4) [Electric Potential Energy] A double-helix of DNA (deoxyribonucleic acid) acts like a spring and compresses upon becoming ionized and charged at the both ends. The double helix is 2.17μm long

before charging, and compresses 1.5% after the DNA double helix becomes charged with negative charge of -3e at one end and a positive charge of +2e at the other end. For simplicity you may assume the DNA in vacuum. (a) What are the magnitude and direction of the electric field at the middle of the double helix in the compressed state? (b) What's the electric potential energy of the double helix when the compression just starts and after the compression completes? (c) If the compressed state is a mechanical equilibrium, what are the magnitude and direction of the elastic force at the two ends in the compressed state?