Electricity

Question 1

What occurs in the battery with electrons and protons

Answer 1

Negative terminal in the battery with an abundance of electrons that want to move to the positive terminal as they have protons with a lack of electrons.

Question 2

What is the terminology of current (Amperes) and Voltage (Volts) and Resistance

Answer 2

Current - the rate that electrons (electricity) flows past a point in a circuit

Voltage - the potential difference in charge between two points in an electrical field

Resistance - how much something opposes current passing through it. (Height of a water fall, higher the waterfall the more potential energy the water has and the more energy it will have when it hits the bottom)

Question 3

What is typically the flow of electrons and protons and what is it in batteries?

Answer 3

Positive (protons) to electrons but in a battery the opposite due to the negative terminal having a lack of electrons

Question 4

What is ohms law

Answer 4

Voltage = amperage x resistance

Question 5

Explain the basic characteristics of an electrical circuit.

Answer 5

If a circuit is not complete, current will not flow.

Question 6

Describe the structure of a:

(a) two-wire circuit;
(b) grounded ‘earth return’ electrical circuit.

Answer 6

(A) Electrical circuit consists of some form of generator, a load which uses electricity and conductors that are connect the two and a switch for control.

Open circuit means current will flow. Closed is opposite

(B) grounded circuits are used to save wire and reduce weight. In a grounded circuit, the metal part of aircraft is used as a conductor.

Question 7

Distinguish between direct and alternating current.

Answer 7

DC - flow of electric current in a single direction, used for control circuits and emergency backup supply
AC - current changes direction, which is produced by an alternating current such as a Alternator.

Question 8

Explain the terms:

(a) frequency (Hertz);
(b) rectification.

Answer 8

Frequency - number of cycles per second measured in Hertz

Rectification - change of AC to DC

Question 9

Explain the properties of magnetism, including:

(a) polarity;
(b) attraction;
(c) repulsion

Answer 9

Polarity - the attractive force the aligns magnetic items and aligns itself parallel with the lines of the earths magnetic field

Attraction - unlike charges attract (N-S)

Repulsion - like charges repel (N-N)

Question 10

Distinguish between:
temporary and permanent magnets;
the properties of ‘soft iron’ and ‘hard iron’ magnets.

Answer 10

Temporary - known as soft iron which are susceptible to being magnetised but have poor retentivity

Permanent - known as hard iron, low susceptibility, high retentivity

Question 11

Describe the terms:

(a) magnetic field;
(b) magnetic flux;
(c) permeability.

Answer 11

Magnetic field - space around a magnet in which its influence can be detected.

Made up of lines of magnetic force which represent the direction in which a magnetic North Pole would move if free to do so.

Magnetic flux - number of magnetic lines of force set up in a magnetic circuit.

Permeability - resistance of a material against the formation of a magnetic field

Question 12

What are the properties of magnetic flux (4)

Answer 12

Always form a closed loop
Always start north end south
Never intersect
Magnetic lines of forces that are the same (south and south) repel

Question 13

Explain electromagnetism.

Answer 13

It is the strength of a magnetic field thats proportional to the amount of current flowing .

Question 14

Describe the lines of magnetic force around a;

(a) straight conductor;
(b) current carrying coil.

Answer 14

(A) when current flows in a wire, a magnetic field is formed around the wire, if the wire is straight, the field will form around the wire (Right hand rule)

(B) when current flows in a coiled wire, magnetic field is formed around the wire.

Question 15

Describe the construction and the principle of operation of an electromagnetic switch (relay and solenoid)

Answer 15

RELAY
Consists of an electromagnet ( coil around soft iron core) and a pivoting arm of soft iron called armature.

When electromagnet is energised, contact points are closed and current flows through the circuit, and when contact opens when control switch is off the magnetism is gone.

SOLENOID
Same build. When coil is energised a soft iron plunger is magnetically attracted into the coil and the contact disc bridges contact and closes circuit.

Question 16

Describe typical examples of the use of electromagnetic switches in aircraft electrical circuits.

Answer 16

Battery solenoids - when you turn master switch on

Starter solenoid - low amperage, which then allows a higher current to operate the starter motor

Battery to master solenoid to bus bar to starter switch to starter solenoid to starter motor.

Question 17

Explain the principle of electromagnetic induction and the requirements

Answer 17

the creation of an electromagnetic field in a conductor by means of of magnetism.

Requirements:
A magnetic field (magnet)
Conductor (wire coil)
Movement between them

And the voltage produced will depend on the strength of the magnet and speed of movement.

Question 18

Describe the principle of operation of a simple alternator.

Answer 18

An alternator produces electricity via electromagnet induction, If you turn a magnet inside a coil of wire, you will make electricity this is referred to as an alternator

Question 19

Describe:

(a) the features of a practical alternator;

Answer 19

A practical alternator uses an electromagnet (“rotor”) spinning inside a stator (made of coils of wire)

For the alternator the electromagnet requires power to excite and can only get this from the battery.

Dead battery means it wont work

Question 20

Describe how the AC output is normally rectified to provide DC and why

Answer 20

Natural output of the alternator is AC, aircraft require DC.

Rectification is done by diodes to become DC.

Question 21

Describe the principle of operation of a simple generator.

Answer 21

Generators rotate a coil of wire (rotating loop) inside a magnet which generate AC.

Question 22

Describe:
the features of a practical generator;
how the output is passed through a commutator to provide DC

Answer 22

Features - rotating loop conductor which is made to rotate within a magnet where the magnetic field created is stationary.

Output - fitting a commutator (a ring with gaps inside into the generator which inverts the AC into DC

Question 23

Explain:

(A) the need for voltage regulation for both alternators and generators;
(B) why a generator also requires a current regulator and a reverse current relay

Answer 23

(A) if voltage output is too high from an alternator or regulator, this will cause damage to the items and the electrical systems

(B) current regulator - regulates the amount of Current flow. When electric loads are high and battery is low, current regulator will prevent the generator output from exceeding its safe maximum.

Reverse current relay - stop current form the battery discharging back into the battery.

Question 24

Explain the principle of operation of a battery.

Answer 24

A battery produces electricity via a chemical reaction.

Done by the electrolytes reacting with the electrodes, causing electrons to be attracted to the negative terminal.

Question 25

What is the construction of the battery

Answer 25

Two electrodes (anode and cathode) basically metal rods

Electrolyte liquid which basically let electrons stick to anode and not go to the cathode making it proton heavy.

Question 26

With respect to batteries, distinguish between:

(a) primary and secondary cells;
(b) wet cells and dry cells;
(c) lead-acid and nickel-cadium (and similar) types.

Answer 26

Primary - cannot be recharged

Secondary - rechargeable

Wet cell - liquid electrolyte, original type of rechargeable battery and is used in GAA

Dry cell - uses a paste electrolyte and can be rechargeable

Lead acid - uses lead electrodes and sulfuric acid, rechargeable, still widely used not in aviation really though

Nickel-cadmium - more compact and lighter than lead, uses nickel oxide hydroxide and cadmium as electrodes.

Question 27

Explain the meaning of:

(a) battery capacity;
(b) thermal runaway.

Answer 27

(A) the product of current that is drawn from the battery while the batter is able to supply the load until its voltage is dropped to a certain level e.g.

60 ampere hour rating = 60 amps an hour

Or 10 amps for 6 hours etc

(B) basically when battery becomes uncontrollable and enters a self heating state as a result of the battery releasing electrons causing them to heat up and making resistance drop.

Drop in resistance allows more current to flow, leading to further heating.

Question 28

Explain the advantages of NiCad batteries.

Answer 28

Becoming more common in aviation
•Can withstand more charge-discharge cycles than other batteries
•Continues to give a constant voltage (1.2 V per cell) until almost totally ‘flat’
•Can deliver their full capacity at a high rate