Everything in the world is made up of atoms, which are the fundamental building blocks of materials and molecules. The central part is the nucleus, which contains positively charged protons and neutrally charged neutrons. The nucleus is surrounded by electron shells, on which negatively charged electrons orbit. Different charges attract each other and similar charges repel each other. In metals, the electrons on the outer electron shells are less attracted and are easily displaced.
29 protons
36 neutrons
29 electrons (1 free)
When voltage is applied to a piece of copper wire, electrons from the outer electron shell, which already move freely inside the material, start moving in the same direction. Copper is one of the best conductors along with silver, gold and aluminium. In non-metals, ions flow instead of electrons (e.g. lithium ion in some batteries).
Symbol: Q
Unit: Coulomb [C]
1 Coulomb = 6 242 000 000 000 000 000 Electron's charge (6,242 · 1018)
Symbol: V
Unit: Volt [V]
Work is required to separate charges, and the more charges are to be separated, the more work is needed. This concept is related to potential energy stored in a system where charges are separated. Voltage is the measure of how much work would be done to move a unit charge (1 C) between two points.
$ V[V] = \dfrac{W[J]}{Q[C]} $
Voltage is always between two points. Voltage measured with respect to a reference point is called potential, so voltage is a potential difference between two points. The reference point is called zero potential or ground.
Symbol: I
Unit: Amper [A]
The amount of charge flowing through a wire of a given cross-section in a given time. It is driven by voltage, and can occur in metals, certain liquids (electrolytes) and ionized gases. It is also called Amperage.
$ I[A] = \dfrac{Q[C]}{t[s]} $
Symbol: R
Unit: Ohm [Ω]
The property of a conductor to oppose the movement of charge carriers. It varies depending on the material, size and temperature of the conductor. Ohm's law states that resistance is the ratio of voltage to current.
Ohm's law:
$ R[Ω] = \dfrac{V}{I} $
The quality of the material as a conductor is characterised by its electrical resistivity. The lower the resistivity, the better the conductor. Shorter or thicker wires also have lower resistance, less energy is lost within them.
$ R = \rho ⋅ \dfrac{l[m]}{A[mm^2]} $
The smaller the $ \alpha $ coefficient, the less the resistance changes with temperature. The resistance of conductive materials also decreases as their temperature decreases, and when they heat up, their resistance increases.
$ R = R_0 ⋅ (1 + \alpha ⋅ \Delta T) $
Symbol: W
Unit: Watt-hour [Wh]
Electricity consumed or produced between two points in a circuit. The electricity bill is calculated on the basis of the kilowatt-hours consumed during the year, and the capacity of batteries is also given in kWh or Wh.
$ 1[Wh] = 1[V] ⋅ 1[A] ⋅ 1[h] $
Symbol: P
Unit: Watt [W]
Electricity consumed or produced at a given moment. When the power of a device is specified, it usually refers to the maximum power, such as the power used when a cooling fan operates at its highest speed setting.
$ P[W] = V ⋅ I $
The ratio of useful power to absorbed (total) power. The efficiency of a solar panel is around 25%, so around 75% of the sun's energy is not converted. In a power supply with 90% efficiency, one tenth of the absorbed power is lost, for example due to heat. Electric cars are almost five times more efficient than ICE cars.
$ \mu[\%] = \dfrac{P_u}{P_t} $