# i must be really stupid

#### oli p

Joined Nov 24, 2008
7
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

#### KL7AJ

Joined Nov 4, 2008
2,229
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
Ohm's Law is your friend: E=IR.

#### oli p

Joined Nov 24, 2008
7
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

#### KL7AJ

Joined Nov 4, 2008
2,229
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
Yeah...E stands for "Electromotive Force"....same as voltage.

#### oli p

Joined Nov 24, 2008
7
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:

#### Wendy

Joined Mar 24, 2008
23,123
If you have current you plug current and resistance into the equation, and out pops the voltage.

#### oli p

Joined Nov 24, 2008
7
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

#### Wendy

Joined Mar 24, 2008
23,123
Yep, but you want individual voltages across each resistor. The 2 voltages (2 resistors) should add up the the power supply feeding them.

#### floomdoggle

Joined Sep 1, 2008
217
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

#### Audioguru

Joined Dec 20, 2007
11,248
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
No.
1A through 5 ohms produces a drop of 5V, not 7V.
1A through 7 ohms produces a drop of 7V, not 5V.

#### SgtWookie

Joined Jul 17, 2007
22,229
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
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.

#### Wendy

Joined Mar 24, 2008
23,123
It is always tricky with homework problems, you want to give clues without giving the answer outright.

#### dyeraaron

Joined Oct 27, 2008
57
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

#### floomdoggle

Joined Sep 1, 2008
217
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

#### scubasteve_911

Joined Dec 27, 2007
1,203
Can't believe they are still teaching the same story problems from 30 years ago.
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

#### SgtWookie

Joined Jul 17, 2007
22,229
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
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.

#### floomdoggle

Joined Sep 1, 2008
217
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

#### Audioguru

Joined Dec 20, 2007
11,248
Maybe the teacher has Ohm's Law mixed up.