Simple circuit help

lamborg · Feb 23, 2009

Hi everyone,
I just started to take an online electronics course.
I have a question in exam, which I can't find an answer for.
I was wondering if you'd be so kind to help me out.
I attached 2 jpg files to this post with questions. Please take a look
Correct me if I'm wrong but the way I'm thinking, the answers should be:
7.D
8.A
9.D
10.C

Your help is very much appreciated!

RadioRon · Feb 23, 2009

Can you explain the reasons why you made those choices?

lamborg · Feb 23, 2009

RadioRon said:
Can you explain the reasons why you made those choices?

By your question I see that I'm wrong
Well,
When just S1 is closed the current can't go further than left diode.
When S1 and S3 are closed, the current will go through the left diode, through S3 and light up the light bulb.
If only S2 and S3 are closed, the circuit isn't closed, because the S1 is still open.

The only thing I don't understand, what is purpose of the right diode?

Torben · Feb 23, 2009

lamborg said:
By your question I see that I'm wrong

Are you sure about that?

The only thing I don't understand, what is purpose of the right diode?

It may (or may not) just be there to see whether you understand what it would do if it were there--or just to confuse the issue. Test writers love to do that.

Cheers,

Torben

P.S. I think that an argument could be made that in questions 7, 8, and 9 the difference between choice A and choice B could be important, since the schematic gives no indication that the battery has enough voltage to light the lamp, even if current does flow. However, I might be overthinking it, since question 10 implies that if current flows then the lamp will light up.

colin55 · Feb 23, 2009

I would just add: 8. A & B

RadioRon · Feb 23, 2009

lamborg said:
By your question I see that I'm wrong
Well,
When just S1 is closed the current can't go further than left diode.
When S1 and S3 are closed, the current will go through the left diode, through S3 and light up the light bulb.
If only S2 and S3 are closed, the circuit isn't closed, because the S1 is still open.

The only thing I don't understand, what is purpose of the right diode?

I asked the question to make sure you weren't just guessing and had put the effort in to figure it out.
Your assessment of when current flows and when it does not flow, as you stated, is correct. However, in question 8, you say that the answer is A. But how can the answer not be B as well in that case? So, if A happens, then B must happen. And, for that matter, did you know that, typically, filament light bulbs have much lower resistance when cold than when hot. In that case, if the bulb lights up, then C is also correct. So, I believe that in question 8, A, B and C will all occur.

I had not considered the possibility that Torben brings up that the battery voltage may not be enough to light the bulb. My gut reaction is that this is indeed overdoing the second guessing.

colin55 · Feb 23, 2009

Very sneaky:
8 A & B & C

Are all correct. I bet the examiner did not go into the question in this depth.

That's why you need more than 1 person to do anything.

Anyone who thinks they do something on their own is a fool.

You can always learn.

Torben · Feb 23, 2009

RadioRon said:
I asked the question to make sure you weren't just guessing and had put the effort in to figure it out.
Your assessment of when current flows and when it does not flow, as you stated, is correct. However, in question 8, you say that the answer is A. But how can the answer not be B as well in that case? So, if A happens, then B must happen. And, for that matter, did you know that, typically, filament light bulbs have much lower resistance when cold than when hot. In that case, if the bulb lights up, then C is also correct. So, I believe that in question 8, A, B and C will all occur.

Good point. My point was essentially the same, except with the addition that. . .

I had not considered the possibility that Torben brings up that the battery voltage may not be enough to light the bulb. My gut reaction is that this is indeed overdoing the second guessing.

. . .which I may have been, except we only *know* that current will flow. We don't know that the bulb will light up. But I do agree with you and Colin that the chances that this is what the test writer intended are pretty slim.

Cheers,

Torben

lamborg · Feb 24, 2009

RadioRon said:
I asked the question to make sure you weren't just guessing and had put the effort in to figure it out.
QUOTE]

Oh no, I'm not guessing. This is very important to me
However, this is the first exam in the Basics of the Basics. The book explained very shortly how the components work but nothing about circuits like this one. Since it's the Electronic's Basics, I'm pretty sure the answers, they are looking for, must be as simple as well. In addition, there is only one right answer.

Can you please explain to me, what is the diode on the right side is for?
Thanks

RadioRon · Feb 24, 2009

lamborg said:
Oh no, I'm not guessing. This is very important to me
However, this is the first exam in the Basics of the Basics. The book explained very shortly how the components work but nothing about circuits like this one. Since it's the Electronic's Basics, I'm pretty sure the answers, they are looking for, must be as simple as well. In addition, there is only one right answer.

Can you please explain to me, what is the diode on the right side is for?
Thanks

The diode on the right side has no purpose other than to befuddle the student.

lamborg · Feb 24, 2009

RadioRon said:
The diode on the right side has no purpose other than to befuddle the student.

That's what I thought too but wanted to recheck
btw, just passed the exam - 100
Thanks to all

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