# PLEASE SOLVE these.. I cant get the solution..

#### John Ramelb

Joined Feb 2, 2014
26
1.Two resistors A and B are connected in series across 220V DC source. When a voltmeter with a internal resistance of 10kΩ is connected across resistor A, the instrument reads 100V and when connected across resistor B, it reads 80V. Find Resistor A and B.

2. A potential divider of resistance of 50Ω is connected across a 100V Dc source. A load resistance of 10Ω is connected across a tap in the potential divider and the negative terminal of the source. If a current of 4A flows towards the load, What is the current supplied by the source?

PLEASE I REALLY NEED HELP SOLVE THESE TWO PROBLEM.. W/sol'n please thanks

BTW WHO HAVE A PDF OF MCGRAWHILL BOOK IN ELECTRICAL CIRCUITS THANKS !!!

#### spinnaker

Joined Oct 29, 2009
7,830
Oh no homework is due and John wasn't paying attention in class again!

When I was in school, these helped a lot:

Always had one in my notebook.

They have fancier ones now that I don't remember seeing back in the day:

Looks like that site has some excellent explanations of Ohms Law BTW.

http://www.electronics-tutorials.ws/dccircuits/dcp_2.html

And you have the whole internet at your disposal today. Students today sure are lucky.

#### spinnaker

Joined Oct 29, 2009
7,830
Also the other thing that would help me is if I drew the schematic. I hate word problems. Pictures are so much easier to figure out.

#### spinnaker

Joined Oct 29, 2009
7,830
And there are simulators today to help out. Most people here like spice but a bit difficult to use. Yenka is a easy one to use but not the best for true accuracy. Probably OK for simple circuits and learning.

Wow I wish I had all of those tools years ago.

#### John Ramelb

Joined Feb 2, 2014
26
@bertus
@spinnaker

I read the site that you give. Thank you.. I buy the book " Schaum's Outlines Basic Electriciy II Edition Milton Gussow" but I'm looking for the maybe the OLD Edition of that book 3000 Fully solved Problems in Circuits 1...

Can you help me elaborate some facts that I want to know like the " internal resistance" because my Prof already explained the internal resistance only in the battery but I dont know in the Voltmeter,

I have an idea solving the problem but I dont know if I doing it right. "hehe"
My Engr. Prof. Didn't give the figure but some how i think this the figure In my own analysis.

#### John Ramelb

Joined Feb 2, 2014
26
In q.2 what is the potential divider resistance?

#### bertus

Joined Apr 5, 2008
22,276
Hello,

The picture does not show to me.
Attachments and Images

Bertus

#### John Ramelb

Joined Feb 2, 2014
26
here is the image

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Last edited by a moderator:

#### John Ramelb

Joined Feb 2, 2014
26
@bertus

thats what i think in problem no. 1 ..thank you Engr.

#### John Ramelb

Joined Feb 2, 2014
26
@bertus
@spinnaker

sir/engr. what is a potential divider resistance?

#### John Ramelb

Joined Feb 2, 2014
26
@to6afre

thank you so much it is just like getting Voltage at Base in BJT

thank you so much ENGR!

#### John Ramelb

Joined Feb 2, 2014
26
Thank you so much !!

#### John Ramelb

Joined Feb 2, 2014
26
ONLY THE q. 1 i only need explanation

#### bertus

Joined Apr 5, 2008
22,276
Hello,

Looking at the diagrams, you have two voltage dividers:

In the upper diagram, we have Ra in parellel with Rmeter, causing a voltage drop of 100 Volts across Ra.
In the lower diagram, we have Rb in parallel with Rmeter, causing a voltage drop of 80 Volts accross Rb.

With these two sets of parameters you can calculate tha values of Ra and Rb.

Bertus

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#### John Ramelb

Joined Feb 2, 2014
26
in Fig. 1 I use

Vout = Vin ( RL / RL + Ra )

100 V = 220 V ( 10k / 10k + Ra )

Ra = 12k ohms

for Fig. 2

Vout = Vin ( Rb / Rb + 12k )

Vout = 6.857k

is this is correct my answers? THANK YOU SO MUCH SIR/ Engr.

#### bertus

Joined Apr 5, 2008
22,276
Hello,

In the upper schematic Rb gets 10 mA more current as without the meter.
With this higher current the voltage across Rb is 220 - 100 = 120 Volts.
In the lower schematic Ra gets 8 mA more current as without the meter.
With this higher current the voltage accross Ra is 220 - 80 = 140 Volts.

Bertus

#### MrChips

Joined Oct 2, 2009
30,794
You have two unknowns Ra and Rb.
You have two situations which provide you with two equations.
You can therefore use the two equations to solve for the two unknowns.

#### shteii01

Joined Feb 19, 2010
4,644
Problem 1.

I came up with:
Ra=2558.7 Ohm
Rb=2450.5 Ohm

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