In the following figure(Figure 1) , r1 = 5.00 , r2= 3.00 , and r3= 7.00 . The battery has negligible internal resistance. The current i2 through r2 is 3.00 .
I1=1.8A
What is the current i3?
What is the emf of the battery?

Question

In the following figure(Figure 1) , r1 = 5.00 ,  r2= 3.00 , and  r3= 7.00 . The battery has negligible internal resistance. The current i2 through r2 is 3.00 .
I1=1.8A
What is the current  i3?
What is the emf of the battery?

shane_b · Answer

That's better :)

anonymous · Answer

sorry my copy didnt copy the R's lol

shane_b · Answer

Ok, the current through I3 will simply be the sum of the currents through I1 and I2.

To find the EMF, you need to know 2 things: the total current (which will be the I3 current) and the total resistance.  Can you calculate the total resistance?

anonymous · Answer

4.8 is i3
and since the r1and 2 are parralel and r3 is series is it 

1/r12 +1/r3?

shane_b · Answer

$$I_3=I_1+I_2=1.8A+3.0A=4.8A$$$$R_{total}=\frac{R_1R_2}{R_1+R_2}+R_3=\frac{(5\Omega)(3\Omega)}{5\Omega+3\Omega}+7\Omega=8.875\Omega$$$$Emf=V=I_{total}R_{total}=(4.8A)(8.875\Omega)=42.6V$$

shane_b · Answer

Ok, now ask questions :)

anonymous · Answer

where r 12=8?

shane_b · Answer

For two resistors in parallel, the shortcut formula is:$$Rt=\frac{R_1R_2}{R_1+R_2}$$So since R3 is in series with that, you simply add R3 to the total as I showed above.

anonymous · Answer

thanks i get it now

shane_b · Answer

Cool, good luck :)