EMF of the current source is 2V, internal resistance is 1 Ohm. Determine the resistance of the external
July 24, 2021 | education
| EMF of the current source is 2V, internal resistance is 1 Ohm. Determine the resistance of the external circuit if 0.75W of power is released in it.
Let us derive the formula for the power of the external circuit.
Circuit current
I = E / (R + r).
Dedicated power
P = U * I.
Voltage
U = I * I.
Replace and get the expression for the power on the external circuit
P = I² * R = (E / (R + r)) ² * R = E² * R / (R + r) ².
We substitute the known ones and calculate
0.75 = 2² * R / (R + 1) ²;
0.75 = 4R / (R² + 2R + 1);
4R = 0.75 * R² + 1.5 * R + 0.75;
3R² – 10R + 3 = 0;
D = 25 – 9 = 16.
The determinant is greater than zero, there are two roots
R1 = (5 – 4) / 3 = 0.33;
R2 = (5 + 4) / 3 = 3.
Answer: 0.33 ohm; 3 ohm.
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