The cross section of a long coaxial cable is shown in the figure, with radii as given. the linear charge density on the inner conductor is -30 nc/m and the linear charge density on the outer conductor is -90 nc/m. the inner and outer cylindrical surfaces are respectively denoted by a, b, c, and d, as shown. (ε 0 = 8.85 × 10-12 c2/n · m2) the radial component of the electric field at a point that 43 mm from the axis is closest to

2.1 Electric Charge There are two types of observed electric charge, which we designate as positive and negative. The convention was derived from Benjamin Franklin’s experiments. He rubbed a glass rod with silk and called the charges on the glass rod positive. He rubbed sealing wax with fur and called the charge on the sealing wax negative. Like charges repel and opposite charges attract each other. The unit of charge is called the Coulomb (C). The smallest unit of “free” charge known in nature is the charge of an electron or proton, which has a magnitude of e = 1.602 !10"19 C. (2.1.1) Charge of any ordinary matter is quantized in integral multiples of e . An electron carries one unit of negative charge, !e , while a proton carries one unit of positive charge, +e . In a closed system, the total amount of charge is conserved since charge can neither be created nor destroyed. A charge can, however, be transferred from one body to another. 2.2 Coulomb's Law In this section we will simply state Coulomb’s Law. This is the path followed by almost all introductory textbooks. But you will get a much better understanding of this law at an intuitive level in reading Section 2.11 below, where we explain how Faraday thought of this law, in terms of his lines of force (see also Section 1.1). 

A. they hold atoms  together with a molecule.its a force that exist between molecules. hope this .