= Solution
The main generic costs of strongly confining a charged semiflexible <polymer> are bending it away from its preferred straight shape, crowding neighboring segments, and reducing its available configurations. Closely approaching charged strands also repel through electrostatic and hydration interactions; mobile <ions> screen or modify these interactions, while counterion organization contributes to the free-energy balance. The bending cost is especially large in regions of high <curvature>. A <pressure> on the confining wall and a <force> resisting further insertion are related thermodynamic responses, but are not identical quantities with the same units.
At the level of measurement principles, an <optical tweezers> apparatus acts as a calibrated compliant <force> sensor. A displaced trapped bead experiences approximately $F_{\rm trap}=k_{\rm trap}\Delta x$. A tether transmits <tension> between that sensor and the material under load; the opposing support fixes the other end. The conceptual <force balance> is
$$
\text{compliant sensor}\ \longleftrightarrow\ \text{tension-bearing tether}\ \longleftrightarrow\ \text{confined polymer system}.
$$
The extension and restoring <force> supply a mechanical record of the load. This is a force-sensing schematic, not a biological preparation or packaging protocol. The measured <tension> can include bending, interstrand interactions, dissipation and any other mechanically coupled contribution; it cannot be identified with only the bending term without additional assumptions. In a reversible generic model $F_{\rm conf}=\partial G/\partial L$ at specified geometry, while an internal <pressure> is $-\partial G/\partial\mathcal V$ at specified <polymer> content. These derivatives clarify what a <force> measurement can and cannot determine.
\Image[/past-exam-of-the-biological-physics-course-of-the-university-of-oxford/2016-compliant-force-sensor.png]
{title=A compliant optical force sensor and a generic confined-polymer load}
{description=Original force-balance schematic. The sensor displacement reports transmitted tension; the drawing supplies no biological preparation or packaging procedure.}
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