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

In summary, the textbook says that the starter motor needs 1200W to start the engine, but the method in the second section says that the starter motor needs 1000W. Which is it?
  • #1
halcyondays
2
0
Hello everyone, sorry if this is posted in the wrong section; I just 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!
 
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  • #2
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
Oshada said:
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...
 

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

1. What are simple power calculations?

Simple power calculations are mathematical calculations used to determine the sample size needed for a study or experiment. These calculations take into account factors such as the desired level of significance, effect size, and statistical power.

2. How do I perform simple power calculations?

To perform simple power calculations, you will need to know the desired level of significance (typically 0.05), the effect size (small, medium, or large), and the statistical power (typically 0.80 or 0.90). You can then use a power calculation tool or formula to determine the necessary sample size for your study.

3. Why are power calculations important?

Power calculations are important because they ensure that a study is adequately powered to detect a significant effect. If a study has a small sample size, the results may not be reliable or generalizable to a larger population. Adequate power also helps to avoid Type II errors, where a true effect is not detected.

4. What factors should be considered when performing power calculations?

When performing power calculations, it is important to consider the desired level of significance, effect size, statistical power, and the research question or hypothesis being tested. Other factors such as the type of statistical test being used and potential confounding variables should also be taken into account.

5. Can power calculations be performed after data collection?

While it is ideal to perform power calculations prior to data collection, they can also be performed after data has been collected. In this case, the actual effect size and sample size can be used to calculate the statistical power and determine if the study was adequately powered. However, it is always best to plan and perform power calculations before starting a study to ensure that it is properly designed and powered.

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