ASVAB Electronics Information, circuits and current

What is the primary function of a transformer in an electrical circuit?

1. To convert AC to DC
   Changing AC to DC is rectification, done with diodes; a transformer works only on AC and puts out AC.
2. To change voltage levels
   A transformer steps AC voltage up or down through two coils on an iron core; the ratio of their turns sets the change.
3. To store electrical energy
   Storing energy is the job of a battery; a transformer only passes energy through.
4. To generate electricity
   A generator makes electricity from motion; a transformer only changes the voltage of power it already receives.

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Hydroelectric, coal and nuclear power plants all produce electricity by

1. chemical reactions in giant batteries
   Batteries store energy chemically; none of these plants makes power from batteries.
2. converting light with solar cells
   Solar cells turn light straight into electricity, but water, coal and nuclear plants all work through moving machinery instead.
3. heating thermocouples directly
   Thermocouples make tiny voltages from heat and are used as temperature sensors, not as a power plant's main source.
4. turning a generator with a turbine
   Falling water, or steam from burning coal or from fission, spins a turbine, and the turbine turns the generator that produces the current.

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In a properly working house circuit, the equipment ground wire normally carries

1. the return current from every load
   Return current flows on the neutral (white) wire; current on the ground in normal use means a wiring fault.
2. no current unless there is a fault
   The bare or green ground wire is a safety path; it carries current only when a hot wire touches metal, long enough to trip the breaker.
3. half of the circuit's load current
   Sharing load current with the ground is a wiring error; the neutral normally carries all of the return.
4. current only while a switch is on
   Switching on a load sends current through the hot and neutral wires; the ground stays idle either way.

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What is the main difference between an analog and a digital signal?

1. Analog signals can carry only sound, not data
   Analog signals carry sound, video and sensor readings, and modems once sent data over them; what a signal carries is not what separates the two.
2. Digital signals cannot travel over long distances
   Digital signals cross the world over fiber and radio links; distance is not what separates the two.
3. Analog signals use far less power than digital signals
   Power use depends on the circuit, not on whether the signal is analog or digital.
4. Analog varies smoothly but digital changes in steps
   An analog signal can take any value along a smooth curve; a digital signal is limited to set levels such as on and off.

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A relay is best described as

1. a transistor that amplifies a signal
   Amplifying a signal is the classic job of a transistor; a relay is an electrically operated switch.
2. a resistor that varies with heat
   A resistor whose value changes with temperature is a thermistor.
3. a switch worked by an electromagnet
   Current in a relay's coil makes a magnetic field that pulls the contacts together, so a small current can switch a larger one.
4. a diode that passes current one way
   Letting current flow in only one direction is what a diode does; a relay opens and closes contacts.

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Unlike a fuse, a circuit breaker

1. never trips on a short circuit
   A short circuit draws a very large current, which is exactly what trips a breaker fastest.
2. protects against low voltage
   Breakers react to too much current, not to low voltage.
3. can be reset after it trips
   A breaker opens a switch when current is too high and can be switched back on once the fault is cleared.
4. must be replaced after it trips
   Needing replacement after one use is the drawback of a fuse, whose metal link melts.

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If a radio station's broadcast frequency is doubled, the wavelength of its signal

1. doubles
   At a fixed speed, frequency and wavelength move in opposite directions; doubling one cannot double the other.
2. drops to one-fourth
   One-fourth would follow from a squared relationship; wavelength is simply inversely proportional to frequency.
3. is cut in half
   A radio wave's speed is fixed, and speed = frequency × wavelength, so doubling the frequency halves the wavelength.
4. stays the same
   The wavelength could stay the same only if the speed doubled too; radio waves always travel at the speed of light.

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An AC signal has a frequency of 50 Hz. Its period is

1. 0.02 second
   Period is 1 divided by frequency: 1 ÷ 50 = 0.02 second, so one cycle lasts 20 milliseconds.
2. 0.2 second
   0.2 second is 1 ÷ 5; it comes from dropping a zero from the 50 during the division.
3. 20 seconds
   20 is the period in milliseconds (1000 ÷ 50); writing it as seconds makes it a thousand times too long.
4. 50 seconds
   50 is the frequency itself; the period is its reciprocal, not the same number in seconds.

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In a speaker crossover, which part placed in series with the tweeter keeps the low (bass) frequencies out of it?

1. A fuse
   A fuse protects against overload but passes every frequency until it blows.
2. An inductor
   An inductor does the reverse, passing bass and blocking treble; it goes in series with the woofer.
3. A diode
   A diode passes current one way only and would chop up the audio signal, which keeps reversing direction.
4. A capacitor
   A capacitor blocks low frequencies and passes high ones, so in series with the tweeter it lets only the treble through.

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A battery charger's output must stay at 13.8 V even as the load changes. The circuit that holds it there is a

1. rectifier
   A rectifier turns AC into pulsing DC; it does nothing to hold the voltage steady.
2. transformer
   A transformer sets the voltage by its turns ratio, but its output drops under a heavier load.
3. regulator
   A voltage regulator senses the output and adjusts continuously, so the voltage stays put as the load changes.
4. voltage divider
   A voltage divider of two resistors gives a set fraction of its input, but its output sags as soon as a load draws current.

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A piezoelectric crystal, like the one in a gas-grill igniter, produces a voltage when it is

1. squeezed or struck
   Squeezing or striking a piezoelectric crystal produces a voltage; the igniter's spring hammer strikes the crystal to make a spark.
2. heated by a flame
   Heating makes a voltage only in a thermocouple (the thermoelectric effect), not in a piezo crystal.
3. placed near a magnet
   A magnet induces voltage only in a moving conductor or coil; a crystal sitting near a magnet makes none.
4. exposed to bright light
   Voltage from light is the photovoltaic effect of solar cells.

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A TV's remote-control receiver picks up the infrared beam from the remote with a

1. light-emitting diode
   A light-emitting diode is the sender inside the remote; it emits the beam rather than detecting it.
2. thermistor
   A thermistor responds to temperature, far too slowly to follow the fast pulses of a remote.
3. microphone
   A microphone picks up sound, and a remote's signal is infrared light, not sound.
4. photodiode
   A photodiode lets current flow when infrared light strikes it, turning the remote's pulses into an electrical signal.

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What is the term for unwanted electrical signals that can interfere with desired signals?

1. Inductance
   Inductance is a coil's opposition to changes in current, a property of a part, not a stray signal.
2. Noise
   Noise is any unwanted electrical signal, such as hum or static, that mixes with and masks the wanted signal.
3. Resonance
   Resonance is the frequency at which a circuit responds most strongly; tuned circuits use it on purpose.
4. Impedance
   Impedance is a circuit's total opposition to alternating current, a property, not an unwanted signal.

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Adding small amounts of impurities to pure silicon to change how it conducts is called

1. doping
   Doping adds tiny amounts of elements such as phosphorus or boron to silicon to make N-type or P-type material.
2. plating
   Plating coats a surface with a thin layer of metal; it does not change the inside of the crystal.
3. tinning
   Tinning coats a wire or soldering tip with solder before soldering.
4. annealing
   Annealing is heating and slowly cooling a material to relieve stress; it adds no impurities.

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On a solderless breadboard, the five holes in one short numbered row are

1. connected to one another
   Each short row of five holes shares one metal clip, so parts plugged into the same row are connected.
2. joined to the power rails
   The long power rails along the edges are separate strips, connected to a row only with a jumper wire.
3. joined to the row across the gap
   The center gap separates the two halves so a chip can straddle it without its pins shorting together.
4. insulated from one another
   If the holes were insulated, no two parts could be joined without extra wires; sharing a clip is the point of the row.

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