Help explaining the general terms of electricity to a student pilot (and help understanding it myself)?

Question

Hey.

As a flight instructor in training I'm trying to get a good grasp as possible on the things I have to explain to my students.

One of these things, that are not really practiced in ground school or in any other aspect of flight training is electricity.

Basically I'm trying to explain the avionics and electrical system of an airplane. Involved in these schematics and numbers are terms like amperes, ampere-hours and volts.

Now I've had some degree of physics in high school, and a general like for electricity in my childhood so I think I understand enough of it to get a grasp myself, however, I do not really have a good enough understanding to teach it. Therefore I'm trying to come up with good analogies and explanations for the questions that might arise, and to be able to explain to a student with no previous experience with electrical terms to at least get a basic understanding of it.

For analogies using amperes and volts, I usually use the "garden hose analogy", where volts is the pressure, and ampere is the actual flow of water, you can also say the size of the hose is resistance, but that's not really covered in any aspect of flight training.

Now onto the questions, can people help me explain these things:

1. Why do an electrical system - that includes a battery and an alternator - have the alternator at a higher voltage than the battery.

Example: in one airplane you have the alternators running at 14V and the battery at 12V. My logical answer would be that the battery needs to run at a lower voltage to be charged when the alternator is running, but why does the battery only charge when it's at a lower voltage if that's the case?

2. What is a solenoid? In many electrical diagrams in airplanes I see the term solenoid being thrown around, but I've never actually figured out what their purpose is

3. Can you say that the voltage is what determines how much electricity can potentially flow in a system? If yes (or if no), why do electrical systems in cars and airplanes run at a relatively low voltage like 12 or 24. Wouldn't the electrical system be more effective at a higher voltage like triple digits?

Would a 12volt system run through a specific circuit have a limit on the amount of amps it could carry in relation to a higher voltage system?

4. An alternator produces amperes (a measure of electrons passing by a given point over a given time, right?), however, overvoltage from the alternators is always a concern in airplanes. What causes an overvoltage, and what in itself determines the voltage in an electrical system, and how can you change or set the voltage in a circuit?

Finally, why is a higher voltage bad for an electrical system, and what is the damage potential and causes?

5. Demand. How does an alternator, or a device connected to an electrical circuit draw power?

Say you have a device connected to an alternator that at its peak can generate 60 amps. The device only needs 10 amps.

Will only 10 amps flow through the circuit? Or will 60 amps flow through and only 10 of them be used (same example, only a normal household outlet, same result?).

How does the alternator "know" how much electricity to produce to the devices connected, do they actually "draw power" through the circuits in some way?

Hope these questions are not all to far off, and thanks for any answers you guys might have!

Answer 1

Water is often good for helping someone under stand electricity.

Start off explaining voltage is like the pressure that makes water move when you open the faucet. Current is the movement of water. Resistance is what keeps all the water from coming out of the hose at once.

1. Voltage is a difference in potential. When you open the faucet, the current flows from the high pressure to a lower pressure. A battery is a storage device. to get current into the battery, the alternator has to provide a higher pressure (voltage) than the battery. This causes the current to flow into the battery, charging it up. Basically you are moving the electrons from the negative side of the battery to the positive.

2. A solenoid is an electromagnet with a metal rod that is used to move something. When you put current through the wire of the electromagnet, magnetism moves the metal rod.

3. yes. The voltage is the pressure that causes current to flow. Resistance is the opposition to current flow. (current equals voltage divided by current) A garden hose will allow a certain amount of current to flow. Increase the pressure and you get more current. You could also decrease the resistance ( a fire hose) and more current will flow.  The voltage in vehicle is sufficient for the needs.

4. Over voltage can be caused by a high current draw on the system. This causes the alternator to work harder to produce current. Then when the need for the high current goes away, there is a lag before the alternator cuts back. this causes an over voltage. The components in the circuit are designed to handle a certain amount of power (power equals voltage times current). Too much power can burn the damage the components. Damage potential can be a failed system or fire.

5. The alternator has a regulator that that controls he output voltage level. As the resistance of the circuit decreases (the same as if you open all the water faucets in the house), it draws more current , the pressure (voltage) would drop, the regulator allows more current to flow thereby maintaining the voltage.

It is important to understand Ohms law. E=IR and P=IE. P=power, E=voltage, I=current, and R=resistance. The way this works together, if you have a 120 watt (power) bulb connected to your house 120 volt, P=IE or 120 W=120V * 1A. So you have 1 amp current. The to find resistance, 120v = 1a time 120 ohms.

If you have a 12 volt system drawing 10 amps, it is 120 watts. or 20 amps would be 240 watts.  

Answer 2

1)  For the alternator to charge the battery, the current must flow INTO the battery.  For that to happen, the alternator voltage must be higher.

2) In general, a solenoid is any tubular coil of wire, but it is used to describe a mechanical actuator in which that tube of wire has a (soft iron) metal core that is moveable.  When current is passed through the coil, the metal core is attracted into the coil.  This motion can be used to open and close valves, switches etc.

3)  Your analysis is basically correct.  You get the same power from a higher voltage with lower current, since power is voltage times current.  Since power losses increase with increasing current, higher voltage circuits can be more efficient.  Cars used to have 6v batteries, now use 12;  airplanes will use 24 or 48v.  The reason higher voltages are not used come from battery design limitations.

4)  Alternator voltages depend on the RPM, and you have to get enough voltage at low RPMs but that can produce too much voltage at high RPMs.  Therefore, voltage regulators are part of an alternator circuit.  The regulator actually sets the voltage.   Many electrical components will break down at voltages higher than what they were designed for.  All insulators have breakdown fields.  Semiconductors and other electronic components can also be damaged by excessive voltage.  High voltages produce strong internal electric fields which can "tear apart" the atoms or molecules of the materials.  Sometimes the effect of high voltages is to generate excess current, which causes overheating and damage to components

5)  A  good regulator design does not draw any more power from the source than necessary for its operation.  For a simple example consider the old-fashioned mechanical voltage regulator.  It contains a voltage-sensitive switch that opens if the ouput voltage exceeds a certain value, disconnecting the source from the load.  Of course, the output voltage will immediately drop and the switch will close reconnecting the load.  This happens repeatedly at a high rate, but the average voltage will be the switch set point.  Current flows from the source only when the switch is closed.  So if the load is light, the switch is open more time than it is closed.   As load current increases, the close time increases drawing more current from the source.  So the regulator always maintains the right voltage and supplies only the current needed by the load.