From Ideas To Implementation

Question 1

What do moving charges in a magnetic field experience?

Answer 1

A force.

Question 2

What did Heinrich Geissler invent in 1855?

Answer 2

A vacuum pump efficient enough to reduce the pressure in a glass tube to small fraction of normal air pressure (0.01%). By placing metal electrodes in these tubes it was possible to get electricity to flow through the tubes

Question 3

What are striation patterns?

Answer 3

As the pressure is reduced in a discharge tube, different striationn patterns were produced that depended on the gas pressure.

Question 4

At what pressure does a green glow appear in the glass tube behind the anode and opposite the negative electrode (cathode)?

Answer 4

~0.01 kPa (kilopascals).

Question 5

What did Crookes deduce in 1875 about the green glow in the glass tubes?

Answer 5

The green glow was found to be caused by negative electrons emanating from the negative electrode.

Question 6

Charged particles can be affected by what?

Answer 6

Both electric and magnetic fields,

Question 7

What is the property of the Maltese Cross cathode ray tube?

Answer 7

The rays travel in straight lines.

Question 8

What is the property of the Paddle Wheel cathode ray tube?

Answer 8

The rays carry energy and momentum.

Question 9

What is the property of cathode ray tube when a magnet is brought near the fluorescent screen?

Answer 9

The rays are deflected by magnetic fields.

Question 10

When and by who was the nature of cathode rays - whether they are a wave or particles determined by?

Answer 10

In 1887 J.J Thomson determined their nature as being particles, some years later his son G.P Thomson showed cathode rays to be waves.

Question 11

Why is a parallel plate capacitor a useful device?

Answer 11

Because it provides a uniform electric field, the intensity of which is calculated from the equation E=V/d.

Question 12

What ratio did J.J Thomson successfully measure?

Answer 12

The charge to mass ratio for cathode rays.

Question 13

What are the three main components of a cathode ray tube?

Answer 13

The electron gun, deflecting system and fluorescent screen.

Question 14

What are cathode ray tubes found in?

Answer 14

Electron microscopes, oscilloscopes and TV’s.

Question 15

What is an oscilloscope used for?

Answer 15

To ‘view’ electrical signals or waveforms. Its invention has had a significant impact on experimental physics.

Question 16

When Hertz measured the speed of radio waves what did he find it to be?

Answer 16

The same as the speed of light. He thus deduced that light is a transverse electromagnetic wave.

Question 17

When studying the generation of electromagnetic (radio) waves what did Hertz discover?

Answer 17

The photoelectric effect, but failed to investigate it further.

Question 18

What are black bodies?

Answer 18

Perfect emitters and absorbers of radiation.

Question 19

Could black body radiation curves be explained by classical physics?

Answer 19

No.

Black body radiation curves, which relate the maximum frequency of emission to the temperature of the black body, are unable to be explained by classical physics (the physics of Newton and Maxwell).

Question 20

In 1900, what did Maxwell Planck propose in an attempt to explain the black body radiation curves?

Answer 20

Max Planck proposed that emission and absorption of electromagnetic radiation for a black body is quantised. Meaning that the energy is not continuous but is in discrete amounts. These amounts are given by E=hf where E is energy, h is Plancks constant (6.626x10^{<span>-34</span>} J.s) and f is the frequency of the light.

Question 21

What is the photoelectric effect?

Answer 21

The emission of electrons from substances when irradiated (exposed to radiation) with light (that is, electromagnetic radiation).

Question 22

What was the major conflict between the results of experiments on the photoelectric effect and classical physics?

Answer 22

The experimental results indicated that emission was frequency dependant, that is, below a certain frequency no electrons were emitted regardless of the light intensity.

Classical physics predicted no frequency dependence.

Question 23

How did Einstein explain the photoelectric effect, and in doing so what did he prove?

Answer 23

In 1905, Einstein explained the photoelectric effect and in doing so, proved the particle nature of light. He proposed that energy is concentrated in ‘bundles’ called photons with energy given by the Planck relationship E=hf.

Question 24

The photoelectric effect is put to use in solar cells. What are solar cells?

Answer 24

Solar cells are employ the semi-conductor silicon and are used to convert solar energy directly into electrical energy.

Question 25

The photoelectric effect is put to use in photocells. What are photocells?

Answer 25

Photocells are photoelectric cells in which the electron of an electric current are produced by the photoelectric effect; they are used in devices such as electric ‘eyes’, radiation detectors and light meters.

Question 26

What happens when two or more atoms are brought near to eachother?

Answer 26

Their energy levels split. As a result, electrons can be shared between atoms. For a finite amount of matter containing billions of atoms, the individual energy levels form a continuum resulting in energy bands.

Question 27

In the band model of matter, what are the two bands called?

Answer 27

The energy bands are the valence band and the conduction band, seperated by the forbidden energy gap.

Question 28

Do conductors, semiconductors and insulators differ in their band structure?

Answer 28

Yes, as shown in the diagram.

Question 29

Is it possible for an electron to jump from the lower (valence) band to the higher (conduction) band?

Answer 29

In some circumstances it is possible.

When the electron jumps from band to band, it leaves a (positive) hole behind. Holes represent the absence of electrons in a nearly full band.

Question 30

Can both holes (positive charge) and electrons (negative charge) carry electric current?

Answer 30

Yes.

Question 31

Do conductors have more or less free electrons than semiconductors?

Answer 31

More.

Conductors have many free electrons, semiconductors have a few and insulators have almost no free electrons.

Question 32

What is meant by ‘doping’ a material?

Answer 32

When a Group III or Group V atom is substituted for a Group IV atom of germanium or silicon we say the material has been doped.

Question 33

What does doping a semiconductor achieve?

Answer 33

Doping a semiconductor alters its electrical properties. The electrical conduction can be improved significantly because it provides additional charge carriers.

Question 34

What are p-type semiconductors?

Answer 34

In p-type semiconductors, a Group III atom is substitued so holes are the majority carriers (and electrons are the minority carriers).

Question 35

What are n-type semiconductors?

Answer 35

In n-type semiconductors, a Group V atom is substituted and so electrons are the majority carriers (and holes are the minority carriers).

Question 36

What was the first semiconductor material and why?

Answer 36

Germanium because of its relative ease of purification.

Question 37

What is now the preferred semiconductor material and why?

Answer 37

Silicon as it is more abundant and hence cheaper, and it retains its semiconducting properties at higher temperatures than germanium.

Question 38

What is thermionic emission?

Answer 38

The spontaneous emission of electrons from solids when heated to high temperatures.

Thermionic valves rely on this effect for their operation.

They include the diode, triode and pentode, the latter two being used as amplifiers in old-style (and bulky) radios, televisions, etc.

Question 39

What are solid-state devices?

Answer 39

Solid-state devices have all but replaced thermionic devices. This is because solid state devices can be made much smaller (miniaturised), use less power and reacy faster.

Question 40

Why have transistors had a significant impact on society?

Answer 40

They have led to the computer driven ‘information age’ that has revolutionised communication.

Question 41

What did the Braggs father-son team investigate?

Answer 41

The internal structure of crystals by using X-rays to form diffraction patterns. They were able to show that crystals had atoms arranged in a regular pattern and were also able to determine the interatomic spacing.

Question 42

What is X-ray crystallography?

Answer 42

An important investigative technique into the structure of materials.

Question 43

By using X-ray crystallography what were scientists able to show?

Answer 43

That metals possess a crystal lattice structure.

Question 44

How many electrons do metals tend to have in their outer shell?

Answer 44

One, two or three electrons which are only loosely held by the atoms and hence are able to move.

Question 45

What is the electron sea model?

Answer 45

The electron sea model of metals pictures them as having a lattice of positive ions surrounded by a ‘sea’ of electrons.

Question 46

How does electricity conduction occur in metals?

Answer 46

Electricity conduction in metals results from the flow/drift of a large number of electrons through the lattice.

This drift results when a potential difference is applied between the ends of the metal conductor and is superimposed on the otherwise random wanderings of the electrons.

Question 47

Why is heat generated during conduction?

Answer 47

Heat is generated as the lattice impedes the electrons.

In fact it is the imperfections in the lattice and the presence of impurity atoms that restricts the movement. This restriction of the flow of electrons is what gives a conductor its resistance.