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Electrical Engineering?
A 1.9 mF capacitor has been charged to12 V. The energy stored in the capacitor is used to light up a 0.8 KOhm lamp. Find the current flowing through the lamp 2.8 sec after the switch was turned on.
2 Answers
- qrkLv 75 years agoFavorite Answer
The resistance of an incandescent lamp filament will vary with applied voltage (actually, with temperature of the filament). This question is difficult to answer since you need to know the resistance of the filament with temperature and you need to know the thermal response of the filament. I would tell the prof that he didn't supply enough information about the filament to answer this question.
If you just use a fixed 800 ohm resistor for the load, you simply use the RC charge/discharge equation for a capacitor to find the voltage across the capacitor after 2.8 seconds. Once you know the voltage, use ohms law to calculate the current.
Vc = Vi * e^(-t/RC)
where Vc = voltage across the capacitor, Vi = initial voltage across the capacitor.
No stinking algebra needed to solve this one.
- ?Lv 65 years ago
The Time Constant is (resistance) x (capacitance). V(t) = Vmax*e^(t/(R*C)). Plug in 1.9 x 10^-3 F (if it really is millifarad? that is a big cap). And 0.8 x 10^3 ohm, and 2.8 seconds, and then you will know the voltage across the cap (which will also be across the resistor). Now I = V/R, and solve for current.
