The energy view of charge
Lifting a ball stores gravitational potential energy; let go and gravity does work turning it back into motion. Charges do the same in an electric field. Push a positive charge toward another positive charge — against the repulsion — and you store electric potential energy; release it and the field does work driving it away. Because the electric force, like gravity, is conservative, this energy depends only on position, not on the path taken.
For two point charges, the stored energy is U = k\,q_1 q_2/r — note this is $1/r$, one power gentler than the 1/r^2 force. It is positive for like charges (you had to do work to bring them together, energy waiting to fly them apart) and negative for opposite charges (they fell together, releasing energy — you would have to supply energy to separate them).
Electric potential: energy per unit charge
Just as we divided force by charge to get the field, divide energy by charge to get the electric potential V — a property of a point in space, whatever charge you later put there. Its unit, the joule per coulomb, is the volt (V). Everyday 'voltage' is just a difference in electric potential between two points.
Potential is potential energy per unit charge; a point charge makes a potential kq/r; and moving charge q through a potential difference ΔV changes its energy by qΔV.
Equipotentials: contour maps of voltage
Connect all the points at the same potential and you get an equipotential surface — the exact analogue of a contour line on a hiking map, where every point sits at one altitude. Two rules make them intuitive: field lines always cross equipotentials at right angles, and no work is done moving a charge along an equipotential (you walk the contour, never climbing). The surface of any conductor in equilibrium is one big equipotential.
Storing charge and energy: the capacitor
Put two conductors near each other, charge one +Q and the other -Q: that is a capacitor, a device for storing charge and energy. The charge it holds is proportional to the voltage across it, and the constant of proportionality — how much charge it banks per volt — is its capacitance C, measured in farads (F).
Capacitance is charge per volt; for parallel plates it grows with plate area A and shrinks with gap d; a charged capacitor stores energy ½CV².
Slide an insulating slab — a dielectric — between the plates and the capacitance rises by a factor \kappa (the dielectric constant): the slab's molecules polarize and partly cancel the field, letting the plates hold more charge at the same voltage. That is why real capacitors are packed with dielectric, not empty air.
Putting it together, and where it leads
Worked example. A parallel-plate capacitor has plates of area A = 0.50\ \text{m}^2 separated by d = 1.0\ \text{mm} of air, charged to V = 100\ \text{V}. Find its capacitance, stored charge, and stored energy. Step 1: C = \varepsilon_0 A/d = (8.85\times10^{-12})(0.50)/(1.0\times10^{-3}) \approx 4.4\times10^{-9}\ \text{F} = 4.4\ \text{nF}.
Step 2: Q = CV = (4.4\times10^{-9})(100) \approx 4.4\times10^{-7}\ \text{C} = 0.44\ \mu\text{C}. Step 3: U = \tfrac12 CV^2 = \tfrac12(4.4\times10^{-9})(100)^2 \approx 2.2\times10^{-5}\ \text{J}. Sense-check: nanofarads and microcoulombs are the right everyday scale; the tiny energy warns that ordinary capacitors store little energy per unit size — which is exactly why batteries, not capacitors, run your phone.