EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: troutnut68 on March 04, 2021, 07:39:02 pm
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Hi Guys,
I am having trouble with this question from Electricity Demystified v 1. Is there anyone who can explain the answer?
The power formula should be used multiplied by the Time.
P = I2R ; P = IE ; P = E2/R.
Thanks,
Alan
PS The correct answer is C but I tend to get answer B when I try to plug in the numbers.
According to the book, Energy is just Power multiplied by Time.
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With the added resistance, the voltage on the lamp drops in half. So does the current. So half the current times half the voltage gives one fourth of the power. The entire circuit now draws one half the power compared to before, and half of that is wasted in the timer, leaving one fourth for the lamp.
This is impractical, since lamps change resistance with current change so it's not a real world situation.
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You don't know the lamp's I V characteristic so it can't be answered. The lamp is not a linear resistor.
It's a misleading question.
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You don't know the lamp's I V characteristic so it can't be answered. The lamp is not a linear resistor.
It's a misleading question.
If the author wanted the complication, answer D would be correct. As C is purported to be the correct answer, bob91343's answer is correct.
But you're right, the lamp is truly non-linear and, in a perfect classroom, answer D would be correct. I just don't think the complication is warranted at this level of learning.
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So does the length of time 2 hrs have anything to do with the equation? Energy = Power X Time as explained in the book. I have to admit that this question seems a bit too vague or difficult at the time it was introduced.
Alan
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The relationship between power and energy is defined by time. If you utilize 1/4 the power for any period of time you will consume 1/4 the energy.
100W light bulb for 8 hours -> 800 watt-hours energy - 100W of power over 8 hours
25 W light bulb for 8 hours -> 200 watt-hours energy - 25W of power over 8 hours.
https://enphase.com/en-us/support/what-difference-between-watt-and-watt-hour
It is vitally important to understand the difference between watts (rate of consumption) and watt-hours (total consumption).
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So does the length of time 2 hrs have anything to do with the equation? Energy = Power X Time as explained in the book.
Not really. The same length of time is used in both comparisons. And since it's a constant it can be removed from both calculations and you can just compare power (watts).