1. **Problem statement:** A block of mass $m$ and charge $q$ is connected to a fixed point $O$ by an inextensible string on a horizontal table. An electric field $E$ is applied perpendicular to the string and in the plane of the table. We need to find the tension in the string when it becomes parallel to the electric field. 2. **Understanding the forces:** When the string is parallel to the electric field, the block experiences two main forces in the horizontal plane: - The tension $T$ in the string, directed along the string. - The electric force $F_e = qE$, acting on the charge in the direction of the electric field. 3. **Equilibrium condition:** Since the block is in equilibrium (or moving with constant velocity), the tension must balance the electric force: $$T = qE$$ 4. **Explanation:** The tension in the string provides the necessary force to counteract the electric force acting on the charged block. When the string aligns with the electric field, the tension equals the magnitude of the electric force. **Final answer:** $$T = qE$$