Atoms, charge and current | PLS Foundation

Start with particles and an element

An atom has a nucleus containing positive protons and usually uncharged neutrons, with negative electrons around it. The proton count identifies the element. A neutral atom has equal numbers of protons and electrons. An electron diagram is a model: small circles need not represent little balls following planetary orbits.

Source: OpenStax: atomic structure and ions ↗

Silicon has atomic number 14, so its nucleus has 14 protons. Neutral silicon therefore has 14 electrons. Silicon isotopes have different neutron counts. Changing the electron count changes the atom’s net charge; it does not change its element into a different one.

Source: Royal Society of Chemistry: silicon ↗ · OpenStax: atomic structure and ions ↗

Count positive and negative charge

A proton and an electron carry equal magnitudes of charge with opposite signs. Like charges repel and unlike charges attract. When objects gain or lose electrons, charge is transferred: the total charge of an isolated system is conserved. “Positive” and “negative” name a physical property, not good and bad kinds of electricity.

Source: OpenStax: charge and conservation ↗

For a counting example, keep 14 protons and remove one of 14 electrons. Fourteen positive units and thirteen negative units leave one positive unit: +e. The result is a positive silicon ion. Adding an electron instead would leave −e. Here e denotes one elementary charge, not the electron’s negative sign.

Current has a direction convention

In a metal conductor, some electrons can move through the material. The metal does not have to become strongly charged overall for a current to flow. An insulator does not offer the same easy movement of charge. A circuit depends on both its materials and its connections.

Source: OpenStax: conductors and insulators ↗

Conventional current points in the direction positive charge would move. In a DC metal wire, negative electrons drift in the opposite direction. Label the carrier and the arrow before comparing diagrams. Neither convention is wrong.

Source: OpenStax: electric current and its direction ↗

Conceptual electron-cloud/count model: dots are not literal positions or planetary orbits; neutrons are omitted. The lower panel is a separate metal wire, not current through the atom.

Semiconductors also use a hole description: a missing electron behaves as a positive carrier. Electrons and holes can both contribute to current. A hole is not a proton travelling through the crystal. Doping controls carrier populations; n-type and p-type do not mean the whole material carries a large net charge.

Source: OpenStax: electrons, holes and doping ↗

Measure how much charge passes each second

Average current is charge passing a cross-section divided by elapsed time: I = ΔQ ÷ Δt. One ampere is one coulomb per second. The unit C means coulomb here, not the chemical symbol for carbon. A current measurement describes flow through a chosen place, not how much charge is sitting in the entire wire.

Source: OpenStax: electric current and its direction ↗

Suppose 0.060 C passes in 3 s. I = 0.060 ÷ 3 = 0.020 A = 20 mA. If that average continues for 5 s, the transferred charge is Q = I × t = 0.020 × 5 = 0.100 C. Keep units on each line so that milliamperes do not accidentally become amperes.

These are supplied arithmetic values. They do not specify a battery, component rating or experiment to wire up. First practise a prediction with a notebook: identify charge, time, current and direction separately.

Check a claim before moving on

Put the model to work

Make a prediction, run one small check, and record the result and its limits. Explain a mismatch before changing several things at once.

Explore working models →Build a circuit in Logic Lab →

Sources and further reading

Original diagrams, examples and self-check questions. Each model states its assumptions; named source lessons provide further depth.