Part 102: Conductors and Insulators Assignment - Answers and Commentary

Question 1

What does valence mean in terms of shells?

The valence shell is the outer-most electron shell in an atom.

Question 2

What does valence mean in terms of electrons?

The valence electrons are the electrons that are in the valence shell.

Question 3

What group of atoms are usually conductors?

Metals are all conductors of electricity.

Question 4

How many electrons are found in the outer shell of a good conductor?

The valence shell needs to have less than four electrons. The answer is less than 4, or 1, 2, or 3.

Question 5

What is the unit of resistivity?

The unit is Ωm (ohm-metre). Do not confuse this with \( \rho \), the algebraic symbol for resistivity.

Question 6

Is an electron positively charged?

No. An electron is negatively charged.

Question 7

What charge is a neutron?

A neutron is neutral. It is not charged.

Question 8

What is an ion?

An ion is an atom or molecule that has either a surplus or deficit of electrons. This gives an ion an electrical charge.

Question 9

How cold is absolute zero?

Absolute zero is 0 K, or -273.15°C.

Question 10

What element has an atomic number of 29?

Copper, symbol Cu.

Question 11

Loosely bound electrons refer to what group of elements?

Metals.

Question 12

Define conventional current.

Conventional current is defined as the flow of current from positive to negative. This may also be thought of as the flow of positive charge.

Question 13

How does the valence shell appear in a silicon atom.

Silicon is in group 14 (or group IV), meaning it has four valence electrons.

Question 14

Is silicon a base material or a donor material?

Silicon is a base material. Silicon neither has holes, nor donates extra electrons. This makes it a base material.

Question 15

Define an acceptor material.

An acceptor material is a material that accepts electrons in a semiconductor crystal lattice. To do this, it creates electron vacancies, or holes. An acceptor material may also be thought of as a material that "donates" holes (electron vacancies).

Question 16

What does p type mean?

A p-type material is an acceptor material. It is also called p-type because it has fewer valence electrons than the base material and creates holes. The hole may be thought of as a "positive charge", hence "p-type".

Question 17

What does doping mean?

Doping means the deliberate addition of an impurity to a base material to modify its electrical properties.

Question 18

Name a pentavalent material.

A pentavalent material is any non-metallic or semi-metallic material in group 15 (or group V) of the periodic table. These elements are ⁷N (nitrogen), ¹⁵P (phosphorus), ³³As (arsenic), and ⁵¹Sb (antimony). The superscript number before the chemical symbol refers to the atomic number, or protons in the nucleus.

Question 19

Define PTC.

PTC means positive temperature coefficient. A quantity that increases with temperature. For example, a PTC resistive material has increasing resistance with temperature.

NOTE: This question is quite broad, as while it can apply to resistance, it is also applicable to any number of other physical phenomena.

Question 20

Where are covalent bonds found?

Covalent bonds are usually found in molecules containing non-metallic and semi-metallic elements. These molecules have atoms that bond together by sharing electrons. Examples include hydrogen gas H₂, methane CH₄, nitrogen gas N₂, carbon dioxide CO₂, phosphorus pentoxide P₂O₅, and sulphur S₈.

Question 21

Is carbon PTC or NTC

Carbon is slightly NTC (\( \alpha = \) -0.0005°C^-1 at 20°C).

Question 22

What is Nichrome?

Nichrome is an alloy predominantly containing nickel (~80%), chromium (~20%) and other alloying elements (balance).

Question 23

Where is tungsten used in the electrical industry?

Tungsten is used for filaments e.g. light bulb filaments.

Question 24

Why is copper so popular in the electrical industry?

Copper has the best "all around" combination of price, conductivity, and workability.

Question 25

Why is aluminium not used as much as copper for conductors?

Aluminium is not as good a conductor as copper, and is often more difficult to work.

NOTE: This question is quite broad, and means little without greater context. Aluminium is widely used in distribution and transmission lines because it is cheaper and lighter for the same amount of conductivity. For small conductors (under 16 mm²), aluminium is not as widely used because it is not as conductive (making cables bulkier for the same level of resistance) as workable as copper (at least the same alloys as used for transmission lines).

Question 26

Why are rubber cables being replaced with PVC?

Rubber cables are prone to perishing, causing the insulation to crack. Heat and exposure to oxygen compounds this effect.

NOTE: This answer applies to natural rubber insulation. Synthetic rubbers such as neoprene or silicone rubber (used in high temperature cords) is not subject to perishing.

Question 27

Why is carbon good for brush material in rotating machines?

Carbon is self-lubricating.

Question 28

Varnish usually covers what conductor in which application?

Varnish is used when the maximum amount of conductor must be contained in a particular volume. A prime example of this is transformers and motor windings.

Question 29

Name a fluid insulator commonly used in the electrical industry.

A fluid is any liquid or gas. Air is a common fluid insulator, but in terms of other fluids deliberately chosen, the following apply (this is not an exhaustive list): mineral oil, castor oil, silicone oil, nitrogen N₂, carbon dioxide CO₂, hydrogen H₂, sulphur hexafluoride SF₆.

Question 30

Name the most common insulator used for cables in buildings.

The most commonly used insulator, at least by installed cable length, would be PVC (polyvinyl chloride).