Power vs. Energy test question

[dana1028]

I recently got an answer wrong and I disagree with the instructor's answer.

The equations we used:
Power = Watts = Volts x Amps
Energy = Watt-hours - Watts x hours

The test question: Two (2) 75w light bulbs are used for 4 hrs. How much power was used? [ or something to the affect]

Possible answers:

600w
600 wh
150 w
150 wh

I answered 600w....instructor said the correct answer was 150w.... because power over time = energy.

I understand that wh is used as a measure of enery consumption - used by utlities, etc. to charge you for your usage.

However there is no way you could turn on (2) 75w light bulbs for 4 hrs. and only use 150w.

Could someone please enlighten me?

Thank you

 

[gar]

100730-1350 EST

Read the question very closely.

The 4 hrs is extraneous information and in the question only to see if you know the difference between power and energy.

150 W is the correct answer assuming the voltage was constant and the lamp resistance was constant.

Also note the answers 150 W and 150 WH. The WH is there to trap you.

.

 

[bob]

To answer the question you must know the difference between energy and power. As you said

The equations we used:
Power = Watts = Volts x Amps
Energy = Watt-hours = Watts x hours

The amount of power being used buy the 2 bulbs is 150 watts but the amount of energy used is 150 watts x 4 hrs = 600 watt-hours.

However there is no way you could turn on (2) 75w light bulbs for 4 hrs. and only use 150w.

I hope this clears up your question.

 

[dana1028]

Thanks for the replies.

 

[LarryFine]

However there is no way you could turn on (2) 75w light bulbs for 4 hrs. and only use 150w.

Sure, there is. 4 seconds, 4 hours, or 4 years, only 150 watts is being used.

Now, 150 watts over a specified time; that's what's measured in watt-hours.

 

[charlie b]

Suppose that apples cost $1.00 per pound. You put five of them on the scale, and check the weight, and see that you have 2 pounds of apples. Then you leave them there for four hours, and check the weight again. How much will they weigh now?

All practical aspects aside (like getting a grocery store to agree to this experiment and hoping a bunch of flies don't start eating part of the apples), what is relevant here is that the weight is not influenced by time. Neither is power. 150 watts of light bulbs will still be 150 watts of light bulbs, no matter how long you leave them on.

 

[skeshesh]

Sound analogy Charlie, but I like your analogies better when they involve beer and foam! :grin:

 

[charlie b]

Sound analogy Charlie, but I like your analogies better when they involve beer and foam! :grin:

Many thanks, though I prefer bourbon myself. But for the record, the beer and foam analogy (i.e., for explaining power factor) is not of my creation. I don't know who to credit (or blame) for coming up with that one.

 

[zog]

Power is defined as the [rate] of doing work.

The question is the same as this. You were driving 30 MPH, now you are going twice as fast, at a constant speed, in 4 hours, how fast are you going?

 

[LarryFine]

You were driving 30 MPH, now you are going twice as fast, at a constant speed, in 4 hours, how fast are you going?

0 mph. Ran out of gas.

 

[Bilbo111]

75 watts is a rating. The actual power consumed would depend on the applied voltage and resultant current, and the configuration of the circuit.( in series, or parallel.) Assuming 120V applied, and bulbs in parallel, then 150W as noted.