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# determine the value of R?

the resistor in the figue recieves 20W power.

### 2 Answers

- roderick_youngLv 74 weeks agoFavorite Answer
Total resistance of the branch on the right = 40 + R [1]

Resistance of 30 ohms in parallel with that = 30(40+R)/(30 + 40 + R) = 30(40+R)/(70+R) [2]

The 5 ohm and [2] form a voltage divider, so voltage on the non-battery side of the 5 ohm is

75 * (30(40+R)/(70+R)) / (5 + 30(40+R)/(70+R)) [3]

the 40 ohm and R form a voltage divider, so the voltage across R is R/(40+R) of [3], or

V = 75 * (30(40+R)/(70+R)) / (5 + 30(40+R)/(70+R)) * R/(40+R) [4]

But the power in R is 20 W. Remembering that P = V^2/R,

20 = V^2 / R [5]

R = V^2/20 [6]

Substitue [6] for R into [4], then solve for V. Once you have V, sustitute back into [6] to find R.

- billrussell42Lv 74 weeks ago
thevenin equivalent, terminals at R

Rth = 5 parallel 30 in series with 40

Rth = 5•30/35 + 40 = 30/7 + 40 = 44.3 Ω

Vth = 75(30/35) = 64.3 volts

Voltage across R = 64.3R/(44.3+R)

current thru R = V/R = 64.3/(44.3+R)

power is the product

64.3R/(44.3+R) x 64.3/(44.3+R) = 20

(64.3/(44.3+R))²(R+1) = 20

(4133/(1961+66.6R+R²))(R+1) = 20

20(1961+66.6R+R²) = 4133(R+1)

39220 + 1332R + 20R² = 4133R + 4133

20R² – 2801R + 35087 = 0

using a quad. solver

R = 126, 13.9 ohms

but check my math.