Help with Electric Circuits Problem

[ashwinkandel]

I have two problems of electricity:
1.
http://i56.tinypic.com/2a6rnzs.jpg"

i.Use Kirchoff's first law to find the current through the 4.00 ohm and 8.00 ohm resistors
ii. Calculate the emf E1
iii. Calculat the emf E2
iv. Calculate the current through the 12.00 ohm resistor

2.
A student builds the circuit in the diagram, using a battery of negligible internal resistance. The reading on the voltmeter is 9.0V.

http://i55.tinypic.com/ic5vgi.jpg"

i.The voltmeter has a resistance of 1200 ohm. calculate the emf of the battery.
ii. The student now repeats the experiment using a voltmeter of resistance 12 Kiloohm. Show that the reading on this voltmeter would be 9.5V
iii. Refer to your answers in i. and ii. and explain why a voltmeter should have as high a resistance as possible.

 

[danielakkerma]

Hi,
It could really have been useful for you to show some of your attempts first, just to get a hint at where you stand, and where you're experiencing difficulty.
The first query you have there, is one of the fundamental formulaies of Kichoff's rules, which states that: The sum of all currents meeting in a closed loop(signed, naturally) is always equal to zero.
In other words,
$\large<br /> \displaystyle<br /> \sum_{n=1}^{N} I_n = 0$
For any point of intersection; Try marking, around the first cycle there, the currents as I1(known and given), (I2, also presented), and the sought I3, and simply compute arithmetically.
Likewise the same applies to the Voltages and EMFs...
Give it a shot, and let us know how it works out,
Daniel