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Simple power calculations (or so I thought) PLEASE HELP EXAM ON TUES

  • #1
Hello everyone, sorry if this is posted in the wrong section; I jsut signed up.

Homework Statement



Okay, so the question in my textbook is:
"A car engine needs 1000W to start it. The starter motor supplies this power. The starter motor is 80% efficient and it runs off a 12V battery.
a) Calculate the current the stater motor draws from the battery to start the engine.


Homework Equations


Efficiency% = (output power / Input power) X 100 [eq. 1]
V=IR (ohm's Law)

The Attempt at a Solution


Ok, so basically, I know that I need to find the actual power that needs to be drawn, then find out current (I) through V=IR.

The only part I am having difficulty with is finding the power that needs to be drawn.
I thought that (from equation [1]) that 80%= (power output / power input) x 100, then rearrange this to find power input. Given from information that power output has to be 1000, rearranging the equation will give power input needed as 1250W (please check this for me).

However, the answers in the book shows that I should simply do this: power drawn = 1.2(1000) = 1200W..... this is a complete different answer to the one above! Which one do I really use?

To make matters worse, there is another question int he book "A generator can provide 5000W of electrical power. An engine drives the generator, and the generator is 80% efficient. How much mechanical power must the engine provide?"
The answer in the book solves this by (5000W / 80%) = 6250W (which is basically using the power efficiency equation... why can't i use 5000W X 1.2= 6000W to solve this???

Thanks in advance for any help!
 

Answers and Replies

  • #2
41
0
1250W seems to be right. Your textbook must have messed up somehow! The current is easily obtained from P = VI.

Basically: Second section's right -> Do the same for first.
 
  • #3
1250W seems to be right. Your textbook must have messed up somehow! The current is easily obtained from P = VI.

Basically: Second section's right -> Do the same for first.
Thank you for your help. I also thought that maybe the textbook messed up, but the method of 1000(1.2) still seems plausible if you think about it...

Really appreciate it! I'm going to wait for a few more answers just to confirm...
 

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