From Ideas to Implementation

FROM IDEAS TO IMPLEMENTATION

1. Increased understandings of cathode rays led to the development of television

- Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves

Cathode rays were observed to have inconsistent behaviour with regards to its nature – particle or wave.

The English physicists believed that cathode rays were a stream of negatively charged particles. Cathode rays were observed to cause a paddlewheel to rotate, demonstrating that they had momentum and thus mass. They were also observed to deflect in a magnetic field and travelled more slowly that light did, all evidence for the particle nature of cathode rays.

However, German physicists including Heinrich Hertz believed that cathode rays were a form of wave. They found that an opaque Maltese cross placed in the path of the cathode rays caused a shadow to appear in green light, and they travelled in straight lines, both properties like light. Furthermore, they were found to be able to pass through thin films of metal without damaging them, something that no particle had ever been observed to do. Hertz believed they were waves because there was no observable deflection by an electric field; it was later found that this was caused by an insufficiently low pressure in his cathode ray tube.

The debate was eventually settled by Thomson’s experiment in 1897, and the particle nature of cathode rays proven without doubt.

- Explain that cathode ray tubes allowed the manipulation of a stream of charged particles

When a high voltage is applied across two parallel plates in a vacuum tube, cathode rays are emitted from the cathode and accelerate towards the anode. The glass behind the anode is hit by the cathode rays and causes a glow. Different pressures in the vacuum tubes were found to produce different fluorescent results. The cathode rays can also be manipulated by deflection in an electric or magnetic field.

- Identify that moving charged particles in a magnetic field experience a force

Charged particles passing through a magnetic field experience a force, the direction of which can be determined by the right hand palm rule.

- Identify that charged plates produce an electric field

Oppositely charged plates will produce and electric field, the field lines going from the positive plate to the negative plate.

- Describe quantitatively the force acting on a charge moving through a magnetic field

F = Force (N)
q = Charge (C)
v = Velocity (ms-1)
B = Magnetic field strength (T)
θ = Angle between direction of velocity and magnetic field

- Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates

A positive and negativepoint charge will create an electric field, as will oppositely-charged parallel plates. The strength of the electric field at any point is defined as the force per charge, and the direction is the direction of force on a positive charge when placed at that point. The force on a charge in an electric field is given by:

F = Force (N)
E = Electric field strength (NC-1)
q = Charge (C)

- Describe quantitatively the electric field due to oppositely charged parallel plates

E = Electric field strength (NC-1, Vm-1)
V = Voltage between plates (V)
d = Distance between plates (m)

- Outline Thomson’s experiment to measure the charge/mass ratio of an electron

J.J. Thomson conducted an experimentin 1897to determine the charge to mass ratio of cathode rays. This experiment proved the particle nature of cathode rays.

Part 1

Firstly, the magnetic field supplied by the coil and the electric field supplied by the electric plates were turned on in such a way as to cancel each other out and allow the cathode ray to pass undeflected.

Thus;

Part 2

Next, the electric field was turned off so that the cathode ray was deflected from the magnetic field. Since the cathode ray entered the magnetic field at 90°, the deflection was in a circular arc.

Thus;

Substituting from the first part;

Since the electric and magnetic field strength were known and the radius of the arc of the cathode ray could be measured, the q/m ratio could be found. It was found to be 1.76x1011 C/kg regardless of the cathode materials used, proving without a doubt their particle nature. This high q/m ratio suggested that an electron was either very light or very highly charged.Thomson believed that it was very light and that it was a constituent of an atom, leading to his plum pudding model.

Later experiments (notably Millikan’s oil drop experiment) showed that the charge of an electron was 1.6x10-19 C. Using this and the q/m value, the mass of the electron was found to be 9.11x10-31 kg. This mass is less than a thousandth of the mass of an H atom and confirmed that electrons are a part of atoms.

- Outline the role of the followingin the cathode ray tube of conventional TV displays and oscilloscopes:

Electrodes in the electron gun
A cathode in the electron gun is heated up by a separate voltage supplyor heating filament to emit electrons via thermionic emission. These electrons accelerate towards the anodes. A third electrode between the cathode and the anode, called the grid, can be used to control the velocity of the electrons by making it more positive or negative. This controls the brightness of the screen.In a black and white TV there is one electron gun while a colour TV has three.
The deflection plates or coils
The accelerated electrons travel towards the deflection system, which deflect the electrons in the desired direction towards the screen.In cathode ray oscilloscopes, the deflection is provided by electric fields fromtwo pairs of parallel plates, the X and Y plates, while in TVs magnetic fields from coils are used for larger and more efficient deflections.

The fluorescent screen
The inside of the glass on the end of the vacuum tube is coated with fluorescent material which releases light when struck by electrons. Once the beam moves on from a pixel (or phosphor dot), the fluorescence fades after a short time.In a colour TV, each pixel has three sub-pixels for red, green and blue. In a CRO, a beam of cathode rays scans across the screen relatively slowly to trace out a single dot or line. In TVs, the electrons scan aseries of horizontal lines across the entire screen 50 times a second. The odd linepixels are scanned first followed by the even lines. The scanning pattern is called araster.

2. The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation

- Describe Hertz’s observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate

In 1887, Heinrich Hertz verified the existence of electromagnetic waves, the first person to do so after Maxwell’s prediction.

In his experiment, he used an induction coil to produce sparks across the gap between the electrodes of a transmitter. The current oscillated back and forth, generating an e-m wave that was emitted. Hertz found that when sparks jumped across the gap in the transmitter, sparks would also jump across the gap in the receiver, even though it was not connected to a source of current. Hertz concluded that the sparks generated in the receiving loop were induced by an e-m wave that was produced by the transmitter.

Hertz then showed that these waves could be reflected, refracted, could be polarised and travelled at the speed of light, a strong confirmation of Maxwell’s theory. He also noticed that the intensity of the sparks in the detecting loop increased when illuminated by UV light, but did not investigate further.

- Outline qualitatively Hertz’s experiments in measuring the speed of radio waves and how they relate to light waves

Hertz was able to determine the velocity of the radio waves by measuring its frequency and wavelength (since v=fλ). The frequency was known since it was the same as the frequency of the oscillating current, while the wavelength was found by analysing the interference pattern of two waves with slightly angled paths. He found that the speed of the radio waves was equal to the speed of light.

- Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised

Classical physics predicted that as the wavelength of black body radiation decreased, the intensity of the radiation would increase. As the wavelength decreased to the UV region, the intensity would increase without bound and approach infinity, contradicting experimental results. This was known as the ultraviolet catastrophe.

In 1900, Max Planck proposed a theory that was able to reproduce the experimental graphs. His theory was based on a radical assumption that the energy emitted from a black body was not continuous, but rather quantised and emitted as packets of energy called quanta. This suggested that the energy of any radiation could only be a multiple of a minimum value, equal to hf.

- Identify Einstein’s contribution to quantum theory and its relation to black body radiation

In 1905, Einstein extended Planck’s quantum assumption by proposing the particle nature of light – that light consists of quanta called photons, the energy of which was proportional to the frequency of light. Einstein used the photon theory of light to explain the photoelectric effect, for which he was awarded a Nobel Prize.Planck is believed to have been the initiator of quantum physics; however, when Planck first proposed the idea of the quantisation of energy, it was thought to be radical and even Planck himself could not be convinced that this was true. However, when Einstein used it to successfully explain the photoelectric effect, it provided convincing evidence to back up this radical hypothesis. Einstein’s contribution to quantum theory led to the beginning of a new era of modern physics.

The photoelectric effect is the phenomenon in which electrons are liberated from a metal surface struck by electromagnetic radiation with frequency above a certain value. Einstein explained that liberation of electrons is due to the collision of photons in the light with energy more than the work function of the metal.

ϕ = Work function (J)
h = Planck’s constant (6.626x10-34Js)
f0 = Threshold frequency (Hz)

Electrons are only emitted if the energy of the incident radiation is higher than the work function of the metal. The intensity of the light, or the photons per unit area, determines the output current. The liberation of electrons follows the all or nothing principle – all of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else no energy is absorbed. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy.

= Maximum kinetic energy of an emitted electron(J)
h = Planck’s constant (6.626x10-34Js)
f= Frequency of incident light (Hz)
ϕ = Work function (J)

- Explain the particle model of light in terms of photons with particular energy and frequency

Based on Planck’s quantum hypothesis and Einstein’s photon theory, the energy of light is quantised in packets called photons, the energy of which is proportional to its frequency. This phenomenon whereby light can act as a wave in some situations and a particle in others is known as wave-particle duality.

- Identify the relationships between photon energy, frequency, speed of light and wavelength

E = Energy (J) [1 eV = 1.6x10-19 J] h = Planck’s constant (6.626x10-34Js)
f= Frequency (Hz)

c = Speed of light (3x108 ms-1)
f= Frequency (Hz)
λ = Wavelength (m)

- Identify data sources, gather, process and present information to summarise the use of the photoelectric effect in photocells

A photocell is a device that uses the photoelectric effect to convert light energy into electrical energy. They consist of an anode and a cathode from which photoelectrons are liberated. Photocells are used in burglar alarms, automatic doors and other applications needing “electronic eyes”.

When incident light strikes the cathode, photoelectrons are emitted from the cathode due to the photoelectric effect and current flows. However, when the beam of light is interrupted, such as by a burglar passing by or a person walking towards an automatic door, the beam of light is interrupted and the intensity of the light falls. Consequently, the current drops, and an electronic sensor in the circuit sounds an alarm or opens the door.

- Process information to discuss Einstein’s and Planck’s differing views about whether science research is removed from social and political forces

Both Planck and Einstein lived in Germany during the early 20th century, and they were close friends. However, during World War One, Einstein’s pacifist views became clear as he openly criticised German militarism. Planck, however, supported the German cause, signing the Manifesto of the 93 intellectuals declaring their support for German military actions. After Einstein immigrated to America, he deeply regretted his advice to President Roosevelt to research nuclear bombs, due to his fear that the Germans were developing nuclear technology, which led to death of many people in Japan. However, Planck continued his research in Germany during WW2, issuing a “persevere and continue working” slogan and concentrating on his research.

3. Limitations of past technologies and increased research into the structure of the atom resulted in the invention of transistors

- Identify that some electrons in solids are shared between atoms and move freely

In metals, metal cations are surrounded by a sea of delocalised electrons that can move freely and act as charge carriers.

- Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance

When atoms are packed closely together in a crystalline lattice, the energy levels of the electrons vary slightly and wider energy bandsare formed. The valence band is made up of the energy levels of the valence electrons, while the conduction band is the empty band above the conduction band, in which electrons are free to move.

In a conductor, the valence band is partially filled and the conduction band overlaps with the valence band. The forbidden energy gap is non-existent, allowing electricity to be conducted. As temperature increases, resistance increases since the lattice has more energy to vibrate and cause collisions with electrons. In a semiconductor, the energy gap between the valence and conduction band is small, and electrons can easily jump to the conduction band to conduct electricity. As temperature increases, resistance decreases as more and more electrons have sufficient energy to jump into the conduction band. In an insulator, the energy gap between the valence band and the conduction is too great for electrons to jump into the conduction band and conduct electricity.

- Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current

In a semiconductor, when an electron gains sufficient energy to jump into the conduction band, it leaves behind a hole. In the valence band, the electrons then have room to move into the hole when a potential difference is applied, and the hole thus acts as a positive charge carrier moving in the direction of the electric field. In the conduction band, the freed electron can act as a charge carrier. Thus, both electrons and holes contribute to current flow.

- Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators

Conductors have the highest relative number of free electrons, followed by semiconductors in which some electrons have enough energy to jump to the conduction band at room temperature. Insulators have the least number of free electrons.

- Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity

Earlier solid state devices used germanium as there was only technology to extract and purify Ge to a sufficient degree. Purification methods for silicon were only developed in the 1950s, and it became the preferred semiconducting material because it is more abundant than Ge, being one of the most abundant elements in the earth’s crust, and retains its semiconducting properties at higher temperatures than Ge.

- Describe how ‘doping’ a semiconductor can change its electrical properties

Pure semiconductors are called intrinsic semiconductors, while doped semiconductors are called extrinsic semiconductors. Doped semiconductors have impurities in their lattice structure; an n-type semiconductor is formed with a group V dopant while a p-type semiconductor is formed with a group III dopant.

An n-type semiconductor is formed with group V dopant atoms, with one extra electron not required for bonding. These electrons occupy the donor level, an energy level close to the conduction band, and thus can easily be promoted into the conduction band, thus increasing the number of free electrons to conduct electricity.