Electricity And Magnetism, Electrical System - DC Flashcards
Define: Electrical circuit
A closed path formed by electrical conductors along which the electrons flow = closed circuit
If a circuit is not closed, the current will not flow = open circuit
Define: Conductor
A material that allows the efficient flow of electric current through it with minimal resistance
Define: Earth return circuit (grounded circuit)
An electrical circuit where the aircraft’s fuselage is used as part of the conductors.
Define: Electromotive force (Volt, V)
Voltage = the difference in electric potential between two points in an electrical field
Define: Resistance (Ohms)
How much something opposes current passing through it.
Depends on:
- Material - different materials have different resistances.
- Length - the longer the conductor, the higher its resistance.
- Cross-sectional area - The larger the cross-sectional area of a conductor, the lower its resistance
- Temperature - generally increases with temperature
What is Ohms law?
The current is directly proportional to the voltage and inversely proportional to the resistance.
Voltage = Current x Resistance
Define: Direct current (DC)
A type of electrical current where the flow of electric charge (electrons) is constant and moves in one direction.
What is the main advantage of direct current?
Simplicity and consistency.
- Commonly used in many of the electrical systems, supplies by a battery and/or an alternator or generator.
Define: Alternating current (AC)
Type of electrical current where the direction of the flow of electrons switches back and forth at a regular interval.
What is a cycle?
Two direction changes
What is a frequency cycle measured in?
Hertz (Hz)
1 Hz means the current would go through 1 cycle in 1 second.
What is magnetism?
A physical phenomenon produced by moving electric charges. Results in attractive and repulsive forces between objects.
- Caused by the motion of electrons
- Metals become aligned with a common direction which results in the material becoming magnetised.
What is polarity?
Magnetic field lines flow North to South Pole.
- Cannot isolate a single pole
- Earth acts as a large magnet
- In a compass, the north end of the compass needle is attracted to Earth’ Magnetic pole
Explain attraction and repulsion.
Magnets attract certain materials and repel others.
- Two north poles or two south poles are brought together = repel
- A north and a south pole are brought together = attract
What is magnetic flux?
The number of magnetic lines of forces set up in a magnetic field is called magnetic flux.
What are the properties of magnetic flux?
- Always form a closed loop
- Always start from the north and end in the South Pole
- Never intersect each other
- Magnetic lines of forces that are parallel to each other and are in the same direction repel each other
What is magnetic permeability?
Resistance of a material against the formation of a magnetic field. The higher the magnetic permeability of the material, the more it supports the formation of magnetic fields.
- Measured in Henry per metre
What are temporary magnets known as?
Soft iron
What is soft iron?
- Type of magnetic material that is easily magnetised and demagnetised.
- Low coercivity (requires relatively small magnetic fields to magnetise it, and loses its magnetism quickly)
- High magnetic permeability (conducts magnetic flux well)
What is hard iron?
- Permanent magnets
- Difficult to magnetise and demagnetise
- High coercivity (requires large magnetic fields to magnetise it, retains magnetism)
- Low magnetic permeability (conducts magnetic flux poorly
Explain electromagnetism.
When electrically charged particles interact with each other and with magnetic fields.
- Electric charges: interaction between positive and negative charges makes an electric force.
- Electric current: when electric charges flow through a conductor, creating a current. This movement generates a magnetic field.
What is a straight conductor?
When a current flows through a straight conductor, it creates a magnetic field around the conductor.
- The lines of magnetic force around a straight conductor are concentric circles around the conductor.
- Strength of the magnetic field decreases as you move farther from the conductor
What is a current carrying coil?
- AKA a solenoid
- Generates a magnetic field when an electric current flows through it
- Magnetic field lines inside the solenoid are nearly straight and parallel to the axis of the coil.
- Outside of the coil they are curved
- The denser (the closer they are together) the magnetic lines the stronger the field
What is an electromagnetic switch (relay)?
An electrical component that allows a low-power control signal to switch a higher-power circuit on or off.
What does a relay consist of?
- Electromagnet ( a coil of wire wound around a soft iron core)
- Armature (a moveable metal contact)
- A spring
- One or more sets of contacts
What is the principle operation of an electromagnet?
- Low-power control signal (voltage) is applied to the coil - creates a magnetic field, turning it into an electromagnet.
- Magnetic field attracts the armature which causes the moveable contact to either make or break the connection with the fixed contact.
- When the control signal is removed, the magnetic field collapses and the armature is returned to its original position by the spring.
What are the three requirements for electromagnetic induction?
- A conductor e.g. wire or a coil
- Magnetic field
- Relative motion between the conductor and the field
What is the process of electromagnetic induction?
- When the conductor moves through the magnetic field it induces an electromotive force (EMF) in one direction.
- Voltage can increase by increasing the speed at which the conductor moves through the field or by increasing the magnetic field.
- Direction of EMF can changed by reversing the movement of the conductor by switching the polarity of the magnetic field.
What is a simple alternator?
Device that converts mechanical energy into electrical energy.
What does a simple alternator consist of?
- Stationary set of coils - stator and rotating magnet called a rotor.
What is Faraday’s law of electromagnetic induction?
A change in magnetic field strength induces a voltage in a nearby conductor.
How does a simple alternator work?
- As the rotor spins, the magnetic field around it changes
- Induces voltage in the stator coils - produces an alternating current (AC) output.
- Alternator is driven by a belt - speed of alternator is directly proportional to the engine speed.
As the engine speed increases, the output voltage and current of the alternator also increase.
What is a practical alternator?
The magnetic field is created using an electromagnet rather than a permanent magnet.
- To create electricity - requires DC current, coming from the battery
- Equipped with voltage regulators to maintain a constant output voltage under varying load conditions.
What is a rectifier?
A device used to change AC to DC.
What is a simple generator?
Converts mechanical energy from the engine into electrical energy
How does a simple generator work?
Rotates a conductor inside a fix magnetic field. Does not require electrical power from the battery as is does not have an electromagnet.
- As the coil rotates, it experiences a changing magnetic field, this causes an electric current to flow through it, generating electrical energy.
What are disadvantages of a simple generator?
- Bigger and heavier
- Generates lower amount of current per RPM, therefore less suitable for low RPM situations.
What is a practical generator?
Used to produce a greater amount of energy - is fitted with multiple coils creating the armature of the generator.
- Permanent magnet is replaced by an electromagnet called a “field coil”.
- Generates AC current.
What is a commutator (AC to DC)?
Designed to convert alternating current (AC) generated by the coil into direct current (DC).
- Is a rotating mechanical switch
What does the output’s voltage of an alternator and generator vary on?
- Speed of the rotor or engine RPM
- Number of conductors
- Strength of the magnetic field
Does this by reading the output’s voltage and then applying a correction on the magnetic field.
What is the principle operation of a battery?
Stores and provides electrical energy.
Three main parts:
1. Negative part. = anode
2. positive part = cathode
3. Substance in between them (electrolyte)
- Electrons flow from the negative part to the positive part through the device
Battery capacity
- The amount of electrical energy that a battery can store and provide when fully charged
- Measured in ampere-hours (Ah) or milliampere-hours (mAh)
What are primary cells?
- Batteries that are designed for single-use and cannot be recharged.
- Once they are depleted, battery is considered dead and must be disposed of.
What are secondary cells?
Rechargeable batteries.
What are wet cells?
- Liquid electrolyte batteries, contain a liquid electrolyte that facilitates the helical reaction required for the battery to function.
- Require maintenance
What are dry cells?
Use paste-like or solid electrolyte instead of liquid.
- Portable
- Less prone to leakage
What is a lead acid battery?
- Wet cell battery that uses lead plates and a sulfuric acid electrolyte.
- High-power output and relatively low cost
- Heavy
- Performance degrades over time
- Environmentally hazardous (toxic)
Nickel-cadmium (NiCd) battery.
- Rechargeable battery that uses nickel oxide hydroxide and metallic cadmium as electrodes.
- Delivers high discharge
- Lighter
- Suffer from “memory effect”, battery loses its capacity if it is not fully discharge.
What is thermal runaway?
When an exothermic reaction within the battery accelerates due to an increase in temperature, leading to a further increase in temperature and ultimately causing the battery to fail, overheat, or even catch fire or explode.
- Dangerous in aviation as a fire while airborne can be catastrophic.
What type of electrical system does GA aircraft predominantly use?
Direct Current
- Simplicity and cost
- Compatibility with batteries
- Weight considerations
What is a bus bar?
Metallic strip which functions as the primary conductor and distributor for power distribution in an electrical system.
What is the function of fuses and circuit breaker?
To protect the electrical system from electrical overload.
When and how long should you wait to reset a circuit breaker?
- If you can smell or see smoke
- After 2 minutes
What is an ammeter?
An instrument used to measure electric current in a circuit.
- Left zero ammeter - indicates the current flow from the alternator/generator only, not the battery.
- Centre zero ammeter - indicates the current flow into or out of the battery.
What is a starter motor?
Draws high amperage current from the battery to start the engine.
What is excessive charge rate?
Over voltage is when there is excessive current being produced.
- Battery overheating
- Damaged electrical and radio systems
What is alternator/generator failure?
Identified by left zero ammeter.
- Does not mean that the engine will stop working.
What is a total electrical failure?
- Due to failed alternator and flat battery, failed wiring, failed master switch, overload or short-circuit.
- Most aircraft have back up systems
What is a technically enhanced aircraft?
An aircraft in which the primary flight instruments and navigation information are displayed electronically.
- Draws loads of 25 Amps
- In the event of a alternator/generator failure, battery will be flat in less that 20 minutes
