# Exam question on Ohm's law.

#### Mah_Student

Joined Oct 15, 2020
2

#### MrChips

Joined Oct 2, 2009
24,610
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?

#### crutschow

Joined Mar 14, 2008
28,170
Have you considered that there is possibly a typo error in the question?
Why do you say that?

Mah_Student, Do you know how to calculate series and parallel resistance?

#### ericgibbs

Joined Jan 29, 2010
14,138
hi Mah,
All the resistors are the same value, so assign an equivalent value to each section.
Say 6R, 3R and 2R.
You know there is 3V across the 3R,,,, consider the voltage ratio's.
E

#### Delta Prime

Joined Nov 15, 2019
1,187
I need help to understand the solution to this question:
The answers are given.do you know about ratios? If not then you must say so,then I will tear it apart to help you understand. #### MrChips

Joined Oct 2, 2009
24,610
Why do you say that?
Sorry, my mistake.
However, to be pedantic, the result is a negative value. I don't see a negative value as an option.

• hrs

#### WBahn

Joined Mar 31, 2012
26,398
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.

#### WBahn

Joined Mar 31, 2012
26,398
Will you please, I need help to understand the solution to this question:
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The question implies that the particular value of R doesn't matter. So pick a value of R that makes things easy for you.

#### 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.

#### dl324

Joined Mar 30, 2015
13,506
Then the potential difference between a and d =(6/R) . (11R/6) = 11 V.
The absolute value is 11V. Problem is poorly worded.

#### Audioguru again

Joined Oct 21, 2019
3,833
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
3,754
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.

#### Audioguru again

Joined Oct 21, 2019
3,833
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.
You are correct. The exam question was written by an idiot.

• Delta Prime

#### sghioto

Joined Dec 31, 2017
2,416
he exam question was written by an idiot.
I wouldn't say that. The circuit in question is hypothetical don't over analyze it. Besides what if the current is AC.

#### WBahn

Joined Mar 31, 2012
26,398
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
26,398
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.

#### WBahn

Joined Mar 31, 2012
26,398
I wouldn't say that. The circuit in question is hypothetical don't over analyze it. Besides what if the current is AC.
It would have the same issue regarding the relative polarity of the two voltages in question.

#### MrChips

Joined Oct 2, 2009
24,610
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.

#### sghioto

Joined Dec 31, 2017
2,416
It would have the same issue regarding the relative polarity of the two voltages in question.
Even with AC current?