Electrons

The d i s cove r y o f t he e l e c t ron was a l andm ar k i n phys i c s and l ed t o grea t t e c hno l ogy advanc es

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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

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.

There is a certain minimum threshold energy which the electron must have to escape.

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Cathode RaysBeams of electrons moving at high speed = Cathode RaysElectrons 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 beamThe deflection of an electron beam can be caused by

1. A magnetic field

2. An electric field

Force on electron

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Cathode Ray OscilloscopeThe CRO has three main parts i.e.

1. Electron Gun2. Deflecting System3. Florescent Screen

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• 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.

Electron Gun

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• Consist of two pairs of parallel plates : X-Plates and Y-Plates

Deflection System

• 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 'saw-tooth'.

• The p.d. applied to the Y-plates is the signal to be examined. With the p.d. across the X-plates, 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 CROA CRO can be used for

1. Measuring Potential Differences2. Displaying waveforms3. Measuring time interval & Frequency

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Potential Difference SignalFrequency signal

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X-RaysX-Rays are produced when high-speed electrons are stopped by matter

X-rays

In an X-ray tube, electrons from a hot filament are accelerated across a vacuum to the anode

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 electric field between anode and concave cathode

X-RaysX-Ray Properties

1. Penetrate matter up to 1 mm of lead2. Not deflected by electric or magnetic field3. Ionize a gas4. Affect a photographic film5. Cause fluorescence6. 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 EffectIn 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.