ASVAB Practice Test – Electronics Information

Total resistance in a circuit that has both series and parallel elements is calculated by the following equations: Total resistance = 85 ohms + 175 ohms + 1/( 1/ 150 ohms + 1/ 25 ohms) Total resistance = 260 ohms + 18.75 ohms Total resistance = 278.75 ohms

The total resistance for the circuit was found in the previous problem (278.75 ohms), so this value is plugged into the equation as well as the voltage from the battery source (80 volts). I = V/ R = 80 V/ 278.75 ohms = 0.29 amps

Voltage lost is expressed in amps because it is referring to a reduction in current, so Choice D, 5.4 volts, is incorrect. Voltage will be lost at the 25-ohm resistor because this resistor is in a parallel circuit, so Choice B, 0 amps, is incorrect. Again, the voltage lost can be calculated by substituting known values into iterations of Ohm’s Law. V = I × R = (0.29 amps) × (18.75 ohms) = 5.4 volts. Then, this is substituted into the following equation: I = V/ R = 5.4 V/ 25 ohms = 0.22 amps

The equation to determine the cost is cost = power (in kilowatt-hours) × time (in hours) × rate (in cents per kilowatt-hour). The important step is ensuring that all values are inserted in the equation with the correct units, so some conversions need to take place. Power is given as 150 watts, which equals 0.15 kilowatt-hours (150/ 1000). The time of 240 minutes is divided by 60 minutes/ hour to yield 4 hours. Therefore: cost = (0.15 kW) × (4 hr.) × (10 cents/ kWh) = 6 cents/ 100 = $. 06 Choice A did not convert the power into kilowatt hours, but left it in watts. Choice B did not convert the time from minutes into hours. Choice C did not correctly convert the cost into dollars.

The physical structure of the atoms that compose matter lends itself to the production of electricity. The arrangement of the subatomic particles and the associated charges— mainly the negatively charged electrons in the cloud— are associated with the ability to create an electric current, which can be harnessed to do work.

The structure of atoms is as follows: The central nucleus contains protons and neutrons. Protons have a positive charge (+ 1), and neutrons are neutral, so they do not carry a charge. Around the nucleus is a cloud, which contains electrons in rings called orbitals. Electrons are negatively charged (– 1), and those in the outermost ring— farthest from the nucleus— tend to be easier to remove from the atom.

Physical properties of atoms are influenced to some degree by the number and arrangement of electrons in the shells around the nucleus. The electrons in the outer shells in metals are free to drift between atoms. This movement potential allows them to jump between atoms, creating an electric current or flow, when subjected to an external force such as a magnetic field.

Choice B added the resistances together as is done in a series circuit, but the given circuit is a parallel circuit. Choice C missed the final step that the resistance found is the inverse of . Choice D simply chose the lowest resistance of the given resistors, but this is not how to calculate the equivalent resistance in parallel circuits.

Electric conductors tend to be metals, such as silver, copper, and aluminum. They “conduct” electricity, which means that they help the electric current flow easily. This is largely because metals have low resistance because the electrons are easily detached from these atoms, so they are then free to jump from atom to atom and create a current. In contrast, insulators (such as wood, plastic, or rubber) have high resistance but low conductivity. Conductivity and resistance are opposite properties; a given single material cannot be highly conductive and highly resistant at the same time.

Insulators provide more resistance than conductors. As mentioned, conductors have little resistance. Insulators, such as plastic or rubber, in contrast, have high resistance. Essentially, they slow the flow of electric current much like friction deters mechanical motion.

The difference in potential between two separate points is a measure of volts. Amperes indicate the number of electrons that pass a specific point per second. Ohms measure the resistance to flow (essentially electrical friction). Watts are a measure of power or the rate at which electrical energy is used or converted into another type of energy. Watts are often converted to kilowatts or, when expressing power rates, kilowatt-hours.

The amount of energy needed to power a certain device is expressed in kilowatt-hours. Therefore, Choices A and D are incorrect. The amount of energy needed is the product of the power in kilowatts multiplied by the time in hours. The air conditioner is 6000 watts or 6 kilowatt-hours multiplied by 3 hours is 18 kilowatt-hours. Choice C did not properly convert the power from watts into kilowatts.

When devices are connected in a series circuit, the voltage is the same across each device (or resistor) in the loop, but the current drops after each device. Recall that for lightbulbs connected in series, this is observed by a decrease in the brightness of each successive bulb. This is one of the disadvantages of a series circuit. In contrast, the current increases with the addition of each resistor in parallel circuits.

In a parallel circuit, more charges will seek the path with least resistance. Because of this, more current passes through circuit paths with lower resistance. If the resistance is the same in both paths, then the same amount of current passes through each resistor. An important feature of parallel circuits is that the voltage is the same across each resistor; it does not drop as it does in series circuits. In series circuits, voltage drops after each resistor, and current is constant, and in parallel circuits, voltage stays the same after each resistor, and current increases. The total current can be calculated as the sum of the electric current in each branch of the parallel circuit.

Most land-based, heavy machinery use AC electricity, not DC, because large amounts of electricity can be transmitted over long distances with AC while incurring significantly less loss of power than with DC electricity. DC circuits have voltage sources that provide a constant voltage, and the wiring on most ships, airplanes, and electronic circuits uses DC electricity.

The coils in the electric motors of AC equipment serve as inductors (not capacitors), which are typically coils of conducting wire in which a magnetic field is created by the electric current. Examples of capacitors are ceramic, Teflon, and air because these are non-conducting materials.

Electrical power is essentially the energy output over a given time. It is found by multiplying the current (in amps) from a voltage source by the voltage of that source. So, P = IV. Ohm’s Law states that the voltage equals current in the circuit multiplied by the resistance (V = IR). Therefore, the power equation can be written two different ways, by substituting known relations from Ohm’s Law: