Semiconductor(Electronics) Electrical Conduction Through Solid Materials – SS3 Physics Lesson Note (First Term)

A gas conducts electricity only at low pressure and high potential difference, and the electrons produced at the cathode are called cathode rays.

Behavioural objectives

1. State the conditions under which a gas conducts electricity.
2. List the characteristics of cathode rays and give one application.
3. Explain thermionic emission.
4. Describe the diode valve and explain why it is used as a rectifier.
5. Describe the cathode ray oscilloscope and state its uses.

What are the conditions for discharge in a gas?

A gas conducts electricity (electrical discharge) when its pressure is very low and the potential difference across it is very high.

Conditions for discharge

• Low pressure: the gas in the discharge tube is kept at very low pressure.
• High potential difference: a very high voltage is applied across the cathode and anode.

How does a gas conduct in a discharge tube?

1. At very low pressure and high voltage, the gas in the tube breaks into ions.
2. Positive ions move towards the cathode.
3. Negative ions and free electrons move towards the anode.
4. The positive ions knock electrons off the metal plate of the cathode.
5. The electrons produced at the cathode are called cathode rays.

• Charge carriers in a gas: ions and electrons carry the current, not neutral molecules.

What are cathode rays?

Cathode rays are streams of fast-moving electrons produced at the cathode of a discharge tube.

What are the characteristics of cathode rays?

• Particles: they consist of streams of fast-moving electrons.
• Fluorescence: they make glass and other materials glow, usually with a greenish colour.
• Straight lines: they travel in straight lines.
• Deflection: electric and magnetic fields deflect them.
• Ionisation: they can ionise a gas.
• Mechanical effect: they turn a light paddle wheel in the tube because they have mass, momentum and energy.
• Energy: they are highly energetic particles.
• Photographic effect: they affect photographic plates.
• X-ray production: they produce X-rays when suddenly stopped by high-density metals.
• Penetration: they can pass through thin metal foils such as aluminium and gold foil.

How do cathode rays differ from electromagnetic waves?

• Cathode rays: streams of electrons with mass and negative charge, deflected by electric and magnetic fields.
• Electromagnetic waves: energy waves with no mass and no charge, not deflected by these fields.

What is an application of cathode rays?

• Fluorescent tubes: used commercially for lighting and display signs.
• How it works: the tube contains mercury vapour at low pressure, which glows when cathode rays pass through it.

What is thermionic emission?

Thermionic emission is the release of electrons from the surface of a metal when the metal is heated to a high temperature.

How does thermionic emission happen?

1. The filament is heated to a high temperature.
2. Its free electrons at the surface gain extra energy.
3. The electrons break through the surface of the metal.
4. They gather outside the metal as an “electron cloud”.

How are electrons freed from a cold cathode and a hot cathode?

What is a diode valve?

A diode valve is an evacuated glass bulb with a hot filament and an anode, and it is a simple application of thermionic emission.

What are the parts of a diode valve?

• Anode: usually a metal cylinder.
• Filament (heater): a tungsten wire that is heated to supply electrons.
• Cathode: the emitting part surrounding the filament.
• Glass bulb: highly evacuated, and it encloses all the parts.

How does a diode act?

1. The battery (E) sends current through the filament and heats it.
2. The hot filament supplies free electrons.
3. The anode is made positive in potential with respect to the cathode.
4. Electrons flow towards the anode and form the anode current (I_a).
5. The milliammeter registers the anode current.

What are the characteristics of a diode?

• Non-Ohmic conductor: the characteristic curve shows that the diode valve does not obey Ohm’s law.
• One-way flow: current flows only in one direction, from cathode to anode, when the anode is positive.

Why is a diode valve used as a rectifier?

• Rectifier: a device that changes a.c. to d.c.
• Reason: the diode allows current in one direction only, so it produces d.c. voltage from an a.c. supply.

What is a cathode ray oscilloscope?

The cathode ray oscilloscope (CRO) is an instrument used to investigate current and voltage in electronic circuits.

What are the parts of a cathode ray oscilloscope?

It is a vacuum tube with an electron gun at one end and a fluorescent screen at the other end.

What does the electron gun consist of?

• Heated filament: supplies electrons by thermionic emission.
• Cathode: the source from which the electrons are emitted.
• Anode: acts as a focusing lens to accelerate the electrons and focus them onto a spot on the screen.

What are the uses of a cathode ray oscilloscope?

• Waveforms: studying all types of waveforms, especially alternating current waveforms.
• Frequency: measuring the frequency of a signal.
• Amplitude: measuring the amplitude of voltage of electronic devices.

Evaluation

1. State the conditions under which a gas conducts electricity.
2. Mention five characteristics of cathode rays.
3. What is thermionic emission?
4. Why is a diode valve used as a rectifier?
5. When a metal is heated to a high temperature and electrons leave its surface, this is known as: (a) photoelectric emission (b) thermionic emission (c) field emission (d) secondary emission.

Assignment

1. (a) Draw a labelled diagram of a cathode ray oscilloscope showing the essential parts. (b) State the function of the hot filament, the anode, the fluorescent screen and the deflector plates. (c) State one way in which cathode rays differ from electromagnetic waves.

Reading: New School Physics for SSS, pages 481 to 483.