Charge is the physical quantity itself, measured in coulombs. Current is the rate of charge flow, \(I = dq/dt\), in amperes. Voltage is the energy available per unit charge, \(V = \Delta E/dq\), in volts.
Current is measured through a point — it is a flow. Voltage is measured between two points — it is a difference. This is why an ammeter goes in series and a voltmeter goes in parallel.
From high potential to low potential, i.e. from the + terminal to the − terminal. Note this is opposite to the direction electrons actually move; the convention predates the discovery of the electron.
Charge — not voltage and not current. A battery rated 24 A·h holds \(24 \times 3600 = 86{,}400\) coulombs.
\(V = I R\) and \(P = I V\). Combining them gives the two other useful forms, \(P = I^2 R\) and \(P = V^2/R\) — use whichever matches what you already know.
\(R = \rho L / A\): the material's resistivity \(\rho\), the length \(L\), and the cross-sectional area \(A\). Longer means more resistance; fatter means less.
Gold does not corrode, so it is used on contacts and connectors. Aluminum is far lighter and cheaper per unit of conductance, which is why transmission lines and aircraft wiring use it. Cost, weight, and corrosion beat raw resistivity.
The exponent is always a multiple of 3, so every result maps onto a standard prefix. \(0.000\,030\,56\) W becomes \(30.56\,\mu\text{W}\) rather than \(3.056\times10^{-5}\) W.
Large, ideally infinite. The meter connects in parallel with the device, so a low internal resistance would draw current and change the very voltage it is trying to measure.
Small, ideally zero. The meter is inserted in series, so any resistance it adds would reduce the current it is measuring. This is also why connecting an ammeter directly across a source creates a short circuit.
\(E = P\,\Delta t = 120\,\text{W} \times 8760\,\text{h} = 1051.2\ \text{kW}\cdot\text{h}\), so the cost is about $105. Note the utility bills for energy, not power.