# Newbie electronic resistance question

Discussion in 'Electronic Design' started by ragtag99, Aug 2, 2006.

1. ### ragtag99Guest

Thanks for taking the time to look at this post. I am new to the field
but we all had to start at some point somewhere right.

I'm trying to study on my own and I came across this web page
http://library.thinkquest.org/10139/small/electro4.html and the very
last question, just scroll all the way to the end and theres a
'challenge question', I don't know where im going wrong. My answers
aren't the same as the answers given. Could someone instruct me on how
to solve that very very primative cirucit.

Also could someone give me a practical example of why you need to know
Ohms law, which i am studying on faith alone right now. Would it be
something along the lines of "I have X component which runs on Y amps
but I have 3Y amps running though, what resistance would i need" or
something like that? Or am i way off.

Thanks for any help.

Jesse

2. ### Jim ThompsonGuest

The answer given for question 5 IS correct.

...Jim Thompson

3. ### Michael A. TerrellGuest

Jessie, newbie questions should be asked in
<

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

4. ### Spehro PefhanyGuest

In multiple choice questions where you have no clue as to what they

Best regards,
Spehro Pefhany

5. ### leepsGuest

with why you should learn ohms law. Actually you shouldnt learn ohms
law you should understand electricity and in turn come to understand
ohms law, it should just be 'common sense'.
If you want some examples with why you would need it.
You want to save money by using the smallest resistor possible.
What do you need .25 watt .5 watt or 1 watt or 5 watt
well if you know the voltage and the resistance you can figure out
current and with the current and the voltage you can thus figure out
wattage watts=volts*amps or watts =resistance*voltage squared
or you have an analog to digital converter that will only take 5 volts
but your signal goes up to 12 volts. What do you do, have no fear, for
ohms law is here!
use a voltage divider and knock that voltage down
divide by 4 now your giving your a/d 3 volts not 12
see it comes in handy
what you should be doing is studying whats a volt, whats an amp,
whats inductance, whats capacitance whats resistance and in doing so
ohms law will become "like yeah DUH i didnt need them to tell me that"

C.

7. ### David L. JonesGuest

You have to essentially work from the inside, outward.
Solve the easiest bits first, and build your way up to the final
circuit. The previous questions are designed to give you this skill.

There are obviously three resistors in parallel (2,3,5ohm), so solve
them first, that gives 0.9677ohms

Next solve the two resistors in series, 10ohm + 12ohm = 22ohm

Now you'll notice that the 3 resistors you solved first are in parallel
with the two series ones you solved second, so solve 0.9677ohms in
parallel with 22ohms, that's 0.9269ohms.

This total value of 0.9269ohms is then clearly in series with the 3
ohms resistor on the right. So 3 + 0.9269 = 3.9269ohms

Hope that helps.

Dave

8. ### ragtag99Guest

yes that helps immensily. I was way out there using the resister on the
right as a series with the middle resister in the center and all other
sorts of lunacy. Now i see where i was wrong in so many situations
thanks a lot.

for now on ill take everyones advice and post in the .basic newsgroup
(this was the first one that google came up with :/ ) Ill be posting
here soon enough though

thanks again

Jesse

9. ### David L. JonesGuest

Great!
Now it's time to move onto the famous resistor cube problem:

*evil grin*

Dave

10. ### John WoodgateGuest

You should post to sci.electroncis.basics
which is specially for helping new people.
What answers do you get? That may help to detect where you are going
wrong.

The three low-value resistors in parallel are 2 ohms, 3 ohms and 5 ohms.

The way to calculate the total resistance in your head is not to use
directly the formula 1/R = 1/R1 + 1/R2 + 1/R3 but an equivalent:

2 x 3 x 5 = 30. 2 ohms = 30/15, so 1/(2 ohms) = 15/30. Similarly,
1/(3ohms) = 10/30 and 1/(5 ohms) = 6/30.

Then 15/30 + 10/30 + 6/30 = 31/30. This is 1/R, so R = 31/30 ohms.

Then the series branch: 10 ohms + 12 ohms = 22 ohms.

22 ohms in parallel with 31/30 ohms: Work in the same way as for the
three resistors: 22 x 30 = 660. 30/660 + (31 x 22)/660 = 712/660, so the
resistance is 660/712 = 0.927 ohms (use calculator)

Add the 3 ohms in series to get the total resistance = 3.927 ohms.
I can understand that view. The trouble is that you need to learn more
about practical circuits before you could understand why and how Ohm's
Law is used. Some examples on the web page, especially fig 3, rarely
come up in real life.
You are way off. Sorry.

11. ### John WoodgateGuest

The mind boggles. DON'T, whatever you do, EVER get a teaching or
training job.

12. ### leepsGuest

Actually i do a pretty good job when the person is in front of me. But
yeah i agree after i read what i wrote i know i didnt do much help. I
hope at the very least he'll take my advice and look up the voltage,
amperage , etc instead of just learning ohms law cause thats what your
supposed to do

13. ### John WoodgateGuest

English may not be your first language but you write it as if you know
it well. So why not go the further step of punctuating properly and

You give the impression of semi-literacy.

14. ### PeteSGuest

Amperage?

Is that the amount of '&' per line?

or did you mean current?

PeteS

15. ### John WoodgateGuest

In message <>,
No, that's 'ampersandwich'
He's just paying ohmage to technician-speak.

16. ### leepsGuest

I suppose I deserve it. The thing is i grew up in the states and spent
the last two years in france and i hate to say it im forgetting some of
my english. im actually starting to put the adjective after the noun
sometimes when i speak english ill say things like food hot or girl
blond. Its actually starting to scare me.
but hey give a guy a break im trying to help

17. ### Lord GarthGuest

One of my pet peeves as well!

18. ### Michael A. TerrellGuest

I didn't think you were supposed to pet peeves?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

19. ### Sjouke BurryGuest

Watch out, they might bite,especially when they
have been estivating.

20. ### ehsjrGuest

A practical example is putting a resistor in series
with an LED, so that you won't burn the LED out.
Say you have a typical LED, which has a maximum current
rating of 30 mA, and you want to light it from a 9 volt
battery. The LED uses 1.8 volts, so you need to drop
7.2 volts in the resistor. E=IR, so 7.2 = .03 R, therefore
R = 7.2/.03 or 240 ohms.

Ed

Would it be