Electron Theory & Resistance Flashcards
What is an atom?
Smallest particle that still retains its chemical properties
What is an element?
A chemical group depending on their individual atoms
What determines an element’s atomic number?
Number of protons in the nucleus
When atoms of an element have a different number of neutrons, what are they called?
Isotope
What is a negatively charged ion and a positively charged ion called?
Anion - Negatively charged
Cation - Positively charged
What is a compound?
A chemical combination of two or more elements
What is a molecule?
The smallest particle a given substance can exist in
What is it called when an atom forms molecules by sharing electrons between their valence shell?
Covalent bond
How many electrons are there in the 1st, 2nd, and 3rd shells?
2, 8, 18
What two factors determine molecular stability?
- Whether valence shell is full
- Whether molecule is electrically neutral
What can happen if the valence shell is not full?
It will attract the opposite charge, forming ions or covalent bonds
How many atoms are there in a valence shell of a conductor?
3 or less
Why are metals good conductors?
They have 1 or 2 electrons on valence shell and are easily released.
- When an electron is introduced, it repels nearby electrons to form positive ions which temporarily join the valence shell and move on.
- VALENCE TO VALENCE FLOW (current)
Order these metals from best to worst conductors:
- Copper
- Aluminium
- Gold
- Silver
- Silver
- Copper
- Gold
- Aluminium
What is an example of a liquid conductor and how does it transfer current?
Mercury
- Ionic Conduction: Not true electron flow but instead charged molecules (ions) moving through the surface.
How many electrons do the valence shells of insulators have?
5 or more
- tries to maintain a full valence shell (no free electrons)
How many electrons do the valence shells of semi-conductors have?
4 valence electrons
What are semi-conductors?
Semi-conductors are insulators with no free electrons, but instead a very loosely bonded electron (loosened w/small voltage)
- Has properties of conductors and insulators
- Can conduct current but only to a limit before damage
- Silicon, Germanium
What is the charge distribution of a discharged and charged battery?
Discharged:
- both plates have equal charge. No voltage, hence, no current.
Charged:
- Electrons are removed from the positive plate and added to the negative plate.
- However, no conductive path = no current.
How does chemical action generate current?
Certain chemical and metal combinations create a concentration of electrons.
- When a conductor is placed between 2 points, electrons move from high concentration to low.
- The negative ions from the solution move towards the negative plate. Eventually the solution loses its ions and current flow stops.
How does friction induce a current?
2 materials rubbing created a concentration of electrons which raises voltage. A conducting path between them raises allows the voltage to discharge.
How can heat induce a current?
Thermocouples generate a voltage due to a heat difference of a junction of 2 dissimilar metals at 2 junctions.
- Heat energy added generated different amounts of free electrons in the 2 dissimilar metals. If hot and cold junction are at the same temp the conductivity is equal and opposite = no current.
- When heat is added, the hot junction generates voltage (free electrons)
What junction is the millivolt meter placed in a thermocouple?
Cold junction.
Does the current generated in an EGT thermocouple indicate true engine temperature?
No, it only represents a temp difference between the hot and cold junction.
- True engine temperature requires temp compensation in the gauge (e.g. bi-metallic adjusting mechanism for ambient temp)
What are typical high temp EGT thermocouples made of?
Alumel-Chromel.
How can light induce a current?
Photovoltaic cells:
- The photons ‘knock loose’ the electrons from the semi-conductor valence shell.
- Electrons only flow in 1 direction, ‘holes’ flow in the opposite direction.
How does magnetism generate a current?
When a conductor moves through lines of flux, a voltage is induced (alternators, generators).
How does pressure induce a current?
Piezoelectric effect:
- Crystals produce voltage when bent or distorted
- e.g. Microphones convert sound waves into electrical signals using a diaphragm to produce AC voltage
- e.g. Gas burner igniters (piezo-lighters) use a spring-loaded mechanism to compress crystals rapidly to generate high voltages for ignition.
What are the 2 biggest problems with static electricity on aircraft?
- Radio frequency interference
- Fuel lines and fuel tank (explosions)
What is Coulombs Law?
Charged objects experience a force between them depending on magnitude and distance squared (separation)
- F=(kQ1Q2) / d^2
How is charge distributed on objects?
Charges distribute evenly on smooth symmetrical surfaces and concentrate around sharp points.
- In hollow objects, the charge concentrated on the outside, leaving the inside free of charges.
What unit is used to measure electrical charge?
Coulomb (Q)
- 1 Coulomb = 6.28x10^18 electrons.
How does static charge conduct?
It does not move, but it can slowly leak through humid air or conductive paths (e.g. trailing edge of wing)
- Can’t be controlled without hazard due to currents and power involved.
How does Ion conduction work?
- Most liquids and gases allow ionic conduction where charges are transported by charged molecules.
- e.g. lightning strikes.
Define 1 Volt
The EMF required to make 1 ampere flow through 1 ohms of resistance.
- V=IR
What 2 factors determine rate of electron flow?
- EMF from battery
- Circuit resistance
Define 1 ampere
1 coulomb of electron per second
Define 1 ohm
the resistance when 1 volt is applied and the circuit produces 1 amp.
What is conductance and what are its units?
Ability to conduct current, opposite to resistance.
- Unit: Siemens (s) or Mhos
- G=(I/R)
What is a short circuit?
When an unintended alternate path is formed for the current to short cut.
- Results in damage due to the overall circuit resistance lowered, allowing high current.
- Prevented with fuses and CBs
