Physics Help Forum power consumption

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum

 May 5th 2016, 04:39 AM #1 Junior Member   Join Date: Jan 2012 Posts: 19 power consumption The question is An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be __________. I proceed with P = VI. I found I (electric current) = 100/220. This I substituted in P = VI. = 110 * (100/220) = 50. But they say the answer is 25 W. Where I went wrong? Kindly enlighten me. with thanks and regards, Aranga
 May 5th 2016, 05:24 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,334 You are assuming, incorrectly, that the current is the same in both cases. It's not. Start by considering that the resistance of the incandescent bulb is a constant. You can calculate the value of the resistance from P_1 = V_1^2/R. Now that you have a value for R, you can use it to calculate a new value of power for a different voltage using P_2 = V_2^2/R. Alternatively you can divide the second equation by the first: P_2/P_1 = (V_2^2)/(V_1^2) From this you can see that if V_2 = (1/2) V_1, then P_2 = (1/4) P_1. Another approach is to recognize from V=IR that if V is cut in half then I must be cut in half (assuming constant R). Then from P_2 = V_2 I_2 you have that both V_2 and I_2 are half their original values. Hence P_2 is 1/4 its original value. Last edited by ChipB; May 9th 2016 at 06:17 AM.
 May 5th 2016, 11:15 PM #3 Junior Member   Join Date: Jan 2012 Posts: 19 Sir, Excellent. Now I understand. I always have the confusion when to use which. P=VI, P = (V^2)/R and P = (I^2)*R. With Best wishes, Aranga
 May 12th 2016, 12:31 AM #4 Junior Member     Join Date: May 2016 Location: STZGT Posts: 4 The wave formula that I would use is speed = wavelenght/period so 170/0,6 = 283

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