as
a consequence of their absorption of energy from
electromagnetic radiation of very short
wavelength, such as ultraviolet light.