ASVAB — All Questions

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.