DC circuits and electronic principles

NEETS Mod. 1 §2-1

NEETS Mod. 1 §2-1 — Cells, electrochemistry, primary vs secondary A battery converts chemical energy directly into electrical energy. Its building block is the cell, consisting of two dissimilar electrodes (a positive and a negative plate) immersed in an electrolyte. Chemical action between the electrolyte and the plates frees electrons at the negative electrode and removes them at the positive, creating a potential difference across the terminals; connecting a load lets current flow. A primary cell (e.g. carbon-zinc, alkaline) is not rechargeable — its chemical action consumes an electrode, and once exhausted it is discarded. A secondary (storage) cell (lead-acid, nickel-cadmium) is reversible: passing current backward through it restores the plates and electrolyte to their charged state…

NEETS Mod. 1 §3-1

NEETS Mod. 1 §3-1 — Ohm's law and electrical power Ohm's law is the foundation of all circuit calculation: the current in a circuit is directly proportional to the voltage and inversely proportional to the resistance, expressed E = I × R, where E is electromotive force in volts, I is current in amperes, and R is resistance in ohms. Rearranged: I = E/R and R = E/I. If any two quantities are known the third is found. Resistance opposes current flow and depends on the conductor's material (resistivity), length (longer = more resistance), cross-sectional area (thicker = less resistance), and temperature (for metals, resistance rises with temperature). Electrical power — the rate of doing electrical work — is measured in watts: P = E × I. Combined with Ohm's law this gives the useful forms P =…

NEETS Mod. 1 §3-2

NEETS Mod. 1 §3-2 — Series circuits and Kirchhoff's voltage law A series circuit provides a single path for current, so the same current flows through every component. Its rules follow from that: the total resistance is the sum of the individual resistances (Rt = R1 + R2 + R3 …); the current is the same at every point (It = I1 = I2 = I3); and the source voltage divides among the resistances in proportion to their size. Kirchhoff's voltage law states that the algebraic sum of all voltage drops around a closed loop equals the applied voltage (or, the sum of all voltages around any closed loop is zero). Thus Et = E1 + E2 + E3. Each resistor's drop is found by Ohm's law using the common current (E1 = I × R1), making series strings act as voltage dividers. A break anywhere (an open) stops all …

NEETS Mod. 1 §3-3

NEETS Mod. 1 §3-3 — Parallel circuits and Kirchhoff's current law A parallel circuit provides two or more paths (branches) for current between the same two nodes, so the same voltage appears across every branch (Et = E1 = E2 = E3). Kirchhoff's current law states that the total current entering a junction equals the total leaving it; thus the source current is the sum of the branch currents (It = I1 + I2 + I3), each branch drawing current per Ohm's law from the common voltage. Total resistance of a parallel combination is always less than the smallest branch resistance, found from 1/Rt = 1/R1 + 1/R2 + 1/R3; for two resistors the product-over-sum form Rt = (R1 × R2)/(R1 + R2) is handy, and for N equal resistors Rt = R/N. Adding parallel branches lowers total resistance and increases total c…

NEETS Mod. 1 §3-4

NEETS Mod. 1 §3-4 — Series-parallel circuits and voltage dividers Most real circuits are series-parallel combinations that must be simplified in steps. The method is to reduce parallel groups to a single equivalent resistance, then treat those equivalents in series with the rest, working from the components outward until one total resistance remains; total current is then found from the source voltage, after which the technician works back inward to find the voltage across and current through each part. Voltage dividers use a series resistance string tapped at points to supply several lower voltages from one source, sized so that the bleeder current is large compared with the load currents to keep the tap voltages stable. Internal source resistance behaves as a resistor in series with the…

NEETS Mod. 2 §2-2

NEETS Mod. 2 §2-2 — RL time constant and inductors in combination The rate at which current rises or falls in an inductive DC circuit is set by the time constant, T = L/R seconds, where L is inductance in henrys and R is series resistance in ohms. In one time constant the current reaches about 63% of its final value; it is essentially at full value after five time constants, and it decays on the same schedule when the source is removed. Inductors in series (not magnetically coupled) add like resistors: Lt = L1 + L2 + L3. Inductors in parallel combine by the reciprocal rule, 1/Lt = 1/L1 + 1/L2 + 1/L3, giving a total less than the smallest. Inductance only opposes changing current, so it has no steady-state effect on pure DC once current is constant, but in an AC circuit — where current is …