Solving Voltage/Current in Resistor Circuits

[alksjadf]

Homework Statement

2ohms 4ohms 3ohms and 9ohms are connected across an 8-V DC source. what is the current through the 9ohm resistor

Another problem is

A 100 V DC signal is applied to four resistors. the values of the resistors are 20ohm 40ohm 60ohm and 80ohm. what is the voltage across the 40 ohm resistor.

They're kind of similar except one is current the other is voltage.

Homework Equations

i don't know

The Attempt at a Solution

i don't know.

 

[kts123]

Remember that ohms are a measure of resistence, whereas voltage is the max flow under zero resistance. What you need to do is see how much electricty is lost as it passes through each resistor.

 

[Hurkyl]

So... you haven't seen any examples of someone solving for quantities in a circuit given other quantities? And you haven't been taught any formulae relating current, voltage, and resistance? If so, then the best way to proceed would be to complain to your school's science department.

 

[alksjadf]

How do I find out how much is lost through each resistor.

 

[kts123]

Doesn't your school provide you with textbooks? The methods should be listed in plain language. I could just blurt out the method, but it's important to be able to use your textbooks / library to get the information you need.

 

[alksjadf]

Can you just show me how this is done because I have no clue whatsoever and showing me how this is done will allow me to do the problems that follow.

 

[Hurkyl]

Can you just show me how this is done

Can, yes. Will, no. We have an explict policy on physics forums that we will not do your homework for you.

 

[alksjadf]

It's just one problem so I see how to it and then I can do the rest.

 

[kts123]

You're being given Voltage and Ohms, you need to find Amperage. Research formula which relate the three measurements. If you learn how they are related, and what Ohms/Amps/Volts are, the solution will become apparant.

 

[alksjadf]

So is the answer for the first one just going to be 8/9?

 

[alksjadf]

And the other one 100volts. That doesn't make any sense to me.

 

[alksjadf]

What do parallel and series look like? And are you sure you're supposed to add the resistors together?

 

[montoyas7940]

What do parallel and series look like? And are you sure you're supposed to add the resistors together?

What book are you using?
Are these questions from that textbook?
Have you read the chapters covering ohms law and simple circuits?

Look at the examples, get some idea of what you need to do.
Otherwise all anyone will be able to do is solve these for you.
That will harm you not help you.

 

[alksjadf]

It's a studyguide for the final.

 

[montoyas7940]

What book are you using?
Are these questions from that textbook?
Have you read the chapters covering ohms law and simple circuits?

Ok, a study-guide. Do you have a resource available besides the study-guide?

 

[alksjadf]

My textbook.

 

[montoyas7940]

What book are you using?
Are these questions from that textbook?
Have you read the chapters covering ohms law and simple circuits?

Look at the examples, get some idea of what you need to do.

Good, now we can make some progress. What textbook?

 

[alksjadf]

http://wps.prenhall.com/wps/media/objects/782/801727/cover.gif

 

[montoyas7940]

OK, look in chapter 21 ( I think) it covers electric current and direct current circuits.

Edit: post again when you have found the correct chapter. There are some examples we can discuss.

 

[alksjadf]

Yeah I was looking at 21-2

 

[montoyas7940]

Ok, there are some examples of series and parallel dc circuits. Do you understand the difference between them? Do you know the meaning of the symbols?

 

[alksjadf]

I just don't know how much is lost when it goes through each resistor.

 

[montoyas7940]

The voltage dropped or lost in each resistor is equal to the current through the resistor times the resistance of the resistor. Do you understand the difference between series and parallel circuits?

 

[alksjadf]

Yes.

 

[montoyas7940]

In your original post you don't specify the type of circuit. It matters, is it series?

 

[alksjadf]

I don't know there's supposed to be a picture but it doesn't come out. I think it would be series.

 

[montoyas7940]

Ok, well, let's just start by assuming it is a simple series circuit. Do you know the formula for equivalent resistance in a simple series circuit?

 

[alksjadf]

V=ir ?

 

[montoyas7940]

No, Req = R1+R2+R3... the sum of all of the resistors in the circuit. So, for your circuit what is Req (or the equivalent resistance).

 

[alksjadf]

18 right.

 

[montoyas7940]

Good, so Req = 2+4+3+9 ohms = 18 ohms.

In a series circuit there is only one path for current to take so all of the current flows through all of the resistors. (Are you sure that you know what a series circuit looks like?)

Using Ohms law I=V/R you can calculate the current in your circuit. Use the voltage you were given and the resistance you calculated. What do you get?

 

[alksjadf]

8/18

If it's in parallel what's the formula for finding the total Resistance?

 

[lukas86]

if resistors are in parallel you want to know the formula for finding the total resistance...?

1/R_eq = 1/R1 + 1/R2 + ... + 1/Rn

 

[alksjadf]

Ok, thank you.