Chapter 4 : Electronics

4.1 Cathode-ray oscilloscope (CRO) 4.1.1Thermionic emission

1. A metal surface has many free electrons.The electrons do not have enough kinetic energy to break free.Will keep binding to the surface of metal.

2. Thermionic emission = emission of electrons from a hot metal surface (cathode)

Current passes through tungsten filament (cathode) > hot

Electrons gain kinetic energy > sufficiently high k.e.e- escape (from surface of tungsten filament to surroundings)

Electrons emitted are accelerated towards anode (by the high potential difference between cathode and anode)

Cathode ray = a beam of electrons moving at high speed in a vacuum

Thermionic emission is like evaporation

Thermionic emission EvaporationEnough energy must be absorbed before electrons are emitted from metal surface

Enough energy must be absorbed before water molecules escape from the water surface

Negatively charged electrons are emitted

Neutral whole molecules escape

Demonstration of thermionic emission (using thermionic diode)

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Chapter 4 : Electronics

Factor Rate of thermionic emissionTemperature of the cathode Temp ( ) , r.o.t.e. ( )

Surface area of cathode S.A. ( ), r.o.t.e. ( )Potential difference

between anode and cathodeP.d. ( ), r.o.t.e. ( )

Emitted electrons accelerate faster towards the anode

Types of cathodeNature of metal oxide

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Chapter 4 : Electronics

Properties of cathode rays Electrons move fast in a straight line in vacuum Carry momentum and energy (due to moving electrons) Produce fluorescence/bright spot (when strike fluorescent material) Charged negative (deflect towards +ve plate by elec. and mag. field)

Energy change in a cathode ray

When a potential difference V is applied across the cathode and anode in a vacuum tube, electrons emitted have electric potential energy

Electric potential energy = eVe = charge on 1 electron = 1.6 × 10-19 C

Emitted electrons will be accelerated by the potential difference V to the maximum velocity of v, hence

Kinetic energy = 12 mv2

m = mass of an electron = 9.1 × 10-31 kgv = maximum velocity of the electrons

By using principle of conservation of energy,

12 mv2 = eV

Maximum velocity of electron, v = √ 2 eVm

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Chapter 4 : Electronics

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