1 Cathode Ray Debate

Physics:
From Ideas to Implementation

1.1Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves:

Observations that fit the Wave model:

Travelled in straight lines
A shadow created when an opaque object was in its path

Observations that fit the Particle model:

Rays left the cathode at right angles to the surface
Deflected by magnetic fields
Did not appear to be deflected by electric fields
Small paddle wheels turned when placed in their path
Travelled considerably slower than light

1.2Explain that cathode ray tubes allowed the manipulationof a stream of charged particles:

The Cathode Ray Tube:

Highly evacuated glass tube (to reduce obstruction/collisions) containing two electrodes
High voltage applied across the electrodes
Cathode rays (streams of electrons) flow from the cathode towards the anode

Manipulation of cathode rays:

Structures built into or around cathode ray tube, manipulating the rays:

Further electrodes built into cathode ray tube creating an electric field, changing the path of the cathode rays
Magnetic fields applied to cathode rays through the glass from outside the tube
Solid objects placed inside tube to block the path of the rays (e.g. Maltese Cross)

1.3Outline Thompson's Experiment to measure the chargeto mass ratio of an electron:

Charge to mass ratio:

Combined these results to obtain the magnitude of the charge to mass ratio
Given by:

From these results, he could conclude that:
The charge on cathode rays was 1800 times larger than charge on a hydrogen ion; or
The mass of cathode ray particles was 1800 times less than that of a hydrogen ion

2 Forces on Moving Charges

2.1Identify that moving charged particles in a magnetic field experience a force & describe quantitatively the force acting on a charge moving through a magnetic field:

Magnetic fields on moving charges:

Magnetic fields exert forces on electric currents (moving charged particles)
If the particle is travelling parallel to the field lines, there is no force experienced
Maximum force is experienced when travelling at 90 degrees to the field lines
The right-hand-palm-rule (RHPR) is used to determine the direction of the force (thumb is in the direction of a positive charge movement)

Formula:

F = qvBsinθ

3 Electric Fields

3.1Identify that charged plates produce an electric field & describe quantitatively the electric field due to oppositely charged parallel plates:

Plates with a potential difference between them produce an electric field running between them - the field lines are parallel and run from the positive plate to the negative plate
Electric fields exist in any region in which an electrically charged objectexperiences a force
Charged plates exert a force on other charged objects indicating anelectric field exists

Formulae:

Electric field strength:

Force on a moving charge:

3.2Discuss qualitatively the electric field strength due to a point of charge, positive and negative charges and oppositely charged parallel plates:

The strength of an electric field at any point is defined as the size of the force acting per unit of charge
The direction of the electric field at any point is defined as the direction of the force a positive charge would experience placed at this point

Point of Positively Charged:

Diminishes with distance from particle
Direction points radially away from particle

Point of Negatively Charged:

Diminishes with distance from particle
Direction points radially towards particle

Between Positive and negative charges: Between oppositely charged parallel plates:

4 TV Displays and Oscilloscopes

4.1Outline the role of electrodes in the electron gun, the deflection plates or coils, the fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes:

Electron gun:

Heated cathode releases electrons by thermionic emission
Positively charged anode develops strong electric field exerting a force

The electron gun produces a narrow beam of electrons - the electrodes (cathode and anode) in the gun accelerate the electrons

Deflection plates/coils:

The deflection plates (in CRO) or coils (in TV) establish an electric field that controls the deflection of the electron beam from side to side and up and down

Fluorescent screen:

The fluorescent screen is coated with a material that emits light when struck by electrons in the cathode ray
This allows the position of the beam to be seen where it strikes the screen

5 Hertz and the Photoelectric Effect

5.1Describe Hertz's observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate:

The Photoelectric Effect: when light of an appropriate frequency is shone onto a metal surface, electrons are emitted from that surface.

Hertz observed that the spark between the gap in the transmitter loop caused an electrical disturbance between the gaps in the detecting loop
Hertz observed that the gap in the detector could be made larger and still generate sparks when the radiation from the transmitting spark shone directly into the gap in the detecting loop
Hertz did not recognise that the UV component in the transmitter spark removed free electrons from the surface of the metal (the photoelectric effect), thus allowing the discharge (spark) to occur across a wider gap

5.2Outline qualitatively Hertz's experiments in measuring the speed of radio waves and how they relate to light waves:

Hertz set up an induction coil connected to the transmitter loop
As sparks were generated across a small gap they induced sparks in a detecting loop a small distance away
This spark was evidence for electromagnetic waves travelling through space from the induction coil to the detecting loop
Hertz was able to calculate the velocity of the waves by reflecting the generated waves off a metal sheet and measuring the wavelength of the standing wave set up by interference
Substituting this wavelength and the known frequency of the wave generator into the general wave equation (v = f λ), Hertz calculated the wave speed at 3 x 108 ms-1
This value was very close to the values for the speed of light earlier estimated by Maxwell

6 Photoelectric Effect Applications

6.1Identify data sources, gather, process andpresent information to summarize the use of the photoelectric effect in solar cells and photocells:

Solar cells:

Uses the photoelectric effect to convert the energy from sunlight into electrical energy
Uses silicon semiconductor – when light (photons) hits the top layer of a solar cell, some of the electrons in the n-type silicon semiconductor are set free
The electrons flow out of the n-type silicon, into the external circuitry (e.g. a light bulb), and return to the cell via the p-type silicon

Photocells (photodetectors):

Uses the photoelectric effect directly to produce an electric current in the circuit attached
Some utilize photoelectric effect to detect presence of light or radiation – used in automatic switching devices (e.g. used to switch street lights on/off)
Also used in burglar alarms, automatic opening doors, entrance buzzers (when entering into a shop), breathalysers, etc.

7 Social and Political Forces

7.1Process information to discuss Einstein and Planck's differing views about whether science research is removed from social and political forces:

Background:

Application of scientific study to ‘the war effort’ was seen as a problem by many scientists
Two groups emerged:
Those against turning their ideas into weapons of destruction
Those agreeing with decisions on the use of scientific study – leftto politicians to decide

Planck's View:

Plank was a nationalist, believed in and worked for his country
Loyal to whatever government decided
One of first German intellectuals to sign the document supporting the war effort
Then devoted his work and research to the war effort

Einstein's View:

Supported no government, had no particular political views
Refused to sign document supporting the war effort, signed peace document instead
Pacifist (peace-keeper); did not want his work contributing to killing others

8 Black Body Radiation

8.1Identify Planck's hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised:

Black body:

An idealized object that absorbs all electromagnetic radiation that falls on it
No electromagnetic radiation passes through it and none is reflected

The amount of energy radiated reaches a maximum at a wavelength that depends on the temperature of the black body

Planck’s hypothesis:

Emission and absorption of electromagnetic radiation for a black body cavity is quantised
Energy is not continuous, can only be radiated or absorbed in small, discrete amounts or packets called quantum

8.2Identify and assess Einstein's contribution to quantum theory and its relation to black body radiation:

Einstein's Explanation of Planck's work:

The energy associated with the radiation from a black body is concentrated in packets of energy that he called photons
He extended Planck’s quanta idea to light and also explained that wave and particle behaviour can coexist in light – dual nature

Photon:

Is the smallest amount of radiation energy possible at a particular frequency
Photon can only transfer all or none of its energy, not part of it
Amount of energy carried by photon is proportional to its frequency (by E=hf)
Einstein determined that the intensity of light is proportional to number of photons

Relation to Black Body Radiation:

The higher the frequency the greater the total energy radiated (E=hf)
Shown by the black body radiation curves (in above graph)

9 Particle Model of Light

9.1Explain the particle model of light in terms of photons with particular energy and frequency:

Particle Model:

Instead of being emitted as continuous waves, light travels in packets of energy called photons
A photon carries an amount of energy that is proportional to the frequency of the radiation/light
All photons of light of a particular frequency have precisely the same amount of energy
The higher the frequency of the light, the more energy the photon possesses, thus photons of ultraviolet light have higher energy than those of blue light, which in turn have higher energy than photons of red light
All photons, regardless of their frequency, have zero rest mass and travel at 3x108ms-1 in a vacuum

9.2Identify the relationships between photon energy, frequency, speed of light and wavelength:

E = hf

c = f λ

10 Electrons in Solids

10.1Identify that some electrons in solids are shared between atoms and move freely:

In some solids, the outer electrons are very loosely bound to particular atoms
These electrons can therefore move across the entirety of the solid

11 Conductors vs Insulators

11.1Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance:

Conductors:

Valence band is only partially filled, thus the conduction and valence bands overlap
Valence electrons easily move along conduction band
Material has low electrical resistance

Insulators:

Valence band is completelyfilled, thus there is a large forbidden energy band/gap
Difficult for valence electrons to move into the conduction band
Material has high electrical resistance

Semiconductors:

Valence band is almost filled, thus there is small, not to wide energy band/gap
At higher temperatures, electrons in valence band gain sufficient energy to cross gap
This reduces electrical resistance of the material

11.2Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semi-conductors and insulators:

Conductors contain high number of free electrons in the conduction band
Under normal conditions, insulators and semi-conductors have far less free electrons
Raising the temperature, using certain lighting conditions or applying a potential difference, can induce electrons in some semiconductors to move into the conduction band

12 Electrons and Holes

12.1Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current:

When an electron in a semiconductor moves into the conduction band it leaves a “hole”,
i.e. an atom with one less valence electron than normal
An electron from a nearby atom in the valence band can move and fill the hole – this then creates another hole, and so on
The creation of holes and the movement of electrons to fill them is equivalent to an electric current in the semiconductor
Electrons flow in one direction – the apparent movement of holes in the opposite direction can be considered as a flow of positive charge

13 Solid State vs. Thermionic Devices

13.1Describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices:

Thermionic Devices:

Contains a cathode that emits electrons only when heated to a high temperature
It requires a separate heating circuit to heat the cathode, which takes time to heat up
Requires a near vacuum to allow electrons to flow between the electrodes, thus they are commonly packaged in an evacuated glass tube
Thermionic devices are larger in size

Solid State Devices:

Uses semiconductors to generate electron flow - no heated circuit required
Functions immediately
Operates at normal pressures
Solid-state devices are smaller in size

Reasons for replacement:

The combined advantages of smaller size, simpler and cheaper construction, higher reliability,less fragility, lower power requirements and speed of operation make solid state devices more attractive to electronics manufacturers than equivalent thermionic devices

14 Semiconductors

14.1Describe how 'doping' a semi-conductor can change its electrical properties

Doping is the addition of an impurity (such as gallium or arsenic) to a semiconductor to change its conductivity
The atoms of the doping element need to fit reasonably well into the semi-conductor lattice structure so as not to distort it and impede electron flow
The doping element needs to have either one more or one less valence electron than the semi-conductor material itself
Doping increases the potential conductivity of the semiconductor – extra electrons or holes to act as charge carriers

14.2Identify differences in p and n-type semiconductors in terms of relative number of electrons and holes:

P-type Semiconductors:

There are more positive holes than negative charge carriers (electrons)
Elements such as aluminium and gallium (3 valence electrons – group 3 elements) are used as doping agents with silicon to produce p-type semiconductors

N-type Semiconductors:

There are more negative charge carriers (electrons) than positive holes
Elements, such as arsenic and phosphorus (5 valence electrons – group 5 elements) are used as doping agents with silicon to produce n-type semiconductors

15 Transistors

15.1Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity:

At first, germanium was widely used as a semi-conductor because it was easier to purify than other known semi-conductors, such as silicon
Silicon eventually replaced the germanium as semi-conducting material of choice in transistors because:
it is the second most abundant element on earth by weight, which means it is relatively cheap
it retains its semiconducting properties at relatively high temperatures (when compared to germanium)
it can handle higher electric currents before overheating (which destroys its semiconductor properties)
processing techniques were developed to produce very pure, single-crystal forms
in single-crystal form (very pure silicon), the molecular structure of the material is uniform, thus ensuring consistency of properties

15.2Discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor:

Problem in the early days of the radio was amplification – the received signal was extremely weak and could not produce a loud sound without being amplified
Achieved amplification by using vacuum tubes (thermionic devices), which were very fragile, large, expensive and required high power consumption
The invention of the transistor solved these significant shortcomings:
Contained 3 semiconductors – NPN transistors and PNP transistors
It was smaller, longer lasting, more reliable, less expensive and required less power
It quickly replaced the vacuum tube in many electronic devices

15.3Assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors:

The discovery of the transistor led to the development of integrated circuits in which many thousands of transistors could be constructed on a tiny microchip
This lead to complex circuitry development which was incorporated into appliances to make many tasks formerly done manually, now done electronically
For society, this meant that fewer manual labour jobs were available, but at the same time, more jobs became available to control and manipulate the new machinery and devices
The development of the microchip made lead to computers and their high accessibility, where in today’s age, every home has at least one computer
Other developments due to microchips and microprocessors contributed to medical diagnosis and treatment as well as entertainment, communications, and much more – affecting societies higher living standards

16 Crystal Structures