# i must be really stupid

Discussion in 'General Electronics Chat' started by oli p, Nov 24, 2008.

1. ### oli p Thread Starter New Member

Nov 24, 2008
7
0
i have started a new course at college as a part of my apprentiship, and have been set an asignment.

for one of the questions i have to find out the voltage across two resistors.

i have found out that
Resistor 1= 5 ohms
R2= 7 ohms
Voltage is 12V
and the current flow is 1 amp

but now i have to find out what the voltage drop across each of the resistors is . . . .
i have no idea how to do it

thanks oli

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
Ohm's Law is your friend: E=IR.

3. ### oli p Thread Starter New Member

Nov 24, 2008
7
0
is that the same as V=IR
????

but the voltage is shown as E??

and i no i do sound really stupid but how do i use that to find the drop?

thanks
oli

4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
Yeah...E stands for "Electromotive Force"....same as voltage.

5. ### oli p Thread Starter New Member

Nov 24, 2008
7
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yeah i thought that

but still, i am quite dense
so how could i use that to get the voltage drop across the two seperate resistors?

thanks

Last edited: Nov 24, 2008
6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
If you have current you plug current and resistance into the equation, and out pops the voltage.

7. ### oli p Thread Starter New Member

Nov 24, 2008
7
0
so

if i go
V=IR

12V=1A x 12ohms

and then
12-12=0

so the voltage drop is 0

is that right??
anyone

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Yep, but you want individual voltages across each resistor. The 2 voltages (2 resistors) should add up the the power supply feeding them.

Jul 17, 2007
22,183
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10. ### floomdoggle Senior Member

Sep 1, 2008
217
2
Are these resistors in series or parallel?
This is not a trick question, but it is a trick. 5 ohm resistor should have 7 volts, 7 ohm, 5volts, should be the voltage drop. Can't believe the Sarge missed this. Please don't shoot me, I'm only the handyman.
Dan

11. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
1A through 5 ohms produces a drop of 5V, not 7V.
1A through 7 ohms produces a drop of 7V, not 5V.

12. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
I didn't miss it. But since it seems to be a homework problem, I didn't simply want to give him the answer.

Since the source voltage is 12, the circuit current is 1A, and there are two resistors valued at 5 Ohms and 7 Ohms, the only possible configuration is that the resistors are in series.

This is rationalized by Ohm's Law: I=E/R, or Current = Voltage / Resistance.
Since E=12 and I=1, R must be configured as 5 Ohms + 7 Ohms = 12 Ohms, so that I=1.

Flipping Ohm's Law around, E=IR, or Voltage = Current x Resistance.

The remainder of the problem should not be too difficult to figure out.

13. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It is always tricky with homework problems, you want to give clues without giving the answer outright.

14. ### dyeraaron Active Member

Oct 27, 2008
57
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you say you have 12 VDC and 2 resistors which are 5 ohms and 7 ohms....with 1 amp for current flow............

well 5 ohms plus 7 ohms = 12 and so if these resistors are in series which i assume they are.....12 volts / 12 ohms = 1 amp...but now you take 1 amp * 7 ohms = 7 volts across that resistor and same for 1 amp * 5 ohms = 5 volts.........Kirchoffs law is you friend as well.......what goes in must come out

15. ### floomdoggle Senior Member

Sep 1, 2008
217
2
If I goofed up giving out the answer, I am sorry. I remember this question from basic electronics back in the 70's. Can't believe they are still teaching the same story problems from 30 years ago. My apologies to all.
Dan

16. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
I used to think this was silly to be learning such old stuff, but now I am glad. I am learning stuff from 1864 right now (Maxwell's equations!). Does this make engineers useful out of school, no! But does it give them potential, yes!

Fortunately for me, I started tinkering at a young age and have a lot of practical experience.

Steve

17. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Dan,
It's not just that you tried to give an answer, you gave an incorrect answer. I don't know how you could possibly have come up with 5v across a 7 Ohm resistor and 7v across a 5 Ohm resistor when the total voltage is 12V and total current is 1A.

The problem is that the answer you gave may have confused the dickens out of the original poster.

Rather than simply solving what might seem to be (an) elementary problem(s), I try to refer students to resources that will enable them to figure out the problem themselves.

18. ### floomdoggle Senior Member

Sep 1, 2008
217
2
Sarge,
It is not that I gave an incorrect answer, it is that this is a trick. I got a B+ instead of an A because of this question. Try it again, I've never figured it out. I say this with all due respect.
Dan

19. ### Audioguru New Member

Dec 20, 2007
9,411
896
Maybe the teacher has Ohm's Law mixed up.