1. **Problem statement:** A voltage source of 20V is applied across a 2.5-ohm rheostat. We need to calculate the power dissipated in the rheostat. 2. **Formula used:** The power dissipated in a resistor (or rheostat) can be calculated using the formula: $$P = \frac{V^2}{R}$$ where $P$ is power in watts, $V$ is voltage in volts, and $R$ is resistance in ohms. 3. **Explanation:** This formula comes from combining Ohm's law ($V = IR$) and the power formula ($P = IV$). Substituting $I = \frac{V}{R}$ into $P = IV$ gives $P = \frac{V^2}{R}$. 4. **Calculation:** Substitute the given values: $$P = \frac{20^2}{2.5} = \frac{400}{2.5} = 160$$ 5. **Answer:** The power dissipated in the rheostat is 160 watts.