If the values of "C1" and "R1" shown in the illustration were 1 microfarad and 100 k ohms respectively, at what listed time would "C" be considered fully charged? See illustration EL-0086.
• RC time constant (τ) for a resistor–capacitor (R-C) circuit • How many time constants it takes for a capacitor to be considered "fully charged" on the charging curve in the illustration • Converting R in ohms and C in farads to get τ in seconds
• Use the formula τ = R × C with R1 and C1 values from the question; what value of τ in seconds do you get? • Look at the graph: along the bottom, time is marked in units of RC time constants; at about how many τ is the capacitor essentially at 100% charge? • Once you know τ and the number of time constants needed for "full" charge, which answer choice time is closest to that total charging time?
• Be sure you convert 100 kΩ to ohms and 1 µF to farads before multiplying • From the curve labeled Vc1 on Charge, note at how many RC constants the curve is almost at 100% (commonly used in electronics texts) • Multiply your τ (in seconds) by that number of time constants and compare the result carefully with each listed answer
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