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Ch 32: AC Circuits
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 32, Problem 63a

Commercial electricity is generated and transmitted as three-phase electricity. Instead of a single emf, three separate wires carry currents for the emfs ε1 = ε0 cos ωt, ε2 = ε0 cos(ωt +120°), and ε3 = ε0 cos(ωt−120°) over three parallel wires, each of which supplies one-third of the power. This is why the long-distance transmission lines you see in the countryside have three wires. Suppose the transmission lines into a city supply a total of 450 MW of electric power, a realistic value. What would be the rms current in each wire if the transmission voltage were ε0 = 120 V rms?

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Step 1: Understand the problem. The total power supplied by the three-phase system is given as 450 MW, and the rms voltage for each wire is ε₀ = 120 V rms. We need to calculate the rms current in each wire. Recall that in a three-phase system, the total power is distributed equally among the three wires.
Step 2: Recall the formula for power in an AC circuit: P = V_rms × I_rms × cos(ϕ), where P is the power, V_rms is the rms voltage, I_rms is the rms current, and cos(ϕ) is the power factor. For simplicity, assume the power factor cos(ϕ) = 1 (ideal case).
Step 3: Divide the total power among the three wires. Since the power is equally distributed, the power per wire is P_wire = Total Power / 3. Substitute the given total power (450 MW) into this formula to find the power per wire.
Step 4: Rearrange the power formula to solve for I_rms. Using P_wire = V_rms × I_rms × cos(ϕ), isolate I_rms: I_rms = P_wire / (V_rms × cos(ϕ)). Substitute the values for P_wire, V_rms, and cos(ϕ) into this formula.
Step 5: Perform the substitution and simplify the expression to find the rms current in each wire. Ensure the units are consistent (e.g., convert MW to W if necessary).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Three-Phase Electricity

Three-phase electricity is a method of alternating current (AC) power generation and transmission that uses three separate conductors, each carrying a current that is phase-shifted by 120 degrees. This configuration allows for a more efficient and stable power supply, as the total power delivered is constant, reducing fluctuations and improving the overall efficiency of the system.
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RMS Voltage and Current

RMS (Root Mean Square) voltage and current are measures used to express the effective value of alternating currents and voltages. The RMS value is equivalent to a direct current (DC) value that would deliver the same power to a load. For example, an RMS voltage of 120 V means that it can deliver the same power as a 120 V DC source, making it crucial for calculating power in AC circuits.
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Power Calculation in AC Circuits

In AC circuits, the total power can be calculated using the formula P = √3 × V_rms × I_rms, where P is the total power, V_rms is the RMS voltage, and I_rms is the RMS current. For three-phase systems, the power is distributed equally among the three phases, allowing for the calculation of the current in each wire when the total power and voltage are known.
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Related Practice
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Textbook Question

A generator consists of a 12-cm by 16-cm rectangular loop with 500 turns of wire spinning at 60 Hz in a 25 mT uniform magnetic field. The generator output is connected to a series RC circuit consisting of a 120 Ω resistor and a 35 μF capacitor. What is the average power delivered to the circuit?

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Textbook Question

You're the operator of a 15,000 V rms, 60 Hz electrical substation. When you get to work one day, you see that the station is delivering 6.0 MW of power with a power factor of 0.90. What is the rms current leaving the station?

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Textbook Question

Commercial electricity is generated and transmitted as three-phase electricity. Instead of a single emf, three separate wires carry currents for the emfs ε1 = ε0 cos ωt, ε2 = ε0 cos(ωt +120°), and ε3 = ε0 cos(ωt−120°) over three parallel wires, each of which supplies one-third of the power. This is why the long-distance transmission lines you see in the countryside have three wires. Suppose the transmission lines into a city supply a total of 450 MW of electric power, a realistic value. In fact, transformers are used to step the transmission-line voltage up to 500 kV rms. What is the current in each wire?

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