Exam question on Ohm's law.

Thread Starter

Mah_Student

Joined Oct 15, 2020
2
Will you please, I need help to understand the solution to this question:
1602768392127.png
1602768432916.png
1602768458697.png
-------------------------------------------------
The examiner say that the answer is 11 Volt. I don't know why. Please help. Thanks
 

MrChips

Joined Oct 2, 2009
22,096
We do not do your homework for you. Show us how you would tackle the problem.
What is the equivalent resistance between a and d?
What is the current flowing from a to d?
What is the current flowing from c to d?

Have you considered that there is possibly a typo error in the question?
 

WBahn

Joined Mar 31, 2012
26,301
Strictly speaking, the answer should be -11 V. Probably just a bit of sloppiness on the part of the person that wrote the question. Interpret it to be talking about the magnitude of the voltage differences between the mentioned points and you'll be fine.
 

Thread Starter

Mah_Student

Joined Oct 15, 2020
2
Thanks for the replies.
I found the explanation:
The equivalent resistance in bc =R/2 and in cd = R/3.
Since the potential difference ib bc = 3 V, then the current intensity = 3/(R(2) = 6/R A.
The total equivalent resistance = R + (R/2) + (R/3) = 11R/6.
Then the potential difference between a and d =(6/R) . (11R/6) = 11 V.
Thanks very much.
 

Audioguru again

Joined Oct 21, 2019
2,061
Why doesn't the question say the resistor values?
Is the student supposed to guess that they are all the same?
This exam question is so simple that I did it without writing down anything.
 

Ian0

Joined Aug 7, 2020
750
Isn't it -3V? No-one said where the power supply was connected. It could quite feasibly be between b and c, and no-one said that a and d were connected to anything at all.
 

WBahn

Joined Mar 31, 2012
26,301
Why doesn't the question say the resistor values?
Is the student supposed to guess that they are all the same?
This exam question is so simple that I did it without writing down anything.
No need to guess -- the resistance of each resistor is R.

And, yes, for people that have experience just a bit beyond where a student would be that is taking this exam this question should be so simple that it can be done by inspection without writing anything down. So what? The question is targeted at people for whom that kind of reasoning is just beginning to come into existence.
 

WBahn

Joined Mar 31, 2012
26,301
Isn't it -3V? No-one said where the power supply was connected. It could quite feasibly be between b and c, and no-one said that a and d were connected to anything at all.
It's 3 V because that is the given condition. If you get to question whether it is -3 V instead, then you can just as validly question whether it is -98.2 V instead. It is a GIVEN that it is 3 V.

As for where the power supply is connected, your observation is valid in strict terms, but immaterial in practical terms. Representing circuit fragments this way with the understanding that the only connections to the rest of the circuit occur at the dangling ends is a nearly universal convention. It would be impossible to communicate anything other than the simplest circuits if this were not the case and if we could not rely on the reader to interpret the circuit fragment accordingly.
 

MrChips

Joined Oct 2, 2009
22,096
When I first saw the question my brain registered "the potential difference between c and d".

Now why would any instructor write down "between c and b" unless he/she was looking for a negative answer?
That is why I assumed there was a typo error somewhere.

If "between c and d" was meant, none of the choices is correct.
If "between c and b" is correct, then the instructor's answer of 11V is incorrect.

The most likely explanation is that whoever set the exam copied a question from a previous exam and simply changed a letter and some numbers, demonstrating typical laziness.
 
Top