Answers and Explanations

Part 6: Electronics Information (EI)

1. B

   Voltage and current are directly proportional. Anytime voltage increases, current will also increase as long as the resistance remains constant, in accordance with Ohm’s Law.

2. C

   As electric current passes through a wire, a magnetic field is generated around the wire. The strength of the field at any given point at a certain distance around a straight wire will be constant for a given current value (and increasing the circumference of a non-coiled circuit will likewise have little effect). However, winding the wire into loops will cause the field to overlap and increase within the loops. Wrapping the wire in a coil around a ferrous (that is, made of iron) object like a nail, will lead to an even greater increase in the magnetic field. That’s because the material’s many microscopic magnets will align with and strengthen it. What about the greater wire length in answer choice (D)? Increasing the length of the wire while holding the current constant will not result in a greater field if the number of loops isn’t increased. In fact, having the same number of loops spread out over a greater wire length will actually result in a smaller increase in magnetic field strength inside the loops.

3. D

   Inductors resist change in current, and thus act as resistors for alternating current, increasing their apparent resistance with the AC frequency. However, for a continuous direct current, an inductor is the one option listed that will offer little resistance to current flow.

4. A

   In the symbol, the number 1 indicates the base, which is the middle piece of semiconductor material inside a transistor.

5. B

   For a simple, parallel circuit with two resistors of equal value, divide the resistance by two to find the equivalent resistance. In this question, two 750 ohm resistors in a parallel circuit gives an equivalent resistance of

6. C

   When current first flows through a coil, the magnetic field builds relatively slowly. This is because the expanding magnetic field generates a voltage in the coil that opposes the original current flow. This is known as counter-emf, and it is produced by a process known as self-induction.

7. C

   Via Ohm’s Law, R = V/I. But before plugging in the current, it must be converted into amperes. 5 mA = 0.005 A. Thus R = = 10,000 ohms. 10,000 ohms is the same as 10 Ω.

8. C

   Resistors R2 and R3 are in parallel with each other, while R1 is in series with them both. Since this simple circuit combines series and parallel loads, it is a series-parallel circuit.

9. B

   Adding resistors in parallel always lowers the resistance, while adding resistors in series always increases it. The rule can be tested or rediscovered by making up a simple question of two resistors in parallel and calculating the effective resistance, but it’s easy to remember if you just imagine more and more barriers in a row (series resistors) adding to the difficulty in moving forward, and the addition of more pathway choices (parallel resistors) making it easier by reducing crowding of electrons.

10. B

    The crystalline structure of pure silicon is very stable. The four valence electrons in each silicon atom bond with the valence electrons in the atoms around it, so no free electrons exist to allow current flow. This can be changed by “doping” the silicon’s crystal structure with phosphorus, arsenic, or antimony. Since these elements all have five electrons in their valence shell, they will bond themselves to the other silicon atoms, but leave one free electron that is able to migrate throughout the crystal. This changes the silicon crystal into an N-type material. This new material is still electrically neutral, but is able to conduct electricity due to the presence of free electrons.

11. A

    There are two types of transistors: an NPN transistor, and a PNP transistor. An NPN transistor is made up of a thin piece of P-type material sandwiched between two pieces of N-type material. A PNP transistor is the opposite: two pieces of P-type material that have a piece of N-type material between them.

12. B

    The reciprocals of the resistance values are and . Since is equivalent to , their sum can be simplified as . The reciprocal is Ω, which can also be -written as 1 Ω.

13. D

    The effective resistance is the sum of the resistances of each resistor in series, so the answer is 3 + 3 = 6 Ω.

14. A

    Adding a resistor when a resistor is already present increases the effective resistance.

15. C

    More than four electrons in the valence shell means that the element is an insulator. Insulators do not conduct electricity well, and therefore are useful for creating electrical barriers. Thus, an element with five electrons in its valence shell is an insulator.

16. B

    An ampere is defined as one coulomb of electrical charge flowing past a point in one second. One coulomb is the amount of charge in 6.25310¹⁸ electrons. This is the same as 6,253,100,000,000,000,000 electrons. If this many electrons flow past a point in a conductor in one second, one ampere of current is flowing.

17. A

    Inductors work exactly opposite to capacitors, in the sense that they allow DC to pass easily, but resist the flow of AC. This is known as inductive reactance, and it will rise in direct proportion to the frequency of the current flowing through the inductor.

18. D

    Only adding a resistor in parallel will decrease the -overall resistance, since it allows more paths for the current to travel through. This means the answer is either (B) or (D). Answer choice (D) works, because , and the reciprocal is =2.5Ω. In contrast, answer choice (B) would result in an effective resistance of 1.6 Ω, so it does not work. You could also have solved this problem using logic. When two loads of equal resistance value are wired in parallel, their effective resistance is half of their individual resistances (likewise, with three identical resistors in parallel, their effective resistance is of their individual resistances, and with four resistors in parallel, effective resistance is decreased to , et cetera). If you know this rule, (D) is the clear choice without doing any math.

19. B

    This is the circuit symbol for an ammeter, which is used to measure current.

20. C

    In a series circuit, the current flow will be the same in all parts of the circuit. The current that leaves the voltage source must return to the voltage source, and since there is only one path for current to follow in a series circuit, current will be the same throughout the circuit. Voltage measured across each of the components, on the other hand, may be different depending on their resistance.