Sorry guys...just need help on two more physics questions?

Question

1) A 12 ohm, a 15 ohm, and an unknown resistance are connected in parallel. The total current of the combination is 12 A. The current across the 15 ohm branch is 4 A. What is the total voltage? What is the total resistance? What is the unknown resistance?


2)A 60 ohm, a 90 ohm and an unknown resistor are connected in series across a 130V battery. An ammeter in the circuit reads 0.67 A. What is the total resistance of the circuit? What is the size of the unknown resistor? What is the power dissipated in each resistor?

One last question plz answer ASAP:

3) A 120 ohm, 60 ohm, and a 40 ohm resistor are connected in parallel and placed across a 120 V potential difference. What is the total resistance of the circuit? What is the total current? What is the current through each resistor?

Answer 1

V = I * R

I = V / R

R = V / I

Problem 1:

The 15 ohm resistor carries 4A. So the voltage across it is:

15 * 4 = 60 Volts

Since the resistors are connected in parallel, this is the total voltage, also applied across the 12 ohm and unknown resistor. 60 Volts on a 12 ohm resistor will drive:

60 / 12 = 5A

The total current is 12A. 4A goes through the 15 ohm, 5A goes through the 12 ohm, leaving 3A for the unknown resistor.

60 / 3 = 20 ohms

The total resistance carries 12A from 60 Volts.

60 / 12 = 5 ohms

Problem 2:

The total resistance carries 2/3 A from 130 Volts:

130 / 2 / 3 =195 ohms

Resistors in series add, so the unknown resistance is:

195 - 90 - 60 = 45 ohms

Power dissipated in a resistor is I^2 * R:

For 60 ohms: (2/3)^2 * 60 = 80/3 Watts

For 90 ohms: (2/3)^2 * 90 = 40 Watts

For 45 ohms: (2/3)^2 * 45 = 20 Watts

Answer 2

1.
voltage is IxR, 4A x 15ohm = 60v. This is true for all 3 resistors because they are in parralel.
Total resistance is V/I, 60v / 12A(given) = 5ohms
unknown resistance is complicated. call it R? the inverse of the total is equal to the sum of the inverses of each resistor
1/5 = 1/12 + 1/15 +1/R?
1/R? = 1/5 - 1/12 - 1/15= 12/60-5/60-4/60 = 3/60
1/R? = 3/60 = 1/20
so R? = 20

2.
Total resistance = V / I = 130/.67 = 194
The unknown R is 194 -60-90 = 44
power = Ix I x R. I squared is .67x.67=.45.
R1 = 60ohm, power for R1 = .45x60=26.9
you can do the other two the same way