Electronics Information · Practice test 1

20 questions · 9 minutes

Answer every question, then tap Check results to see your score and the method behind each answer.

Question 1Not answered

One microfarad is equal to
1. 0.001 farad
2. 1,000,000 farads
3. 1000 farads
4. 0.000001 farad
Correct answer: 0.000001 farad

Micro means one millionth. Since a full farad is an enormous amount of capacitance, practical capacitors are usually rated in microfarads or picofarads.

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Question 2Not answered

Compared with direct current, alternating current is favored for long-distance power transmission chiefly because it
1. never produces any resistive losses in the lines
2. can be stepped up and down efficiently with transformers
3. requires no insulation on the transmission towers
4. flows only during daylight hours
Correct answer: can be stepped up and down efficiently with transformers

Transformers work only on AC, so power can be stepped up to high voltage for transmission, cutting current and line losses, then stepped back down near the customer.

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Question 3Not answered

To measure the voltage across a component, a voltmeter should be connected
1. in series with the component
2. between the component and ground
3. in place of the component
4. in parallel with the component
Correct answer: in parallel with the component

A voltmeter goes in parallel, across the component, because it measures the difference in pressure between two points and has a very high internal resistance. An ammeter is the opposite: it goes in series so all the current passes through it.

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Question 4Not answered

The process of converting alternating current into direct current is called
1. induction
2. amplification
3. insulation
4. rectification
Correct answer: rectification

Rectification uses diodes to let current pass in only one direction, producing DC from an AC supply. Every device with a mains adapter contains a rectifier.

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Question 5Not answered

One kilohertz is equal to
1. 0.001 hertz
2. 1000 hertz
3. 100 hertz
4. 1,000,000 hertz
Correct answer: 1000 hertz

Kilo means one thousand, so a kilohertz is 1000 hertz, or 1000 cycles per second.

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Question 6Not answered

A half-wave rectifier built from a single diode
1. passes both halves of each AC cycle to the load
2. blocks the entire AC waveform completely
3. passes only one half of each AC cycle to the load
4. converts AC to DC without producing any ripple
Correct answer: passes only one half of each AC cycle to the load

With only one diode, current can flow during just one half of the AC cycle, when the diode is forward biased, and is blocked during the other half, wasting that portion of the wave.

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Question 7Not answered

A 250 watt television runs for 4 hours each day. How much energy does it use in that day?
1. 10 kilowatt-hours
2. 1 kilowatt-hour
3. 0.1 kilowatt-hours
4. 62.5 kilowatt-hours
Correct answer: 1 kilowatt-hour

250 watts is 0.25 kilowatt, and running for 4 hours uses 0.25 x 4 = 1 kilowatt-hour.

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Question 8Not answered

A broken filament inside a light bulb is an example of
1. a short circuit
2. a parallel circuit
3. an open circuit
4. a grounded circuit
Correct answer: an open circuit

A broken filament leaves a gap in the only path for current, so nothing flows and the bulb goes dark. This is a textbook open circuit.

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Question 9Not answered

The capacitive reactance of a capacitor decreases as the
1. frequency of the applied AC signal increases
2. frequency of the applied AC signal decreases
3. capacitor's own capacitance value decreases
4. capacitor is switched to a pure DC supply
Correct answer: frequency of the applied AC signal increases

Capacitive reactance falls as frequency rises, the opposite of inductive reactance, because a capacitor charges and discharges more readily as the signal alternates faster, offering less opposition to it.

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Question 10Not answered

A device draws 5 amperes at 120 volts. How much power does it use?
1. 600 watts
2. 125 watts
3. 24 watts
4. 115 watts
Correct answer: 600 watts

Power equals voltage times current, so 120 x 5 = 600 watts. The choice 24 comes from dividing instead, which would give resistance.

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Question 11Not answered

If the resistance of a circuit doubles while the voltage stays the same, the current
1. rises to twice its former value
2. stays exactly the same
3. falls to half its former value
4. falls to a quarter of its value
Correct answer: falls to half its former value

Ohm's law makes current inversely proportional to resistance at fixed voltage, so doubling one halves the other. Power also falls by half, since P = V²/R.

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Question 12Not answered

An inductor's ability to store energy comes from its
1. magnetic field, built up as current flows through its coil
2. electric field, built up between two separated plates
3. chemical reaction, driven by the current through it
4. physical resistance to the current passing through it
Correct answer: magnetic field, built up as current flows through its coil

Current flowing through a coil sets up a magnetic field around it, and that field is where an inductor stores its energy, unlike a capacitor which stores energy electrically.

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Question 13Not answered

In a series circuit, the current
1. divides among the components
2. is greatest at the first component
3. falls to zero at the last component
4. is the same at every point
Correct answer: is the same at every point

A series circuit gives current only one path, so the same current passes through every component. What does divide up in a series circuit is the voltage.

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Question 14Not answered

Impedance in an AC circuit is
1. the resistance of the conductors alone
2. the reactance of the coils alone
3. the power lost as heat in the circuit
4. the total opposition from resistance and reactance together
Correct answer: the total opposition from resistance and reactance together

Impedance combines ordinary resistance with the reactance of inductors and capacitors, and like resistance it is measured in ohms. In a purely resistive circuit the two are the same number.

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Question 15Not answered

A 1500 watt heater runs for 2 hours. How much energy does it use?
1. 3000 kilowatt-hours
2. 750 kilowatt-hours
3. 1.5 kilowatt-hours
4. 3 kilowatt-hours
Correct answer: 3 kilowatt-hours

Convert watts to kilowatts first: 1500 watts is 1.5 kilowatts, and running for 2 hours uses 1.5 x 2 = 3 kilowatt-hours. Forgetting to convert leaves the answer a thousand times too large.

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Question 16Not answered

A circuit operating at 6 volts carries 2 amperes. What is its resistance?
1. 12 ohms
2. 3 ohms
3. 8 ohms
4. 0.33 ohms
Correct answer: 3 ohms

Resistance equals voltage divided by current, so 6 / 2 = 3 ohms.

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Question 17Not answered

Wiring several capacitors in series, rather than in parallel, produces a total capacitance that is
1. greater than the sum of the individual capacitors
2. equal to the sum of the individual capacitors
3. unaffected by how many are added
4. less than the smallest individual capacitor
Correct answer: less than the smallest individual capacitor

Capacitors in series combine the way resistors do in parallel, by reciprocals, so the total is always smaller than the smallest capacitor in the string.

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Question 18Not answered

What voltage appears across a 5 ohm resistor carrying 3 amperes?
1. 8 volts
2. 1.67 volts
3. 45 volts
4. 15 volts
Correct answer: 15 volts

Voltage equals current times resistance, so 3 x 5 = 15 volts. This is the voltage drop across that one component.

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Question 19Not answered

Doping a semiconductor means
1. heating it until it begins to conduct freely
2. adding small amounts of another element to change its conductivity
3. coating it with an insulating varnish
4. passing a large current through it once
Correct answer: adding small amounts of another element to change its conductivity

Deliberate impurities give the material either spare electrons or spare holes, producing n-type or p-type semiconductor. Joining the two types is what forms a diode junction.

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Question 20Not answered

A 6 ohm and a 3 ohm resistor are connected in parallel. What is the total resistance?
1. 9 ohms
2. 4.5 ohms
3. 2 ohms
4. 18 ohms
Correct answer: 2 ohms

Using the product-over-sum rule for two resistors, (6 x 3) / (6 + 3) = 18 / 9 = 2 ohms, less than either resistor alone.

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