# Physics : Circuts lAb kirchhoff's nodes HELP.

alangmix

## Homework Statement

I have just done a lab and im very confused on what to do.

2 batteries 3 resistors - 3 currents -> recieved by testing.

So i have to do the theoretical equations

ive done the lab and got the currents

and so far ive came up with this.

( -V2 + Vt1 - V1 ) for the first loop
-> also (- i 2 R 2) + Vt1 + ( - i 1 R 1) idk if this is correct

and

( V3 - Vt2 + V2)

and ( i 3 r 3 ) - Vt2 + ( i 2 r 2 )

im not sure what im supposed to do. Plug in the givens?

i think i need two kirchhoff equation

1 node equation .

just very confused

Vt1 - 5v
vt2 - 4 v

R1 - 750 ohms
R2 - 100 ohms
r3 - 750 ohms

am i supposed to just find the equation? or is there a variable thats not given?

i want to LEARN how to do it so if someone could explain it would be nice

## Homework Equations

The only one i know is

V = IR

and some others for kirchhoff which i came up with going according:

1. I = current going against "Pattern" (ABCDE) = -

I = current going with the "Pattern" = +

## The Attempt at a Solution

I have given this above

Homework Helper
welcome to pf!

hi alangmix! welcome to pf! (try using the X2 icon just above the Reply box )

before i go any further, is this a letter "B", with two batteries on the left, both facing the same way, and three resistors on each of the three horizontals?

## Homework Equations

The only one i know is

V = IR

and some others for kirchhoff which i came up with going according:

1. I = current going against "Pattern" (ABCDE) = -

I = current going with the "Pattern" = +

yes, that's correct …

for that reason, the most important thing in any question on Kirchhoff's rules is to start by drawing the circuit with arrows showing the current direction, so that you don't get confused about when to use + and when to use - (it doesn't matter if you draw the arrow the wrong way … it just means that that current will come out as minus)
( -V2 + Vt1 - V1 ) for the first loop

i've never seen that notation before (what does the "t" stand for?), but i see what i means, and yes it's perfectly correct -> also (- i 2 R 2) + Vt1 + ( - i 1 R 1) idk if this is correct

and

( V3 - Vt2 + V2)

and ( i 3 r 3 ) - Vt2 + ( i 2 r 2 )

you may have got the sign of i2R2 wrong, but i can't tell without knowing whether the two batteries are pointing the same way or not

and anyway these should be equations, so they must end "= 0" im not sure what im supposed to do. Plug in the givens?

i think i need two kirchhoff equation

1 node equation .

(btw, they're all Kirchhoff equations … KVL and KCL, or loop and node )

yes, those are your two loop equations, and you need a node equation also, only involving the currents …

then you can happily plug and chug alangmix
thanks for the help and
the circuit is like this

_______resistor 1 ___________l___ _______resistor 3 _____________
l l
Battery 1 -------------------- resistor 2 -------------------------battery 2
l l
l______________-_________________l_________-________________l

current for both batteries going upward hence- creating two loops.

did i get what i wrote correct?

what else am i missing ?

Homework Helper
hi alangmix! sorry, but i don't understand your diagram can you just describe the circuit in detail?

alangmix
It goes from battery one to resistor 1 -> while the other battery on the other side attaches its self to resistor 3 and in the middle is resistor 2. ABCDEF as the whole circuit.

Edit :

B -> is not the battery. just simply the format of the circuit board

on the first post. the T stands for Voltage Total. for battery 1 and battery 2

Last edited:
Mentor
Try drawing your ascii circuit diagram using an editor with a fixed width font (for example, use Notepad and a Courier font), then copy and paste it into a CODE block in a Physics Forum reply. A CODE block is like a QUOTE block, only use the word CODE instead of QUOTE in the tags. The formatting shouldn't be messed with inside such a block. For example:

Code:
     +---R1----+-----R2----+
+|         |          +|
--- V1     R3      V2 ---
-         |           -
|         |           |
+---------+-----------+