Download - Electrical Theory
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Electrical Theory turn to chapter 8 in the A & P mechanics general handbook
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All matter can be defined as anything that has mass (weight),
So matter is everything that exists
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The smallest particle of matter is called an atom which is used to make up a molecule.
Water for example is 2 hydrogen atoms and 1 oxygen atom
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Within these atoms we have electrons rotating around protons
electron is an negative charge of electricity
protons which are a positive charge
Animation click
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The atom is considered neutral if the electrons and the protons are equal
If there are more electrons (negative) than protons then the atom has a negative charge.
If there are more protons than electrons it is said to have a positive charge
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When objects have the same charge they will repel (pull away) each other, but if they have opposite charges they will be attracted to each other.
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Electrons flow from a negative point to a positive point that is called current flow. CLICK
It flows because of the difference in pressure between the 2 points.
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This can be compared to the flow of water between 2 tanks that have different pressures (fig 8-12).
When the water in the 2 tanks is the same the flow will stop.
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-If one tank had 10 psi in it and the other had 2 psi, a difference of 8 psi would exist. -The same is true in an electrical circuit as the potential difference between 2 points is measured in 8 volts.
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Direct Current (DC) is when the electrons move in one direction only
AC or alternating current is when the flow is alternating from one direction to another
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Using a garden hose analogy the current would be similar to the moving water in the hose.
Click for flow animation
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Resistance is the ability of an electrical conductor to restrict the flow of electrons
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The unit used to measure resistance is called an ohm . It is said that an ohm is the resistance needed to limit 1 amp to flow from a charge of 1 volt
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The 4 factors effecting resistance are
1) the material
2) The length {see fig 8-14}
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3) cross sectional area ( wire gauge)
4) Temperature { usually increase in the temp of the conductor means an increase in the resistance
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The basics of an electrical circuit are
1) a power source (electrical pressure-battery)
2) Resistance in the form of an energy consuming device
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3) conductors for the current path (usually copper wire).
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- With out resistance in the circuit the conductors would become overheated and burn
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-Variable resisters (fig 8-34) are used when the circuit is in operation. Where would we see this in operation around the house?
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Common symbols for a generator are
Fig 8-19 shows the symbols for a single cell battery and a three cell battery. Battery symbols always include 1) a short vertical line representing a negative terminal,2) a longer line for the positive terminal
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- fig 8-26 shows a typical view of a fuse which is installed in most circuits to protect it against excessive current flow.
Fig 8-27 shows an open switch and a closed switch
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A typical ground symbol is shown in fig 8-28
Notice how the ammeter and voltmeter are show as they need to placed within a circuit
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- Pg 287 shows several series circuit schematics. Fig 8-54 shows the current flowing from negative to positive ( there are 2 schools of thought on this) through a resistance.
8-55 shows a proper circuit with a battery , switch, load & fuse.
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- The figures on pg 276 show:
- A series circuit where the voltage is added together and the current stays the same
- A parallel circuit where the voltage stays the same and the current is added together
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By placing your thumb over the unknown value in the ohms law triangle . The formula to solve becomes apparent
In this case by covering “I” (for amps). The formula volts ÷ resistance will solve for how many amps will be drawn in this circuit
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250 Ω
0.48 Amp
0.48 Amp
0.48 Amp
0.48 Amp
28.8 V
43.2 V
48 V
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16.2 Ω
120 volts
120 volts
120 volts
3.0 amps
2.4 amps
2.0 amps
7.4 amps
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THE END