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T5A

ELECTRICAL PRINCIPLES

Current and voltage: terminology and units, conductors and insulators, alternating and direct current

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T5A011 of 11

Electrical current is measured in which of the following units?

Why Current is the rate of flow of electric charge past a point in a circuit, and its unit is the ampere (amp), equal to one coulomb of charge per second. The other three units measure different quantities: volts measure electrical pressure (EMF), ohms measure resistance, and watts measure power. In Ohm's Law, I = E/R, the I term is always in amperes.
Watch out Volts is the common trap because voltage and current are often mentioned together, but volts measure the electrical force that pushes current, not the flow itself.
Amperes = Amount of charge flowing. Volts push, ohms resist, watts work, amps flow.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A022 of 11

Electrical power is measured in which of the following units?

Why Power is the rate at which electrical energy is used or produced, and its unit is the watt. In a DC circuit, power equals voltage times current (P = I x E), so 12 volts at 2 amperes is 24 watts. Volts measure electrical pressure and amperes measure current flow, but neither alone tells you the rate of energy use.
Watch out Watt-hours is the tempting one, but that is energy: power multiplied by time, which is what an electric bill measures, not power itself.
Watt = rate now; watt-hour = amount over time. Power questions want the plain watt.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A033 of 11

What is the term for the flow of electrons in an electric circuit?

Why Current is the name for the movement of electric charge (electrons) through a conductor. It is measured in amperes, where one ampere equals one coulomb of charge passing a point each second. Voltage is the electrical pressure that pushes the current, and resistance opposes it, related by Ohm's law E = I x R.
Watch out Voltage is the force or potential difference that causes electrons to move, not the movement itself; "amperes per second" and "volts per second" are rates of change, not standard quantities used here.
Water analogy: voltage is the pressure, current is the flow. Flow of electrons = current, measured in amperes.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A044 of 11

What term describes the number of times per second that an alternating current makes a complete cycle?

Why Alternating current reverses direction repeatedly, and one full back-and-forth trip is a cycle. The count of those cycles completed in one second is the frequency, measured in hertz (Hz); 1 Hz is one cycle per second, and 1 MHz is a million cycles per second. Frequency is what you dial in on your radio, for example 146.52 MHz means 146,520,000 cycles each second.
Watch out Wavelength is the physical distance one cycle travels in space, typically in meters, and it is inversely related to frequency (wavelength in meters = 300 divided by frequency in MHz), so it is a length, not a rate.
Frequency = cycles per second = hertz. Wavelength is measured in meters, frequency in Hz.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A055 of 11

A difference in which of the following causes electron flow?

Why Electrons only move through a circuit when there is a difference in electrical potential, and that potential difference is voltage, measured in volts. Voltage is often called electromotive force because it is the "pressure" that pushes charge through a conductor. With no voltage difference between two points, no current flows between them no matter how good the conductor is.
Watch out Ampere-hours measure stored charge capacity, such as how long a battery can supply a given current, not the force that starts the current. Capacitance and inductance describe how components store energy in electric or magnetic fields, and they oppose or shape changes in current rather than cause it.
Voltage is electrical pressure; like water, flow needs a pressure difference.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A066 of 11

What is the unit of frequency?

Why Frequency counts how many complete cycles an alternating waveform goes through each second, and that unit is the hertz (Hz), named for Heinrich Hertz. One hertz is one cycle per second, so a 7.1 MHz signal reverses direction 7,100,000 full cycles every second.
Watch out The henry is the unit of inductance and the farad is the unit of capacitance, both component properties rather than a rate; "epicycles per second" is not a real unit at all.
Hertz = cycles per second. Henry = inductance, Farad = capacitance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A077 of 11

Why are metals generally good conductors of electricity?

Why Electric current is the flow of charge, and in a solid that means electrons moving. Metals have atoms whose outer valence electrons are only loosely bound, so they drift freely through the crystal lattice as a 'sea' of free electrons. Apply even a small voltage and those electrons move easily, which is exactly what low resistance means. Insulators, by contrast, hold their electrons tightly in place.
Watch out Protons sit locked in the atomic nuclei and never move through a wire, so free protons are not a thing in metal conduction. Density and Young's modulus (stiffness) are mechanical properties with no direct bearing on electrical conductivity, though lead is dense and a poor conductor, which shows they do not track together.
Current is electrons moving. Metals are good conductors because their electrons are free to roam.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A088 of 11

Which of the following is a good electrical insulator?

Why An insulator is a material whose electrons are tightly bound to their atoms, so very few charge carriers are free to move and current is essentially blocked. Glass, along with ceramic, rubber, plastic and dry wood, fits that description, which is why glass and ceramic are used for antenna standoffs, feedline spreaders and high voltage insulators.
Watch out The other three are all conductors: sea water conducts because dissolved salt provides mobile ions, stainless steel is a metal with free electrons, and graphite is a form of carbon that conducts well enough to be used in resistors and motor brushes.
Insulators: glass, ceramic, rubber, plastic. If it is metal, salty water, or carbon, it conducts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A099 of 11

Which of the following describes alternating current?

Why Alternating current periodically reverses its direction of flow, moving first one way through the circuit and then the other. In household AC and in RF signals the voltage swings above and below zero, so the current flows positive, then negative, repeating at some frequency (60 Hz for US mains). Direct current, by contrast, always flows in one direction even if its amplitude varies.
Watch out The choices about swinging between one polarity and zero describe pulsating DC, such as the output of a half-wave rectifier: the amplitude changes but the direction never reverses, so it is not truly alternating.
Alternating means the direction actually flips: plus to minus and back. Just varying in size without reversing is still DC.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A1010 of 11

Which term describes the rate at which electrical energy is used?

Why Energy is measured in joules, and the rate at which it is used or delivered is power, measured in watts (one watt = one joule per second). In a circuit you calculate it as P = I x E, current times voltage. Resistance, current and voltage each describe one property of the circuit, but only power tells you how fast energy is being consumed.
Watch out Current is tempting because it is also a "rate," but it is the rate of charge flow (coulombs per second, amperes), not the rate of energy use.
Power = energy per time. Watts = joules per second. Rate of energy use means watts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5A1111 of 11

What type of current flow is opposed by resistance?

Why Resistance is the property of a material that opposes the movement of charge, and it does that no matter which direction or how fast the charge is moving. So DC, AC at power-line frequencies, and RF currents are all opposed by resistance, following Ohm's law E = I x R. What changes with frequency is reactance, from capacitance and inductance, not resistance itself.
Watch out Picking only direct current or only alternating current suggests resistance is frequency-selective, but that describes reactance and impedance; plain resistance is the same for any current.
Resistance doesn't care about direction or frequency: it opposes every kind of current.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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