Who carries the current in each medium?
Current is charge in motion. What is moving depends on the medium.
- Metals: free (conduction) electrons move. The atoms stay in place.
- Electrolytes (salt or acid in water, melted salts): positive and negative ions move in opposite directions.
- Gases: normally insulators. When ionised they carry current by electrons and positive ions.
- Plasma (very hot ionised gas, as in the Sun, lightning, neon signs): full of free electrons and ions, so it conducts very well.
- Semiconductors (silicon, germanium): current is carried by electrons and by holes (missing electrons that act like positive charges). Few carriers at low temperature, more when heated or doped.
The 3D scene shows the first four. For semiconductors see the lesson p-n junction diode.
Metals: resistance and temperature
In a metal the electrons do not race along. They move randomly very fast, but with a slow drift in one direction when a voltage is on. They keep colliding with the vibrating atoms.
Heat the metal and the atoms vibrate more, so there are more collisions. Resistivity rises with temperature:
R = R₀(1 + αΔT), where α is the temperature coefficient (about 0.004 per °C for copper).
For electrolytes and semiconductors the opposite happens: heating frees more ions or carriers, so resistance falls. Alloys like manganin and constantan have a tiny α, so they are used in standard resistors.
Superconductivity
Some materials, when cooled below a critical temperature, lose all electrical resistance. Mercury does this at about 4 K (−269 °C). Some ceramics do it at much higher temperatures, near 100 K and above.
A current in a superconducting ring can flow for years without a battery. Uses: very strong electromagnets (MRI scanners, particle accelerators), maglev trains, lossless cables. A strong magnetic field or a large current can destroy superconductivity.
Electrolytes and the laws of electrolysis
When current passes through an electrolyte, ions reach the electrodes and matter is deposited or released. This is electrolysis. It is used for electroplating, purifying copper and making aluminium.
Faraday's first law: the mass freed is proportional to the charge passed: m = Z q = Z I t. Z is the electrochemical equivalent of the substance (kg per coulomb).
Faraday's second law: for the same charge, masses freed are proportional to the chemical equivalents M/n of the substances. So Z = M / (n F), with the Faraday constant F ≈ 96 500 C per mole.
Water-based electrolytes follow Ohm's law only roughly. The voltage must also beat a small back-voltage at the electrodes.
Gases: discharge and plasma
Air at normal pressure is an insulator because it has almost no free charges. A few stray ions from cosmic rays and radioactivity exist, so a tiny current flows if you apply voltage.
Raise the voltage and the free electrons gain speed, hit atoms and knock out more electrons. This chain reaction is the avalanche. It gives a spark (high pressure, like lightning) or a steady glow discharge (low pressure, as in a neon sign or a tube light). The gas becomes a plasma. The voltage at which this starts is the breakdown voltage. Lower pressure makes it easier, up to a limit.
Vacuum: thermionic emission
A perfect vacuum has no charge carriers, so no current flows. But a hot metal lets electrons escape from its surface. This is thermionic emission. The hotter the cathode, the more electrons escape.
In a vacuum diode a heated cathode (−) sends electrons to the plate (+). Current flows only one way, so it works as a rectifier. With a small voltage the current grows with voltage. With a big voltage all emitted electrons are collected and the current saturates, so then only more heating helps. Old TVs, X-ray tubes and electron microscopes use this effect.
Try it: salty water and a bulb
Join a bulb and a battery in a circuit, with two bare wire ends dipping in a glass. In plain distilled water the bulb stays dark. Add a spoon of salt and stir: the bulb glows. Salt gives ions that carry current. Look for tiny bubbles at the wire ends. In the 3D, pick Liquid and move the voltage slider.
Key formulas and definitions
- R = R₀(1 + αΔT) (metals: α positive)
- ρ = ρ₀(1 + αΔT)
- Faraday 1: m = Z q = Z I t
- Faraday 2: Z = M / (n F), F ≈ 96 500 C/mol
- Charge: q = I t
- Mass deposited by 1 mole of electrons = M / n
Worked examples
1. A copper wire has 50 Ω at 20 °C. Take α = 0.004 per °C. Find its resistance at 120 °C.
ΔT = 100 °C. R = 50 × (1 + 0.004 × 100) = 50 × 1.4 = 70 Ω.
2. A bulb filament has 40 Ω when cold and 480 Ω when glowing. It is on 240 V. Find the current at switch-on and when hot.
Cold: I = 240 / 40 = 6 A. Hot: I = 240 / 480 = 0.5 A. The surge at switch-on is why bulbs often fail then.
3. A current of 1.5 A flows through an electrolyte for 20 minutes. How much charge passes?
t = 20 × 60 = 1200 s. q = I t = 1.5 × 1200 = 1800 C.
4. A current of 2 A runs for 30 minutes in a copper plating bath. Z of copper = 3.3 × 10⁻⁷ kg/C. Find the mass deposited.
q = 2 × 1800 = 3600 C. m = Z q = 3.3 × 10⁻⁷ × 3600 = 1.19 × 10⁻³ kg ≈ 1.19 g.
5. How long must 0.5 A flow to deposit 0.66 g of copper (Z = 3.3 × 10⁻⁴ g/C)?
q = m / Z = 0.66 / 3.3 × 10⁻⁴ = 2000 C. t = q / I = 2000 / 0.5 = 4000 s (about 67 minutes).
6. How much charge deposits 0.54 g of silver? (M = 108 g/mol, n = 1, F = 96 500 C/mol)
One mole of electrons (96 500 C) deposits 108 g. So q = 0.54 / 108 × 96 500 = 482.5 C.
7. The same charge passes through a copper bath (M = 63.5, n = 2) and a silver bath (M = 108, n = 1). Compare the masses.
Chemical equivalents: copper 63.5 / 2 = 31.75, silver 108 / 1 = 108. By Faraday's second law, m(silver) : m(copper) = 108 : 31.75 ≈ 3.4 : 1.
Common mistakes
- Saying electrons carry current in every medium. In electrolytes it is ions; in gas it is ions and electrons; in semiconductors, electrons and holes.
- Thinking electrons move fast along the wire. Their drift speed is tiny (about a millimetre per second); the signal spreads fast, not the electrons.
- Saying heating raises resistance for every material. It does for metals, but lowers it for electrolytes and semiconductors.
- Using minutes in m = Z I t. Convert time to seconds first.