Two simple questions about equivalent resistance and kirchoff laws

  • Thread starter carlos125
  • Start date
  • #1
13
0
two simple questions about equivalent resistance and kirchoff laws!! pleaseeeee! :D

EQUIVALENT RESISTANCE:
Hi everybody . greetings from Peru.I know it's a dumb question but help please .please read below

Homework Statement


For the network shown in the figure, show that the resistance Rab = (27/17). Ω
View attachment er.bmp

The Attempt at a Solution


This is the the soltuion I found on the internet ,
View attachment serway.bmp
but i don't get WHY rules of equivalent resistances in parallel and series cannot be applied to this problem . obvously using those rules I get another answer which is wrong, can ayone help me please?
KIRCCHOFF:
And i have another question can a batery charge another one?
it looks like i have missed something in class because i don't understand .... like in the solution of this problem :

A dead battery is charged by connecting it to the live
battery of another car with jumper cables (Fig. P28.25).
Determine the current in the starter and in the dead
battery.
sol.jpg


the solution says "the current is backward in the dead baterry tending to charge it up" why? ,I mean ¿what direction of current is needed so that a batery can be charged? ¿from high potntial to less potential ? i just don't get it
can someone explaian this to me please , if you have links it would be great too. Thank you very much
 
Last edited by a moderator:

Answers and Replies

  • #2
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,308


but i don't get WHY rules of equivalent resistances in parallel and series cannot be applied to this problem

Because the circuit doesn't contain resistors that directly "in parallel" with another.
 
  • #3
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073


i don't get WHY rules of equivalent resistances in parallel and series cannot be applied to this problem .
Hi carlos125, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

You should be able to see there is no pair of parallel resistors in that figure, nor are there two resistors in series. You need to get it clear in your mind exactly what it means for a pair of resistors to be in parallel.
And i have another question can a batery charge another one?
it looks like i have missed something in class because i don't understand .... like in the solution of this problem :

A dead battery is charged by connecting it to the live
battery of another car with jumper cables (Fig. P28.25).
Determine the current in the starter and in the dead
battery.
If current flows out of the + terminal of a battery into an external circuit, the battery is discharging. If current flows into the + terminal of a battery from an external circuit, the battery is charging. (Assuming we are talking about secondary cells).
 
Last edited by a moderator:
  • #4
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,308


the current is backward in the dead baterry tending to charge it up" why?

Thats the way batteries work. Current flows one way during discharge and the other way during charge.

If you want to know why take a look at the chemistry that goes on in a battery. Rechargable batteries use a reversible chemical reaction that creates a flow of electrons. In order to reverse the reaction you have to "push" electrons the other way, typically using a higher voltage than that of the battery.
 
  • #5
13
0


thank you guys for your quick replies ,but aren't the 1-ohm resistance(upper left) and the 3-ohm resistance in series? . The definition for resitance in series is "Two or more elements are in series if they are cascaded or connected sequentially " ,aren't they sharing the same wire? so why aren't they in series? look:
res.gif

thanks . btw the wire i'm talking about doesn't separate in two branches too , i'm confused
 
  • #6
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,073


Elements are in series iff the exact same current that passes through one also passes through the other/s.

Elements are in parallel iff both of the ends of those elements connect to the same points.

The 1Ω and the 3Ω would be in series iff nothing was ever connected at point 'a', because with nothing connected at point 'a' those two resistors would carry the same current. But point 'a' is where the external circuit connects, delivering to those resistors unrelated and different currents.
 

Related Threads on Two simple questions about equivalent resistance and kirchoff laws

Replies
1
Views
923
Replies
7
Views
4K
Replies
3
Views
897
Replies
1
Views
1K
Replies
10
Views
2K
  • Last Post
Replies
5
Views
1K
Replies
4
Views
2K
Replies
9
Views
5K
Replies
1
Views
482
Replies
9
Views
2K
Top