ASVAB — All Questions

Three resistors of 4 ohms, 6 ohms, and 12 ohms are connected in parallel across a 24-volt source. What is the equivalent resistance of the combination?

• a.22 ohms
• b.2 ohms✓
• c.12 ohms
• d.4 ohms

For parallel resistors add the reciprocals: 1/R = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12, so R = 12/6 = 2 ohms. The equivalent value is always smaller than the smallest branch. Totaling the three values the way series resistors add gives 22 ohms, the usual error.

A 6-ohm resistor and an 18-ohm resistor are connected in series across a 48-volt battery. What is the voltage across the 6-ohm resistor?

• a.16 volts
• b.12 volts✓
• c.24 volts
• d.36 volts

In series one current flows through both, so find it first: I = 48/(6 + 18) = 48/24 = 2 amperes. Then V = I x R = 2 x 6 = 12 volts. The 18-ohm resistor drops 36 volts, and the two drops add back to 48. Splitting the source evenly gives 24 volts and ignores that drops follow resistance.

A current of 3 amperes flows through an 8-ohm resistor. How much power does that resistor dissipate?

• a.192 watts
• b.576 watts
• c.72 watts✓
• d.24 watts

Power from current and resistance is P = I^2 x R = 3^2 x 8 = 9 x 8 = 72 watts. Check it the long way: V = I x R = 24 volts, and P = V x I = 24 x 3 = 72. Multiplying current by resistance alone yields 24, which is the voltage rather than the power; squaring the resistance instead yields 192.

A 1,500-watt space heater is plugged into a 120-volt circuit. How much current does it draw?

• a.12.5 amperes✓
• b.0.08 amperes
• c.1.25 amperes
• d.125 amperes

Power is P = V x I, so rearranging gives I = P/V = 1500/120 = 12.5 amperes. Dividing voltage by wattage instead, 120/1500, gives 0.08 and reverses the relationship. Since 12.5 amperes is most of a 15-ampere breaker's rating, a heater like this usually needs a circuit of its own.

In a series circuit, the voltage drops measured across three resistors are 4 volts, 7 volts, and 9 volts. What is the voltage of the source?

• a.9 volts
• b.6.7 volts
• c.20 volts✓
• d.16 volts

Around a series loop the individual voltage drops add up to the source voltage: 4 + 7 + 9 = 20 volts. Averaging the three drops gives about 6.7 volts, and taking the largest drop alone gives 9 volts, which would only hold in a parallel circuit, where every branch sees the same voltage.

A transformer has 500 turns on its primary winding and 100 turns on its secondary. If 120 volts of alternating current is applied to the primary, what is the secondary voltage?

• a.600 volts
• b.5 volts
• c.100 volts
• d.24 volts✓

Secondary voltage follows the turns ratio: Vs = Vp x (Ns/Np) = 120 x (100/500) = 120/5 = 24 volts. Fewer turns on the secondary makes it a step-down transformer, so the output has to be below 120 volts. Inverting the ratio and multiplying by 5 gives 600 volts, the step-up answer.

An ideal step-up transformer raises 120 volts to 480 volts. If the primary draws 8 amperes, how much current is available at the secondary?

• a.32 amperes
• b.4 amperes
• c.8 amperes
• d.2 amperes✓

An ideal transformer passes power through unchanged, so Vp x Ip = Vs x Is. Here 120 x 8 = 960 watts, and 960/480 = 2 amperes. Voltage rose by a factor of 4, so current must fall by the same factor. Multiplying by the ratio instead of dividing gives 32 amperes and would create power from nothing.

Which component is a coil of wire that opposes any change in the current flowing through it and stores energy in a magnetic field?

• a.resistor
• b.inductor✓
• c.rectifier
• d.capacitor

An inductor is a coil whose changing current builds a magnetic field, and that field pushes back against the change, so current through a coil cannot jump instantly. The capacitor is the electric-field counterpart and opposes a change in voltage instead, while a resistor simply turns energy into heat.

What is the main function of a transistor in an electronic circuit?

• a.to store an electrical charge
• b.to amplify or switch a signal✓
• c.to step AC voltage up or down
• d.to convert AC into DC current

A transistor lets a small current or voltage at one terminal control a much larger current between the other two, and that single property is what makes both amplification and fast switching possible. Storing charge is the capacitor's job, rectifying AC is the diode's, and changing AC voltage is the transformer's.

What does a relay do in an electrical system?

• a.drops a large AC voltage down to a smaller one
• b.stores electrical energy to release it later
• c.conducts current in one direction but not both
• d.uses a small current to switch a larger one✓

A relay is an electrically operated switch: a small current through its coil makes an electromagnet that pulls heavy contacts closed, and those contacts carry the load current. That is how a light dashboard switch controls a starter motor drawing hundreds of amperes without carrying that current itself.

A knob that varies a circuit's resistance smoothly as its shaft is rotated, such as a volume control, is called a:

• a.transformer
• b.photoresistor
• c.potentiometer✓
• d.thermocouple

A potentiometer is a resistive track with a sliding contact, so turning the shaft changes how much of the track is in the circuit, and with it the resistance and the voltage tapped off the slider. A photoresistor responds to light and a thermocouple to a temperature difference; neither follows a shaft.

What is the purpose of the equipment grounding conductor in a household branch circuit?

• a.to keep the neutral conductor from overheating
• b.to carry the normal load current back to the panel
• c.to carry fault current back so the breaker trips✓
• d.to lower the voltage supplied to the appliance

The grounding conductor normally carries no current at all. Its job is to give a fault a low-resistance path back to the source, so a hot wire touching a metal case draws a large current at once and opens the breaker instead of leaving the case energized. Returning normal load current is the neutral's role.

A heating element made of nichrome wire glows red hot while the copper wire feeding it stays cool, even though the same current flows in both. Why?

• a.nichrome is a semiconductor
• b.nichrome stores charge as it heats
• c.nichrome has much higher resistance✓
• d.nichrome has a lower melting point

Heat is dissipated power, and with the same current in both, P = I^2 x R puts the heat where the resistance is. Nichrome's resistivity is roughly 60 times copper's, so nearly all the power is spent in the element. Copper's very low resistivity is exactly why it is chosen to carry current rather than make heat.

A carbon resistor is marked with color bands of brown, black, and red, in that order. What is its rated resistance?

• a.102 ohms
• b.10,000 ohms
• c.100 ohms
• d.1,000 ohms✓

The first two bands are digits and the third is the multiplier, that is, the count of zeros to append. Brown is 1 and black is 0, giving 10; red is 2, so add two zeros for 1,000 ohms, or 1 kilohm. Reading that third band as a third digit instead of a multiplier produces 102 ohms and is the usual slip.

In the American Wire Gauge system, how does a 12-gauge copper wire compare with an 18-gauge copper wire of the same length?

• a.it is thicker and has less resistance✓
• b.it is thicker and has more resistance
• c.it is thinner and has less resistance
• d.it is thinner and has more resistance

Gauge numbers run backwards: the smaller the number, the thicker the wire. Resistance falls as cross-sectional area rises, because current has more room to move, so 12-gauge carries both more copper and less resistance per foot than 18-gauge. Heavier loads therefore call for a lower gauge number.

An alternating current completes one full cycle every 0.004 second. What is its frequency?

• a.25 hertz
• b.250 hertz✓
• c.2,500 hertz
• d.0.004 hertz

Frequency and period are reciprocals: f = 1/T = 1/0.004 = 250 hertz. Check it another way: 0.004 second is 4 milliseconds, and 1,000 milliseconds divided by 4 gives 250 cycles per second. Misplacing one decimal place turns the answer into 25 or 2,500, so count the zeros before dividing.

A standard wall outlet supplies 120 volts RMS. What is the approximate peak voltage of that waveform?

• a.240 volts
• b.340 volts
• c.170 volts✓
• d.85 volts

For a sine wave the RMS value is the peak divided by the square root of 2, so peak = RMS x 1.414 = 120 x 1.414, about 170 volts. Dividing by 1.414 instead gives 85 and reverses the relation. Roughly 340 volts is the peak-to-peak swing, which is twice the peak rather than the peak itself.

Why is electric power transmitted over long distances as alternating current rather than as direct current?

• a.AC travels faster through copper than DC
• b.AC can be stored in batteries but DC cannot
• c.transformers can raise and lower AC easily✓
• d.AC does not heat the wires the way DC does

A transformer works only on a changing current, so AC can be stepped up to very high voltage for the long haul and back down near the house. High voltage means low current for the same power, and line loss is I^2 x R, so losses drop sharply. Batteries store DC, and any current heats a resistance.

A bar magnet is pushed into a coil of wire whose ends are connected to a sensitive meter, and the meter deflects. What causes that reading?

• a.the magnet adds charge to the coil
• b.the coil's resistance drops near a magnet
• c.a changing magnetic field induces a voltage✓
• d.the magnet pushes charge along the wire

Induction depends on change, not on the mere presence of a field: moving the magnet makes the flux through the coil change, and that change induces a voltage. Hold the magnet still inside the coil and the meter returns to zero even though the field is still there. Move it faster and the induced voltage rises.

Which change would make an electromagnet stronger without altering its power supply?

• a.spacing the turns farther apart
• b.reversing the winding direction
• c.adding more turns of wire to the coil✓
• d.winding the coil on a plastic core

A coil's field strength depends on its ampere-turns, the current multiplied by the number of turns, and on what the coil is wound around. With the supply fixed, only more turns can raise the ampere-turns. An iron core concentrates the field, so plastic weakens it, and reversing the winding just flips the poles.

Adding a controlled trace of another element to pure silicon so that it conducts better is called:

• a.doping✓
• b.etching
• c.biasing
• d.plating

Pure silicon conducts poorly. Doping mixes in a few atoms of an element such as phosphorus or boron, which contributes either extra electrons or extra holes and turns the crystal into n-type or p-type material. Biasing means applying a voltage to a finished device, a later step that changes no crystal.

A 60-volt source is connected across two parallel branches, one containing a 10-ohm resistor and the other a 15-ohm resistor. What is the total current drawn from the source?

• a.2.4 amperes
• b.10 amperes✓
• c.6 amperes
• d.4 amperes

In parallel every branch sees the full source voltage, so solve each branch alone and add the currents: 60/10 = 6 amperes and 60/15 = 4 amperes, for 10 amperes total. Adding the resistances as though they were in series gives 25 ohms and 2.4 amperes, far too little; adding a branch always draws more current.

Radio waves travel at about 300,000,000 meters per second. What is the wavelength of an FM signal broadcast at 100 megahertz?

• a.3 meters✓
• b.300 meters
• c.0.3 meter
• d.30 meters

Wavelength equals speed divided by frequency. 100 megahertz is 100,000,000 hertz, so 300,000,000/100,000,000 = 3 meters. Mind the prefix, since mega means million and losing or adding a single zero shifts the answer to 30 meters or 0.3 meter. Higher frequency always means a shorter wave.

An AM station transmits at 1,000 kilohertz and an FM station transmits at 100 megahertz. Compared with the AM signal, the FM signal has:

• a.a lower frequency and a longer wavelength
• b.a higher frequency and a longer wavelength
• c.a lower frequency and a shorter wavelength
• d.a higher frequency and a shorter wavelength✓

Convert to one unit first: 100 megahertz is 100,000 kilohertz, a hundred times the AM station's 1,000 kilohertz. Wavelength is speed divided by frequency and the speed is fixed, so a hundred times the frequency means one hundredth the wavelength, roughly 3 meters against roughly 300 meters.

Once a capacitor in a direct-current circuit has fully charged, what does it do to the current in that branch?

• a.it slowly lifts the branch's current
• b.it passes current with no opposition
• c.it blocks any further steady current✓
• d.it flips the polarity of the circuit

A capacitor is two plates separated by an insulator, so charge piles up on the plates but nothing crosses the gap. Current flows only while the voltage across it is changing, and once it has charged to the source voltage that current falls to zero. This is why a capacitor blocks DC yet passes an AC signal.

During the compression stroke of a four-stroke gasoline engine, the valves are in what position?

• a.Intake open, exhaust shut
• b.Both valves fully open
• c.Exhaust open only
• d.Both valves fully closed✓

Compression only works against a sealed cylinder, so once the intake valve shuts both valves stay closed while the piston rises and squeezes the trapped charge. An open intake is tempting because that valve was open on the stroke just before, but it seats as the piston starts back up.

What keeps the camshaft turning in step with the crankshaft?

• a.The alternator drive belt
• b.The timing belt or chain✓
• c.The transmission linkage
• d.The serpentine drive belt

The camshaft opens the valves, so it must stay synchronized with the pistons; a toothed belt or a chain ties it to the crankshaft at a fixed ratio that never slips. The serpentine belt is tempting because it is the belt you can see, but it turns accessories, not valve timing.

Coolant is pushed through the engine block and out to the radiator by the

• a.heater core.
• b.radiator cap.
• c.water pump.✓
• d.thermostat.

Coolant does not move on its own. The water pump, spun by the engine through a belt or chain, forces coolant through the block and out to the radiator, where airflow strips the heat. The thermostat is tempting because it also controls flow, but it only gates flow; it cannot create it.

A pressure cap on the radiator lets the cooling system

• a.lower the temperature coolant freezes at.
• b.raise the pressure inside the oil galleries.
• c.raise the boiling temperature of coolant.✓
• d.lower the temperature the thermostat opens.

Pressure raises the boiling point of any liquid, so a cap that holds roughly 15 psi lets coolant run well past 212 F without flashing to steam and blowing out. Freezing protection is tempting because it is the other half of what coolant does, but that comes from the antifreeze mixture.

The main job of an ignition coil is to

• a.change the alternator current from AC to DC.
• b.raise battery voltage to thousands of volts.✓
• c.spread the fuel evenly among all the cylinders.
• d.lower charging voltage to protect the wiring.

A spark plug needs tens of thousands of volts to jump its gap, far more than a 12 volt system supplies, so the coil works as a transformer and steps the voltage way up. Rectifying AC into DC is tempting because it is also electrical, but diodes inside the alternator do that.

An oxygen sensor mounted in the exhaust pipe is there to

• a.tell the computer how rich or lean the mixture is.✓
• b.tell the driver how hot the exhaust is running.
• c.burn the unburned fuel still left in the exhaust.
• d.shut off exhaust flow while the engine is cold.

The sensor measures leftover oxygen in the exhaust and reports it to the engine computer, which lengthens or shortens injector pulses to hold the air-fuel ratio near ideal. Burning leftover fuel is tempting because that truly happens inches downstream, in the catalytic converter.

Turning the key produces only rapid clicking, and the headlights dim sharply at the same moment. The most likely cause is

• a.a blown main charging fuse.
• b.a worn starter drive gear.
• c.a failed ignition coil pack.
• d.a badly discharged battery.✓

Rapid clicking is the starter solenoid pulling in and dropping out as voltage collapses under load, and lights that dim at that same instant show the whole system is starved for current. A dead coil is tempting, but it gives a strong steady crank that simply never fires.

A front-wheel-drive car clicks rhythmically at low speed, but only while it is turning a corner. The worn part is most likely

• a.a front brake caliper slide pin.
• b.an outer constant velocity joint.✓
• c.an outer tie rod end and boot.
• d.a front wheel bearing hub unit.

A worn outer CV joint clicks once per wheel revolution and only when it is driving through an angle, which is exactly what a slow tight turn asks of it. A wheel bearing is tempting as the other noisy part in that corner, but it growls steadily and tracks road speed instead.

A car that keeps bouncing up and down several times after crossing a bump most likely has worn

• a.tie rods and ball joints.
• b.springs or torsion bars.
• c.control arm bushings.
• d.shock absorbers or struts.✓

The spring stores the energy of the bump and the shock absorber converts that motion into heat, so a car that oscillates several times has lost its damping. Springs are tempting because they are the part visibly bouncing, but a broken spring makes one corner sit low instead.

In a wheel alignment, camber is

• a.the inward or outward tilt of a wheel top.✓
• b.the angle at which the tires point inward.
• c.the forward or rearward lean of the strut.
• d.the height of the body above the pavement.

Stand in front of the car and look at the wheel: camber is how far its top leans away from vertical, positive leaning out and negative leaning in, and too much of either wears one edge of the tread. Toe is tempting as the other common setting, but toe is how far the tires point in or out.

Antilock brakes help a driver in a hard stop mainly because they

• a.shorten the pedal travel needed to stop the car.
• b.let the tires keep rolling so steering still works.✓
• c.raise the hydraulic pressure at the master cylinder.
• d.apply the rear brakes harder than the front ones.

A locked tire slides, and a sliding tire cannot steer. ABS senses a wheel about to lock, then releases and reapplies that wheel many times a second so it keeps turning and the driver keeps steering. A shorter stop is tempting, but on gravel or snow ABS can lengthen the distance.

In a drum brake, what forces the shoes outward against the drum?

• a.Wheel cylinder✓
• b.Brake caliper
• c.Backing plate
• d.Return spring

Pressure from the master cylinder reaches a small wheel cylinder inside the drum, and its two pistons push the shoes apart until the linings drag on the spinning drum. A caliper is tempting because it does the same job, but a caliper squeezes pads onto a disc brake rotor.

Steady blue-gray smoke from the tailpipe of a fully warmed engine most likely means that

• a.engine oil is being burned in the cylinders.✓
• b.coolant is leaking past the head gasket.
• c.the fuel mixture is far too rich.
• d.the air filter is badly clogged.

Oil slipping past worn rings or hardened valve stem seals burns along with the fuel and tints the smoke blue. Coolant is tempting because it also smokes, but burning coolant gives white sweet-smelling steam and a rich mixture gives black soot; the color separates the three.

Before drilling a hole in a steel plate, the layout mark is tapped with a center punch in order to

• a.leave a burr for the countersink to remove.
• b.harden the steel around the rim of the hole.
• c.keep the drill bit from wandering off line.✓
• d.clear the chips out ahead of the drill bit.

The chisel edge at the tip of a twist drill skates across smooth metal, so a punched dimple gives that point something to sit down in and the hole starts exactly on the layout mark. Chip clearing is tempting as a real drilling concern, but the flutes handle that.

A machine screw differs from a wood screw in that a machine screw

• a.has a smooth shank below a threaded point.
• b.runs one diameter, for a nut or tapped hole.✓
• c.has a tapered shank that cuts its own thread.
• d.carries a coarse thread meant for soft wood.

A machine screw is the same diameter its whole length and its thread is a standard size, so it mates with a nut or a threaded hole in metal. A tapered shank is tempting because that is the shape most people picture for a screw, but tapering is what lets a wood screw form its own thread.

The most direct way to check whether the sawn end of a board is square to its edge is with a

• a.marking gauge.
• b.bevel gauge.
• c.spirit level.
• d.try square.✓

A try square is a fixed 90 degree blade set into a stock: hold the stock tight on the edge, slide the blade against the end, and any light under the blade shows the cut is out of square. A sliding bevel gauge is tempting because it looks similar, but its blade pivots to copy any angle.

Which fastener is chosen when only one side of the joint can be reached?

• a.A hex head lag screw
• b.A blind pop rivet✓
• c.A bolt and a locknut
• d.A square head bolt

A pop rivet is set by a tool that pulls a mandrel through it, swelling the far end into a head against the back of the work, so nobody has to be behind the panel. A bolt and nut is tempting because it clamps harder, but somebody has to reach around to hold that nut still.

A split lock washer is placed under a nut in order to

• a.keep the nut from backing off from vibration.✓
• b.seal the joint against water getting under it.
• c.keep the bolt from turning inside its hole.
• d.spread the clamping load over a wider area.

The washer is split and sprung open, so tightening flattens it and drives its two sharp ends into the nut face and the work, adding friction that resists loosening on machinery that shakes. Spreading the load is tempting, but that is the job of a plain flat washer.

Radial lines stamped on the head of a bolt tell you its

• a.thread pitch in inches.
• b.overall length in inches.
• c.plating and finish type.
• d.tensile strength grade.✓

Head marks are a strength code: three radial lines mean SAE grade 5, six lines mean grade 8, and a bare head means grade 2, so more lines take more torque and more load. Thread pitch is tempting because it is also standardized data, but pitch is called out in the part size, not on the head.

A hacksaw blade used on thin sheet metal should have

• a.few coarse teeth so the chips clear faster.
• b.many fine teeth so several stay in the cut.✓
• c.long teeth so the stroke removes more metal.
• d.a wavy set so the kerf comes out narrower.

The working rule is at least three teeth touching the work at all times. A coarse blade drops one lone tooth into thin stock, and that tooth catches and strips or snaps the blade. Coarse teeth are tempting because they really do cut faster, but that holds for thick bar, not sheet.

Boards are being glued edge to edge into a wide tabletop. The right clamp for the job is a

• a.metal spring clamp.
• b.locking C clamp.
• c.wooden handscrews.
• d.pipe or bar clamp.✓

An edge glue-up needs even pressure pulled across the whole width of the panel, and a pipe or bar clamp spans several feet and draws the joints shut with a screw. A C clamp is tempting because it squeezes hard, but its throat is inches deep and it can only pinch near an edge.

Soldering differs from welding in that soldering

• a.joins the parts by melting their edges together.
• b.needs far more heat than welding does to work.
• c.makes a joint stronger than the metal it joins.
• d.joins the parts without melting the base metal.✓

In soldering only the filler alloy melts; it wets the heated surfaces and grips them as it freezes, which is why a solder joint carries far less load than the parts themselves. Melting the edges together is tempting because it sounds like joining metal, but that describes welding.

A dovetail joint is used at the corners of a drawer mainly because it

• a.can be cut faster than a butt joint.
• b.uses less wood than a butt joint does.
• c.resists being pulled straight apart.✓
• d.hides the end grain along both faces.

The flaring pins and tails of a dovetail interlock like wedges, so pulling on the drawer front cannot draw the joint open and only the other directions rely on glue. Speed is tempting, but a dovetail is slower to cut than a butt joint; that time is the price paid for the strength.

Wood stain and polyurethane varnish differ in that stain

• a.colors the wood but gives little protection.✓
• b.raises the grain so it can be cut back off.
• c.fills open grain before the first sanding.
• d.builds a hard film that resists water well.

Stain is pigment or dye carried in a thin liquid: it soaks into the fibers to change the color and the excess is wiped away, so almost nothing is left standing on the surface. A tough film is tempting because both go on with a brush, but the film is the varnish coat that follows.

An adjustable wrench should be turned so that the pulling force bears against

• a.the fixed jaw.✓
• b.the worm screw.
• c.the movable jaw.
• d.the jaw opening.

The movable jaw rides on a worm screw and is the weaker half of the tool, so the handle is pulled toward the open side of the jaws and the load lands on the solid fixed jaw. Pulling the other way pries the movable jaw wider, the wrench slips, and the nut corners round off.