NEETS Module 1 – Chapter 3: Direct Current

Basic Circuits, Ohm's Law & Power

A basic electric circuit contains three elements: a source (furnishes electrical energy), a load (device that converts electrical energy to another form), and a switch (controls current delivery). The schematic diagram is a "picture" of a circuit using standardized symbols — the technician's primary aid for troubleshooting.

Ohm's Law

Current in a circuit is DIRECTLY proportional to applied voltage and INVERSELY proportional to circuit resistance. Doubling the voltage doubles the current; doubling the resistance halves the current.

FIND CURRENT

I = E / R

Amps = Volts ÷ Ohms

FIND VOLTAGE

E = I × R

Volts = Amps × Ohms

FIND RESISTANCE

R = E / I

Ohms = Volts ÷ Amps

GRAPHICAL ANALYSIS

When resistance is held constant and voltage is varied, current changes linearly — a straight-line graph (volt-ampere characteristic). When voltage is held constant and resistance is varied, current varies inversely — a curved graph. Both relationships illustrate Ohm's Law visually.

Power (P)

The instantaneous RATE at which work is done. Basic unit: the WATT. Power in watts equals voltage times current. Three equivalent power formulas allow calculation from any two of the four quantities (E, I, R, P).

POWER — BASIC

P = E × I

Watts = Volts × Amps

POWER — VOLTAGE/R

P = E² / R

Power varies as square of voltage

POWER — CURRENT/R

P = I² × R

Power varies as square of current

      POWER RATING & EFFICIENCY
      Resistors have wattage ratings; a 50% safety factor is used in practice (e.g., a 2W resistor uses a 3W-rated part)
Carbon resistors: 1/8, 1/4, 1/2, 1, 2 W — larger physical size = higher wattage rating
Wirewound resistors: 5–200 W; special types handle more than 200 W
Efficiency = Power converted to useful energy ÷ Power consumed. Always less than 1.00 due to losses. Example: 0.95 efficiency = 95 W useful output per 100 W input
1 horsepower = 746 watts

    

🧪 Section 1 Knowledge Check

1. A circuit has 12 volts applied across a 4-ohm resistor. What is the current?

2. Which formula correctly expresses electrical power using only current and resistance?

✅ Section 1 Complete!

Series Circuits & Kirchhoff's Voltage Law

A series circuit contains only ONE path for current flow. All components are connected end-to-end; current must pass through every component.

      FIVE RULES FOR SERIES DC CIRCUITS
      The same current flows through EACH PART of a series circuit.
Total resistance = SUM of individual resistances: RT = R₁ + R₂ + R₃ + … Rₙ
Total voltage = SUM of individual voltage drops: ET = E₁ + E₂ + E₃ + … Eₙ
Voltage drop across a resistor is PROPORTIONAL to its ohmic value (larger resistance → larger drop).
Total power = SUM of individual powers: PT = P₁ + P₂ + P₃ + … Pₙ

    

Kirchhoff's Voltage Law (KVL)

"The algebraic sum of the voltage drops in any closed path in a circuit and the electromotive forces in that path is equal to zero." In practice: the sum of all voltage sources equals the sum of all voltage drops. Use assumed current direction to assign polarities; a negative result means the assumed direction was wrong (not that the answer is wrong).

      APPLYING KVL — PROCEDURE
      Assume a direction of current through the circuit
Assign polarities to all resistors based on assumed current direction
Place correct polarities on all voltage sources
Trace the closed loop; record voltage and polarity of each component in succession
Set the sum equal to zero and solve for the unknown
A negative result for current means the assumed direction was opposite to actual flow

    

SERIES AIDING vs. OPPOSING SOURCES

Series aiding: Sources driving current in the same direction — voltages ADD. Series opposing: Sources driving current in opposite directions — effective voltage is the DIFFERENCE; current flows in the direction of the larger source.

🧪 Section 2 Knowledge Check

1. Three resistors — 10 Ω, 20 Ω, and 30 Ω — are connected in series with a 120 V source. What is the total current?

2. When Kirchhoff's Voltage Law is applied and the calculated current is NEGATIVE, this indicates:

✅ Section 2 Complete!

Reference Points, Open & Short Circuits, Source Resistance

Reference Point (Ground)

An arbitrarily chosen point to which all other circuit points are compared. The reference point is always considered to be at ZERO potential. The Earth (ground) is at zero potential; the symbol for ground shows this common point. Moving the reference point changes the apparent polarity of other points but does NOT change actual circuit voltages or currents. Provides both positive AND negative voltages from a single source.

🔓

Open Circuit

A BREAK in the conducting path. No current flows. R_T = ∞. The full source voltage appears across the open. Can be caused by blown fuses, broken wires, or burned-out resistors.

⚡

Short Circuit

An accidental path of near-ZERO resistance. Current increases drastically. Usually caused by improper wiring or broken insulation. Can destroy components and cause fires.

Source Resistance (Internal Resistance, R_i)

All voltage sources have some internal resistance that causes the terminal voltage to DROP when current flows. The battery voltage measured with no load is higher than when supplying current. Internal resistance cannot be measured directly with a meter. It is represented in a schematic as a small resistor in series with an ideal battery.

      MAXIMUM POWER TRANSFER & EFFICIENCY
      Maximum power transfer occurs when the load resistance EQUALS the internal (source) resistance
At maximum power transfer, efficiency is exactly 50% — half the power is lost in the source
High efficiency is achieved when load resistance is MUCH LARGER than source resistance (most power goes to the load, less is wasted internally)
Practical compromise: communications circuits favor max power transfer; power systems favor high efficiency

    

🧪 Section 3 Knowledge Check

1. A voltage source has an internal resistance of 5 Ω. What load resistance produces maximum power transfer to the load?

2. An open circuit condition results in:

✅ Section 3 Complete!

Parallel Circuits & Kirchhoff's Current Law

A parallel circuit has MORE THAN ONE current path connected to a common voltage source. Each separate current path is called a branch. Two or more branches form a network.

      FIVE RULES FOR PARALLEL DC CIRCUITS
      The SAME VOLTAGE appears across each branch — equal to source voltage.
Branch current is INVERSELY PROPORTIONAL to branch resistance (more resistance → less current).
Total current = SUM of individual branch currents: IT = I₁ + I₂ + … Iₙ
Equivalent resistance is always LESS than the smallest individual branch resistance.
Total power = SUM of individual branch power consumptions.

    

Kirchhoff's Current Law (KCL)

"The algebraic sum of the currents entering and leaving any junction is equal to zero." Currents ENTERING a junction are POSITIVE; currents LEAVING are NEGATIVE. The total of all currents at any junction must sum to zero.

      THREE METHODS FOR EQUIVALENT RESISTANCE
      Equal resistors: Req = R ÷ N (resistance value divided by the number of equal resistors)
Reciprocal method (general formula): 1/Req = 1/R₁ + 1/R₂ + 1/R₃ + … (find common denominator; take the reciprocal of the result)
Product-over-sum (two resistors only): Req = (R₁ × R₂) ÷ (R₁ + R₂)

    

EQUIVALENT CIRCUIT

Any complex resistor network can be reduced to a single equivalent resistor representing total resistance. This process — called reduction to an equivalent circuit — simplifies analysis. The equivalent circuit contains the voltage source and a single resistor (R_eq).

🧪 Section 4 Knowledge Check

1. Two resistors — 20 Ω and 30 Ω — are connected in parallel. Using the product-over-sum method, what is the equivalent resistance?

2. In a parallel circuit, the equivalent resistance is always:

✅ Section 4 Complete!

Combination Circuits, Opens & Shorts in Parallel Networks

A combination (series-parallel) circuit contains both series and parallel elements. Solving it uses equivalent circuits: reduce parallel sections to a single equivalent resistor, redraw as a series circuit, then apply series-circuit rules.

      SOLVING COMBINATION CIRCUITS — PROCEDURE
      Identify all parallel branches and calculate their equivalent resistance
Redraw the circuit — replace each parallel network with its equivalent resistor
Calculate total resistance of the resulting series circuit
Find total current using Ohm's Law (IT = ES / RT)
Find voltage drops across each element using the series-circuit rules
Use branch voltages to find individual branch currents and power

    

REDRAWING CIRCUITS FOR CLARITY

Complex schematics are rarely drawn in neat box form. To simplify: trace current paths; label junctions (A, B, C…); recognize that wires have NO resistance and any unbroken wire is at the same potential; "stretch" or "shrink" wires as needed; redraw in stages into a standard box form.

🔓

Open in a Parallel Branch

That branch is disabled. Current still flows through remaining branches. R_T INCREASES. I_T DECREASES.

🔓

Open in a Series Portion

No current flows anywhere. The entire circuit is disabled. R_T = ∞.

⚡

Short in a Parallel Branch

The shorted branch routes all current around the other parallel resistors — entire parallel network is disabled. R_T DECREASES. I_T INCREASES dramatically.

⚡

Short in a Series Portion

That resistance drops to zero. Total resistance decreases. Current increases. Remaining components carry the higher current and may be damaged.

CIRCUIT PROTECTION

Fuses and circuit breakers are connected in SERIES to protect circuits. When current exceeds a set value, the device opens and stops all current flow. Covers for fuse boxes and junction boxes must be kept closed. Only authorized personnel should service electrical equipment.

🧪 Section 5 Knowledge Check

1. In a series-parallel circuit, a short circuit develops in one branch of the parallel network. What happens to that entire parallel network?

2. When redrawing a circuit for clarity, what electrical characteristic do wires in schematic diagrams have?

✅ Section 5 Complete!

Voltage Dividers & Electrical Safety

Voltage Divider

Two or more resistors in series across a source voltage, used to supply multiple voltage levels from a single source. Any desired portion of the source voltage can be "tapped off." Source voltage must be as high or higher than any voltage required by the divider.

      VOLTAGE DIVIDER DESIGN STEPS
      Determine load voltage and current requirements and available source voltage
Select bleeder current using the 10% rule-of-thumb: bleeder current ≈ 10% of TOTAL load current (the current that flows through the divider resistors without going to any load)
Calculate bleeder resistor value: R₁ = Eload1 / Ibleeder
Calculate total current = load current + bleeder current
Calculate remaining divider resistors from the current through them and the voltage across them

    

POSITIVE AND NEGATIVE VOLTAGES FROM ONE SOURCE

By placing the ground (reference point) between two of the divider resistors, a single voltage source can supply both positive and negative voltages. Points above ground are positive; points below ground are negative. The exact placement of ground depends on the voltage requirements of the loads.

PRACTICAL NOTE

Calculated divider resistor values often don't come out to standard values. The bleeder resistor can be rounded to the nearest standard value; the actual bleeder current must then be recalculated. Series, parallel, or series-parallel resistor networks can be used to construct non-standard values.

Electrical Safety — Shock Hazard: Current, not voltage, is the measure of shock intensity. Even very small currents through vital body parts can cause death.

Current (60 Hz AC)	Effect on Human Body
~1 mA (0.001 A)	Shock can be felt
~10 mA (0.01 A)	Shock prevents voluntary muscle control — cannot let go
~100 mA (0.1 A) for 1 second	FATAL — cardiac arrest likely

⚠ CRITICAL SAFETY FACTS

Fatalities have been recorded from voltages as LOW as 30 volts
Body resistance under unfavorable conditions (wet, cut skin) may be as low as 100–300 Ω
ALL live electric circuits must be treated as potential hazards at ALL times
Aboard ship: body may be in contact with metal structure; perspiration lowers body resistance — extra caution required
Use CO₂ extinguishers for electrical fires (nonconductive; least equipment damage)
Do NOT attempt repairs if not qualified; report suspected malfunctions immediately

      FIRST AID FOR ELECTRIC SHOCK
      First: Free victim from electrical contact using dry insulating material (board, belt, clothing) — DO NOT touch the victim directly until source is removed
Check breathing: Place hands on victim's sides at the level of the lowest ribs — feel for movement. Do NOT give artificial ventilation to someone breathing naturally
If not breathing: Begin artificial ventilation (artificial respiration) IMMEDIATELY — do not delay for any reason except freeing the victim
CPR: If both breathing AND heartbeat have stopped, Cardiopulmonary Resuscitation (CPR) is required — learn CPR from Red Cross or Navy Medical
Time is critical: 7 out of 10 shock victims were revived when artificial respiration began within 3 minutes. After 3 minutes, survival chances drop rapidly

    

🧪 Section 6 Knowledge Check

1. In a voltage divider design, the bleeder current is typically set at approximately what percentage of the total load current?

2. When responding to a victim of electrical shock who is not breathing, the FIRST priority action is to:

✅ Section 6 Complete!

🎓

Five 9s Communications

Certificate of Completion

Official Training Record

This certifies that

has successfully completed

NEETS Module 1 — Chapter 3
Direct Current

Date Completed Score StatusPASSED SourceNAVEDTRA 14173

NEETS Module 1 — Chapter 3

Chapter 3: Direct Current

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