[회로이론 1편] 작정하고 만들었습니다. 회로이론이 국민 과목이 되도록. 전기(산업)기사 필기

Main ideas / lessons conveyed

Overview of the course segment (Circuit Theory Part 1)

• The speaker introduces Circuit Theory (전기회로/회로이론) “Part 1” as a topic with many calculation problems.
• They promise to focus on core problems while reducing difficulty, suggesting they’ll cover about ~5 key problems instead of everything.
• Overall message: don’t be intimidated—this is framed as exam-relevant fundamentals, useful for electrical/industrial certifications.

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Methodologies & step-by-step instructions presented

Problem #1: Series RL transient current at a given time

Given / setup

• Switch closed at (t=0)
• DC voltage: 100 V
• Resistance: (R = 10\ \Omega)
• Inductance: (L = 1\ \text{H})
• Find current at (t = 0.01\ \text{s}), i.e., (i(t=0.01))

Key methodology

• Recognize the circuit as series RL.
• Apply the standard series RL transient current formula (referred to as “Formula 2”).
• Match the time substitution to the formula’s exponential form:
  • The speaker treats 0.01 s as (1/100) and simplifies the exponential accordingly.
• Exponential simplification leads to a value such that:
  • The computed ratio is approximately 0.63.

Result interpretation

• At (t = 0.01\ \text{s}), the current is about 63% of the final steady-state value.
• Connection to time constant:
  • Transient period: before the circuit fully settles
  • Time constant: the time at which the RL response reaches about (0.63) of its final value

Lesson

• Once you identify series RL, you can immediately apply the memorized transient-current formula.
• Understanding time constant / transients makes the exponential behavior feel less abstract.

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Problem #2: Reactive power from power factor (P–Q–S relationships)

Given

• “Emergency power” context: 12 kV

• Power factor: 0.81 (subtitles show slight variations like “0.8” and later (\cos\theta \approx 0.81); the intended idea is (\cos\theta =) power factor)

• Asked to calculate reactive power (Q)

Key methodology

• Use the power triangle:
  • (P): active power
  • (Q): reactive power
  • (S): apparent power
• Power factor relation:
  • (\cos\theta = \text{power factor})
• Trig relationship for the triangle:
  • (\dfrac{Q}{S} = \sin\theta)
  • So (Q = S\sin\theta)
• Use the identity:
  • (\sin^2\theta + \cos^2\theta = 1)
  • Typically: (\sin\theta = \sqrt{1-\cos^2\theta})

Result interpretation

• Reactive power computed around 13.2 kVAr.
• (They also note the parallel approach for active power:)
  • (P = S\cos\theta)

Lesson

• If the question asks for reactive power and provides power factor, the fastest route is:
  • power triangle + (\cos\theta =) power factor + trig identity.

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Problem #3: Equivalent resistance ((R_{th})) using open-circuit logic (Thevenin/Norton style)

Concept addressed

• Find equivalent resistance labeled as (R_{th}).
• Goal: simplify the circuit by finding the resistance seen from terminals A–B.

Given / circuit interpretation

• A current source is present.
• Terminals A and B are open (disconnected).
• Use a viewpoint based on where current can actually flow.

Key methodology

• For (V_{th}):
  • Look at the voltage across the resistor that lies in the relevant conduction path.
  • The speaker’s computed claim:
    • (V_{th} = 8\Omega \times 2\Omega = 16\ \text{V})
• For (R_{th}):
  • Since A–B are open, the current source branch is treated as effectively non-contributing from that viewpoint.
  • The speaker concludes:
    • (R_{th} = 8\ \Omega)
• Equivalent model stated:
  • (V = 16\ \text{V})
  • (R_{th} = 8\ \Omega)

How they constrain solving scope

• They explicitly limit the reduction:
  • “For this problem, this is as far as we go…”
  • Further steps would be handled later if the next question required them.

Lesson

• For equivalent resistance:
  • Apply the open-circuit condition and the logic of which paths conduct.
• Disconnected terminals can significantly change which components matter.

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Problem #4: Harmonic current in AC (RMS vs peak + impedance scaling by harmonic order)

Given

• Task: determine the 7th harmonic current corresponding to the method indicated in the subtitles.
• They reference impedance magnitude:
  • (|Z| = \sqrt{R^2 + X^2})
• Set parameters with simplified values such as (R=1\ \Omega) and harmonic-frequency-related scaling.

Key methodology

• Harmonic current magnitude:
  • (i_n = \dfrac{V_{n,\text{RMS}}}{|Z_n|})
• RMS vs peak handling:
  • If the voltage is given in a peak form like (75\sqrt{2}),
  • Convert to RMS by dividing by (\sqrt{2}):
    • (V_{n,\text{RMS}} = 75\ \text{V}) (their computed value)
• Impedance scaling for harmonics:
  • Inductive reactance: (X_L = \omega L)
  • For harmonic order (n), reactance scales by (n):
    • (X_{L,n} = n\cdot X_{L,1})
  • The speaker notes this specifically for their “3rd harmonic” portion in the impedance computation.
• Then use:
  • (i = V/Z)

Result interpretation

• Final numeric claim: the relevant harmonic current becomes 15 (subtitles were noisy, but the endpoint was a clear numeric result).

Lesson

• The solution depends on two common “gotchas”:
  1. Convert voltage to the correct form (RMS)
  2. Scale impedance/reactance using the harmonic order (multiply reactance by (n))

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Problem #5: Resonance/correction circuit (one-step component value using a formula)

Concept

• A correction/compensation circuit where a single formula yields the required resistance (R) (or a related parameter).

Key methodology

• Use the provided resonance/equivalent frequency relationship as a direct formula application.
• Apply component substitutions:
  • capacitor term: ((3R))
  • inductor term: ((2R))
• Emphasize unit consistency:
  • capacitor given as (10 \times 10^{-6}) (i.e., (10\ \mu\text{F}))
  • convert micro-units carefully so powers of 10 match
• Compute using a square-root expression:
  • Evaluating with a calculator gives 14.15 as the final answer.

Lesson

• Correction/resonance problems often reduce to formula substitution.
• Accuracy heavily depends on unit conversion and correct variable placement.

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Calls to action / study strategy (non-problem content)

• The speaker repeatedly encourages:
  • memorize key formulas (especially “Formula 2”)
  • if topics feel difficult (time constant/transients, Thevenin/Norton, harmonics/RMS), refer to their playlists, especially “Electrical Theory” for basic circuit concepts.
• They request feedback in comments and say they’ll improve future versions.
• They preview the next installment: “Parallel Theory Part 2.”

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Speakers / sources featured

• Single speaker/host: the YouTube channel narrator (first-person narration like “I am starting…”).
• Referenced sources (by category, not by named authors):
  • An on-screen concept of a “formula book” (no specific author named).
  • The high-school identity:
    • (\sin^2\theta + \cos^2\theta = 1)
• Video series / playlists referenced:
  • The channel’s earlier works/videos titled “Story,” “Cheonggi,” “Gigi”
  • The playlist “Electrical Theory”
• No other identifiable named individuals appear in the subtitles.