The power \( P \) dissipated in a resistor is calculated using:
\[\nP = \frac{V^2}{R}\n\]
Where:
- \( P = 100 \, \text{J/s} = 100 \, \text{W} \) (power)
- \( R = 4 \, \Omega \) (resistance)
Substituting the values:
\[\n100 = \frac{V^2}{4}\n\]
Solving for \( V^2 \):
\[\nV^2 = 100 \times 4 = 400\n\]
\[\nV = \sqrt{400} = 20 \, \text{V}\n\]
Therefore, the potential difference across the resistor is 20 V.