Discussion in 'General Electronics Chat' started by NeedAnswer, Jan 11, 2009.

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Jan 11, 2009
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Please see attachment for the question.

Thank you

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2. ### Wendy Moderator

Mar 24, 2008
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It is a trick question. A and B are the battery terminals.

Jan 11, 2009
3
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Hi Bill,
I appreciate your answer, and thanks for the quick response. But one of the listed answers is the correct answer. So, being that you said A and B are the battery posts does that mean that answer C, ('it's the same as the battery voltage'), would be the correct answer?

Thanks again

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Since this is homework I'm trying to allow you to work it out somewhat. If A and B are the battery voltage, then no other statement applies, other than current draw, which was not asked.

Jan 11, 2009
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OK, thanks Bill. I think that pretty much sums it up. I appreciate your help.

6. ### duffy Active Member

Dec 29, 2008
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Crappy question. Needs to start off with "Assuming no change to the float voltage..."

7. ### flat5 Active Member

Nov 13, 2008
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The question is fine as is. A & B will always represent the battery voltage. This is not on a test for third year electronics students.

8. ### duffy Active Member

Dec 29, 2008
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It is a crappy, misleading question.

You don't have to be third year anything to notice a change in loaded vs unloaded battery voltage. I was pretty young when I got my first meter, and like most kids the first thing you do is measure a battery. Then you measure a battery when something's connected to it...

...and you discover that "what you can assume about the voltage" actually DOES "depend on whether the light is on or off" and "depends on whether switch 1 is open or closed".

Last edited: Jan 12, 2009
9. ### steveb Senior Member

Jul 3, 2008
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469
When I looked at this question I concluded that answers A, B and C are valid. Option D should say, all of the above.

10. ### flat5 Active Member

Nov 13, 2008
403
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I stand by my statement. A-B will always show the battery voltage. The point of the question is to see if the student can follow a simple circuit.

"Assuming no change to the float voltage..."
Requires a much more advanced understanding.

There is ONE answer allowed. C is the best fit.

11. ### steveb Senior Member

Jul 3, 2008
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Absolutely not. If this was the intent, then the problem should have shown an ideal voltage source instead of a battery. Batteries are known to be horrible voltage sources with large internal resistance. To propose a battery in the question and then supply answers that relate to an internal source resistance (i.e. answers A and B) is misleading. This is either a mistake, or an unfair trick question.

Last edited: Jan 12, 2009
12. ### SgtWookie Expert

Jul 17, 2007
22,183
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C) is the correct answer, as it covers all cases.
A) and B) are only partial answers.

13. ### steveb Senior Member

Jul 3, 2008
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All three statements are true. If you prefer one answer over another, that's fine. I would pick C too if forced to give one anwser. However, this doesn't change the fact that it's a trick question, if you are only allowed to pick one answer. If the problem said that the battery was ideal and hooked up using superconducting wires, I would change my opinion.

Actually, it can be argued that C is a poor answer since wires will drop some voltage when current is flowing. The bottom line is that the allowed assumptions are not clear, and the given choices of answers do not allow any clarification.

Last edited: Jan 12, 2009
14. ### italo New Member

Nov 20, 2005
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Is definitely C . No matter what is across a battery the battery voltage will prevail as C

15. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Whoever wrote this question should consult (item 12):

http://siteresources.worldbank.org/WBI/Resources/How-to-write-items.pdf

or:

http://www.park.edu/cetl/quicktips/multiple.html

As the second reference says:

"Each item should have only one correct response. When developing items, direct students to select either the "correct" answer or the "best"answer. "Correct" answer directions work best for measuring factual knowledge, while "best" answer directions are well suited for items dealing with interpretation, understanding, or inference."

The question that is the subject of this thread should be included in the references as an example of a bad question.

16. ### steveb Senior Member

Jul 3, 2008
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Only if you assume ideal wires and ideal chassis ground. I know you will say that it is standard practice to assume those schematic symbols (ground, wires, switches) represent ideal devices. Fine. - Then in that case they should use the accepted schematic symbol for an ideal battery, not a pictorial representation that looks like a real car battery. Or, they should state that the battery should be assumed ideal.

The point of asking questions like this is not to understand the ideal case, but to understand the limitations of the ideal schematic representation. In the real world, answer C is wrong, and A and B are correct.

It's just a poor question. - Fine for homework, but unfair on a test.

17. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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And, of course, they didn't say that the battery was charged. As you say, they used a cartoon-like picture of what appears to be a car battery instead of the accepted schematic symbol for a battery. :-(

So how do we know that the battery is charged? Everyone knows that car batteries often get into a totally discharged state.

If it is completely dead, then answer C is the only correct answer. That's the ticket; tell the instructor the battery must be dead.

The argument is now about tests, not electronics knowledge. That's why people who give multiple choice tests should know how to properly make them.

I once took a chemistry test where a question said, "give the formula for periodic acid'. I pronounced the word "periodic" in the same way you would pronounce the word in the phrase, "the sine function is periodic". I couldn't imagine what kind of acid had "repeating" properties. What was meant, of course, was the iodine acid related to perchloric acid.

I asked the professor, "when did we study periodic acid (with the incorrect pronunciation)"? He just smiled at me. I thought it was a bad question.

18. ### thingmaker3 Retired Moderator

May 16, 2005
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The points at A and B will indeed measure the battery voltage. They will do so regardless of whether the battery is loaded down or not, charged, discharged, charging, discharging, overcharged, dead, or transcendental.

19. ### steveb Senior Member

Jul 3, 2008
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C is not the only correct answer if you assume a realistic model for the battery. The question is vague about whether the battery is ideal or real. Thus, it is a bad question. What makes the question worse is that C is not a good answer in the real world. In other words, the idealized circuit represents a car battery in a car with chassis ground with horn and lights. Real car grounds are not perfect and real wires are not zero resistance. Therefore C is not true in the real world, while A and B are true in the real world.

This is an example of a mindless question that dulls the mind. It makes one ignore reality and revert to established dogma. If the question wants you to solve the problem as an idealized situation, then it should make it clear that it is idealized. It has not done that, thus is is a bad question. Very bad indeed.

20. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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No, Steve.

We CAN assume the voltage at the given points is the same as the battery voltage. Our assumption would be in error if the ground wire is loose or corroded, but we CAN make the assumption. Do you assume your real world ground wires are faulty whenever presented with a schematic of them? (You can, of course. But do you?)

We can also assume this poTAYto poTAHto argument will go nowhere fast. As a result, we can assume I'm closing this thread.