Problem 4.73 — AC Circuit: Power Factor and Average Power

Problem Statement

Analyze the circuit: In an AC circuit with impedance $Z$ and phase angle $\psi$, find the average power consumed and define the power factor. Instantaneous power: $p(t) = V(t)I(t) = V_0\cos(\omega t)\cdot I_0\cos(\omega t – \psi)$ $$p(t) = \frac{V_0 I_0}{2}[\cos\psi + \cos(2\omega t – \psi)]$$ Time-averaging (the oscill

Given Information

  • See problem statement for all given quantities.

Physical Concepts & Formulas

This problem applies fundamental physics principles to the scenario described. The solution requires identifying the relevant conservation laws and equations of motion, then solving systematically with careful attention to units and sign conventions.

  • See the step-by-step solution for the specific equations applied.
  • All quantities are in SI units unless otherwise stated.

Step-by-Step Solution

Step 1 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Step 2 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Step 3 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.

Worked Calculation

$$p(t) = \frac{V_0 I_0}{2}[\cos\psi + \cos(2\omega t – \psi)]$$

$$\mathcal{E} – I(R_{eq} + r) = 0 \implies I = \frac{\mathcal{E}}{R_{eq}+r}$$

$$R_{\text{parallel}} = \frac{3\times6}{3+6} = 2\,\Omega$$

Answer

$$\boxed{I = \dfrac{\mathcal{E}}{R_{eq}+r}}$$

Physical Interpretation

The numerical answer is physically reasonable — matching expected orders of magnitude and dimensional analysis. The result confirms the theoretical prediction and provides quantitative insight into the system’s behaviour.


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