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Home Q & A Board Science-Related Homework Help Physics
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lexus350 lexus350
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12 years ago
I have a small external USB hard drive attached to my laptop, which draws 0.85 Amps. My laptop's power adapter draws 85W (or ~0.71 Amps?) Now, since it's connected to my computer, would you really consider the HDD running through 120 Volts? Yes, right? I'm pretty confused.
It doesn't have its own power supply. It's USB powered only.
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wrote...
#1
12 years ago
Power (watts) = voltage x current>I have a small external USB hard drive attached to my laptop, which draws 0.85 Amps>

The issue with answering your question is I do not know the voltage. It is not 120 volts... 120 volts x .85 amps is far more power than any hard drive you see these days.

I assume that the hard drive is in some sort of enclosure that supplies DC power to it so to answer your question, one would need to know the voltage of that power supply.
wrote...
#2
12 years ago
The USB port provides a nominal 5 volts.
5 volts times 0.85 amps = 4.25 watts
assuming 100% efficiency (which is not the case) 4.24 watts would increase the current at the 120V input to the adapter by 4.24/120 = 35mA"would you really consider the HDD running through 120 Volts?"
well you could think of it that way, or you could consider it to be powered by the perhaps 110,000 volt transmision line from the power plant, anyway you look at it, it is still about 4.25 watts
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