Voltage in a 2 battery series circuit

[Dart82]

Homework Statement

What is the magnitude and direction of the current in the following circuit?
[URL=http://imageshack.us][PLAIN]http://img234.imageshack.us/img234/7587/circuituk2.gif[/URL][/PLAIN]
resistance for series: Req=R1+R2+R3+R4...etc
Kirchhoff's rules --> Loop Rule: sum of potential drops must equal sum of potential rises
V=IR

The Attempt at a Solution

1st added all the voltage drops: I(5+27+12+8) = I52
2nd added all the Voltage rises: 30 + 10 = 40V

3rd solved for V when V=IR
40 volts/52 ohms= I
*since resistors are in series they experience the same current right?

my answer .77 Amps clockwise [which is wrong] the right answer is .38 Amps clockwise

My question is this: Why am i not supposed to add the voltages (30+10) together when solving this problem?? If i solve for this problem in the following way, 30V-10V = I(52ohms) then i get .38 Amps clockwise (which is right) but i don't understand why i have to subtract the voltages!

 

[Panda]

Mark the poisitive side of the cells in your image.
Does that give you a clue?

 

[Dart82]

Yes. It seems the only logical explanation i can think of is that when 2 batteries "oppose" each others flow; then one battery will reduce the voltage of the other.

 

[turdferguson]

Yes, the equivalent voltage will either be a sum or (in this case) difference of the 2 voltages. Going in a clockwise loop from A, a particle will go from low potential to high potential (+30V) and then go from high potential to low potential (-10V). If the batteries were arranged + to - in series, you could add the voltages because there would be a rise followed by another rise

 

[Dart82]

i see; i mistakenly counted the 2nd battery as a rise instead of a drop. Thanks again!

 

[louie3006]

i'm staring at the same problem and yet don't get how to solve it ? can anyone offer me any help?! I might have this problem on my final exam this coming monday may 10th , 2010. help please

 

[Phrak]

Which part do you find confusing?

 

[louie3006]

Which part do you find confusing?

the kirchhoffs rule as a whole. I tried solving this problem in the following way :
R= 1/32 + 1/20 = 0.083 ohms
I= V/R = 30/0.083= 360 amps
which I know is wrong so what can I do, I spent the last hour watching a professor from MIt working a similar problem yet he did not show it with numbers or real answers.

 

[Phrak]

Resistance in series adds like this:

R₁=32
R₂=20

R_sum=32+20