exam question help, please.

Thread Starter

accool

Joined Sep 13, 2018
5
hi we got this question on an exam test, and i got it wrong i want to know how to solve it, thanks.

QUESTION:
find Rx so that V0=8v or something along those lines the point is to find Rx

upload_2018-9-13_20-42-2.png

thanks.
 

LesJones

Joined Jan 8, 2017
2,638
Show us how you went about calculating Rx so we can see where you went wrong. I can see a voltage and a current that you can calculate very easily.

Les.
 

Thread Starter

accool

Joined Sep 13, 2018
5
Show us how you went about calculating Rx so we can see where you went wrong. I can see a voltage and a current that you can calculate very easily.

Les.
First i calculated the current at v0=8v by doing 8v/10k(ohms) which equals to 0.8mA

then i calculated the voltage over R1 by doing 0.8mA x 2k(ohms) which equals to 1.6v
i know that since Rx and R2 are parallel they have the same voltage which is 8v
so all i need to do is find the current through Rx so then i can divide it by 8V and get Rx.
right?
 

bertus

Joined Apr 5, 2008
20,535
Hello,

The voltage accross R1 is not the mentioned 1.6 Volts.
The powersource is 10 Volts.
Accross R2 the voltage is 8 Volts.
What is the voltage accross R1?

Bertus

PS I moved your thread from digital design to homework help.
 

Thread Starter

accool

Joined Sep 13, 2018
5
Hello,

The voltage accross R1 is not the mentioned 1.6 Volts.
The powersource is 10 Volts.
Accross R2 the voltage is 8 Volts.
What is the voltage accross R2?

Bertus

PS I moved your thread from digital design to homework help.

ok thanks ill make sure to post in homework help next time.

i found that he voltage is 1.6 volts at R1 why is it wrong?
i didnt undertstand what you said after that im sorry.
 

bertus

Joined Apr 5, 2008
20,535
Hello,

On one side of the resistor R1 there is the powersource of 10 Volts.
On the other side of the resistor R1 there is resistor R2 with a voltage of 8 Volts.
What is the voltage difference accross R1?

Bertus
 

MrChips

Joined Oct 2, 2009
21,090
ok thanks ill make sure to post in homework help next time.

i found that he voltage is 1.6 volts at R1 why is it wrong?
i didnt undertstand what you said after that im sorry.
Because the voltage across R1 plus the voltage across R2 must add up to the supply voltage.
 

Thread Starter

accool

Joined Sep 13, 2018
5
Hello,

On one side of the resistor R1 there is the powersource of 10 Volts.
On the other side of the resistor R1 there is resistor R2 with a voltage of 8 Volts.
What is the voltage difference accross R1?

Bertus
oh i see so my finding 1.6v but its just simply 2v because thats the voltage difference.
but where do i go after that?
 

MrChips

Joined Oct 2, 2009
21,090
If you were to study and apply the rules of KVL (Kirchhoff Voltage Law) and KCL (Kirchhoff Current Law) you will be able to see the solution to your problem.
 

WBahn

Joined Mar 31, 2012
25,750
First i calculated the current at v0=8v by doing 8v/10k(ohms) which equals to 0.8mA

then i calculated the voltage over R1 by doing 0.8mA x 2k(ohms) which equals to 1.6v
i know that since Rx and R2 are parallel they have the same voltage which is 8v
so all i need to do is find the current through Rx so then i can divide it by 8V and get Rx.
right?
You are making one of the classic mistakes in applying Ohm's Law.

You found the current in R2 to be 0.8 mA. Fine so far.

But now you want to find something out about R1 and since you have an equation with a V, and I, and an R you basically said, "Hey, I know and R and I know and I, so I will just throw them at this equation and get a V."

But Ohm's Law is VERY specific -- in order to find the voltage across a resistance you need to use the current flowing in THAT resistor -- you only have the current flowing in some other resistor.

What is the voltage on the left side of R1?

What is the voltage on the right side of R1?

Can you find the voltage drop across R1?
 
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