What is the thermal noise power in a 1 Hz bandwidth at room temperature (290 K)? Use k = 1.38×10^-23 J/K.

• A. 4.0e-22 W
• B. 4.0e-21 W
• C. 4.0e-20 W
• D. 4.0e-19 W

Answer: (B)

Thermal noise power in a resistor over a given bandwidth is described by P = k T B. In a 1 Hz bandwidth, this reduces to P = k T. Using k = 1.38 × 10^-23 J/K and T = 290 K:

kT = 1.38 × 10^-23 × 290 ≈ 4.0 × 10^-21 J.

Since power is energy per unit time and the bandwidth is 1 Hz, this value is directly in watts: P ≈ 4.0 × 10^-21 W.

So, the thermal noise power in a 1 Hz bandwidth at room temperature is about 4 × 10^-21 W.