ASVAB: Electronics Information Review

Circuits

Charge & Current
Charge is a physical property of protons (+1, positive charge), neutrons (0, neutral), and electrons (-1, negative charge), which together make up atoms. Like-charges repel each other (positive and positive or negative and negative), and opposite charges attract (negative and positive). Net charge can be found by adding up the protons and electrons that make up an atom or molecule.
Electric current is the net rate of flow of charge (electrons) past a specific point, as in a wire or circuit. Current (represented by a capital I in equations) is measured in Amperes or Amps (A). A. Ampere is equal to roughly 6.24 x 1018 electrons/second. Ammeters are tools that can be used to measure current.
Electron flow theory states that electrons (e-) flow from the negative terminal to the positive terminal. Electricity is the form of energy the exists statically (potential energy) with an accumulation of protons or electrons or dynamically with their movement (current).

Ohm's Law & Circuit Components
Voltage (V in equations) is the electric potential energy per unit charge between two points which causes current. It is measured in volts, can be generated by batteries or generators, and can be measured by a voltmeter.
Resistance (R in equations) is a material's ability to resist current; it is measured in Ohms which are represented by the Greek letter Ω (omega). Resistors are electrical components designed to resist current in a circuit (a closed loop through which electrons can flow). Resistance can be measured with a tool called an Ohmmeter; it can also be determined for various materials using the following equation because resistance increases with greater material length (L) and resistivity () and decreases with a greater cross-sectional area. Resistivity () is the physical property of resistance of different materials and is usually given (example: aluminum or silver).


Ohm's Law describes the relationship between current (I), voltage (V), and resistance (R) in a circuit and can be written in the following ways:





A. voltage increases or as resistance decreases, current
Example: Find the current through a 6 Ω resistor if the voltmeter reads 12 V across the resistor.









Series Circuits
A series circuit is a loop through which charge can flow along only one path. Resistance of resistors (labelled Rn) lined up in series is additive, so Rtotal=R1+R2+R3+… for all resistors in that series. Current (I) is consistent in all locations throughout the closed series circuit, so Ibattery=IR1=IR2=IR3, regardless of the number of elements in the series. Charge at the positive terminal of the battery experiences a voltage drop (∆V), or loss of potential energy, as it passes through each resistor, such that its voltage will be zero at the negative terminal of the battery. Therefore, ∆Vbattery=∆V1+∆V2+∆V3+…, where each ∆Vn represents the voltage drop across each resistor in series.












Problem Solving with Series Circuits
Using the labelled diagram of a series circuit, solve the following problems:
Example 1: If each resistor is 3Ω and the battery is 9V, what is the current at each of the indicated corners?
 

Step 1:
Given:

;

;


Equation:


Solve:

  Step 2:
Given:


Equation:

  so


Solve:

  Step 3:
Because current is the same throughout the circuit
(Ibattery=IR1=IR2=IR3), current at each corner is the same:
I1 = I2 = I3 = I4
= 1 A
Example 2: If the current at point 3 is 4 Amps, and each resistor is 1Ω, what is the battery voltage?
Given: I3 =
4 A
Equation: Ibattery=I1=I2=I3=I4
Solve: Itotal = 4 A
Given: R1=1Ω; R2=1Ω; R3=1Ω
Equation: Rtotal=R1+R2+R3
Solve: Rtotal = 1Ω + 1Ω +1Ω = 3Ω
Equation: Vtotal=Itotal(Rtotal)  
Solve: Vtotal = (4A)(3Ω) = 12 V

Parallel Circuits
Parallel circuits provide multiple pathways for current to follow, which adjusts the way that current, voltage, and resistance interact when compared to series circuits.
Because current (I) can follow multiple pathways, it is divided among those pathways at each branch, so Itotal=I1+I2+I3+…, where 1, 2, and 3 represent each unique branch. In the example, there are 3.


Equivalent resistance (Req) is the amount of resistance in a parallel circuit, represented as if there was only one resistor in series. All of the resistors in parallel can be simplified using the following equation:


Because a charge will only pass through one resistor in the parallel circuit and not all three, the voltage drop across each resistor will be the same as that of the battery.
Therefore, voltage drops for parallel circuits follow this rule:


Note: If multiple resistors exist on a branch of a parallel circuit, they are in series and should be added together (Rtotal=R1+R2+R3+…) before dealing with them as part of a parallel circuit.

Problem Solving with Parallel Circuits a parallel circuit, solve the following problems:

is 3Ω and the battery is 9V, what is the current at each of the indicated points
(1-8) and each resistor?

Step 1:
Given: R1=3Ω; R2=3Ω; R3=3Ω
Equation:


Solve:


Req = 1


Given: Vbatt=9 V
Equation/Rule: Vbatt=(Itotal)(Req) = ∆V1 = ∆V2 = ∆V3
Solve: Itotal =

 = 9 A
Equation/Rule: ∆V1 = IR1(R1)=
∆V2 = IR2(R2) =
∆V3 = IR3(R3)
Solve: IR1 =

 = 3 A (same for each branch in this example because R1=R2=R3=3Ω
Using this information and the additive properties of current in parallel circuits, the current through the following points and resistors is:
I1 = I2 = I7 = I8= Itotal = 9 A
IR1 = IR2 = IR3= I4 = I5 = 3 A
I3 = I6 = 6 A point 4 is 4 Amps, and R1=3Ω; R2=2Ω; R3=6Ω, what is the total current?
Given: R3= 6 Ω; I4 = IR3
= 4 A
Equation: ∆V3 = IR3(R3)
Solve: ∆V3 = (4A)(6Ω) = 24 V
Given: R1=3Ω; R2=2Ω
Equation/Rule: I =

 
Solve: I1 =

 = 8 A
I2 =

 = 12 A
Equation/Rule: Itotal=I1+I2+I3
Solve: Itotal = 8A + 12A + 4A = 24 A

Electrical Power
Electrical power (P) is the rate energy is produced or absorbed in a circuit. For example, a light bulb absorbs electrical energy and converts it to light energy. Measured in Watts (W), it can be calculated with the formulas:






For example, a 48 W lightbulb in a series circuit powered by a 24 V battery (with no other circuit components) would have a 2 A current and 12 Ω resistance. Using these numbers, practice with the equations.
Note: Horsepower (hp), used when electric power is used for motion, uses the following conversion to Watts: 1 hp = 746 W.

Structure of Electrical and Electronic Systems

AC vs. DC
In the previous examples, the circuits utilized direct current (DC), which is when current flows in only one direction, and voltage and current are constant. Direct current is supplied by batteries and is used in cell phones, ships, and planes. Alternating current (AC), in which current periodically reverses direction, can be produced using generators and is used for land-based applications (including in homes) because less power is lost when electricity travels long distances. In the United States, AC electricity is supplied most commonly at 120 V and 240 V. 

Ground
Grounding is the process of neutralizing the charge of an object by removal of excess electrons to or donation of additional electrons by a much larger object (example: the earth). This ground or large object can seemingly infinitely donate or accept electrons without mathematically significant changes to its net voltage. Examples of grounds include lighting rods and the round component of a three-pronged plug, as used for home appliances. Current shouldn't usually flow through air, but lightning is the visible current that results when charge builds up in clouds; lightning rods provide a safe, low-resistance ground so the high-voltage current is less likely to be destructive. The round component of three-pronged plugs serves as one of the residential applications of this concept.

Electric and Electronic Components

Semiconductors
Resistivity is the physical property of resistance of different materials: metals, which easily conduct electricity, have low resistivity; insulators have high resistivity; and the resistivity of semiconductors falls in between (example: Silicon (Si). Doping is the process of mixing different semiconductor atoms in order to control conductivity of the material.
N-type semiconductors have an excess of electrons as a result of the doping process; when an electric field is applied, a negative pole forms due to the buildup of negatively charged electrons (example: Silicon doped with antimony). P-type semiconductors have a shortage of electrons; when an electric field is applied, a positive pole forms (example: Silicon doped with boron).

Diodes
Diodes are a basic component of circuits, but they can't be described using Ohm's law; they operate as an electrical 'valve', only allowing current to pass in one direction. This allows them to rectify alternating current by converting it to direct current, which is preferred or necessary for some applications. A common example is the light-emitting diode (LED) bulb, which emits light when powered more efficiently than regular light bulbs.
A p-n junction diode is formed by fusing a p-type semiconductor and an n-type semiconductor and is considered a solid-state device, because electricity flows through solid material rather than a vacuum tube. The p-region of the junction is positively charged and the n-region is negatively charged. Reverse bias occurs when the diode is connected such that the positive voltage is applied to the n-region; forward bias occurs when the positive voltage is applied to the p-region.

Transistors
Bipolar junction transistors, p-n-p transistor or n-p-n transistor, are two p-n junction diodes arranged back-to-back with three electrical leads or terminals: the emitter, the collector, and the base.
Transistors are used to boost electrical signals or switch them on or off at high speeds. A large current flows from the emitter to the collector, and a smaller current entering through the base lead can be used to control the larger collector current.
Transistors are smaller and more efficient than vacuum/electron tubes, which are bulky and fragile, their cathode filaments burn out, and they require lots of power. Their uses include early hearing aids, radio, television, and computer memory chips.

Electricity and Magnetism

Capacitors
Capacitors store charge between two conductor components (example: parallel metal plates) with insulation (example: air) between them. Because they store charge between them, capacitors also oppose voltage change across the two plates. Direct current can charge a capacitor but cannot flow through it. Capacitance is the measure of a capacitor's ability to store charge between the two plates.
They are used in circuits and have many applications, including preventing computer memory loss during a short power failure and protecting sensitive component during power surges.

Inductors
When current passes through a wire coil, a magnetic field develops. An inductor is an electrical component made of a coil of wire that can store energy and opposes the rate of change of alternating current flowing through it; however, direct current passes through easily. They can be used as filters, sensors, motors, and transformers.
Inductance is the measure of ability of an inductor to resist changes in current. Faraday's Law of Induction describes the ways voltage or electromagnetic frequency can be generated by changing the magnetic environment in a coil of wire by moving a magnet within it, changing the external magnetic field of the magnet, or changing the area of the coil.

Transformers
Transformers are devices made of two or more coils of wire that use induction to transfer electricity between circuits. A step-up transformer's input (primary) voltage is lower than its output (secondary) voltage; and a step-down transformer's input voltage is higher than its output voltage. This allows for electricity to be to be generated safely at a lower voltage (example: 12 kV), passed through a step-up transformer for transport through power lines at high voltage (example: 400 kV), and passed through step-down transformers for distribution and use (example: 240 V residential use).

Motors and Generators
Motors use electromagnetic induction to convert electrical energy (electricity) to mechanical energy (motion). In a DC motor, torque is produced by magnetic force, which results from current passing through coil located in a magnetic field. They have been around for more than
100 years and are simple, inexpensive, and easy to maintain. In AC (synchronous) motors, torque is produced the same way as in DC motors, but much higher current is required, and they are inefficient. Induction
(asynchronous) motors are more common, and they rotate a magnetic field in order to induce alternating current.
Generators use electromagnetic induction to convert mechanical energy to electrical energy. AC generators mechanically turn a coil in a magnetic field to produce voltage output and induce alternating current.