(e) current as the movement of electrons in metals and movement of ions in…

The idea

Electric current is the rate of flow of charged particles, and the identity of those particles depends on the conductor. In a metal, outer electrons are delocalised from the positive ion lattice, so the charge carriers are electrons; a current of 1 A means 1 C of charge passes a point each second, and since each electron carries about 1.60 × 10⁻¹⁹ C, that is roughly 6.25 × 10¹⁸ electrons per second. In an electrolyte, the liquid or solution contains positive and negative ions that are free to move, so both ion types drift in opposite directions and together carry the current; the electrodes deliver or remove charge, driving chemical change. In both cases the current direction is defined by conventional current, and the carriers move much more slowly than the signal.

What earns the marks

• Current is the rate of flow of charge, I = ΔQ/Δt, measured in amperes where 1 A = 1 C s⁻¹.
• In metals the charge carriers are delocalised electrons; the positive metal ions form a fixed lattice and do not translate.
• In electrolytes the charge carriers are positive and negative ions, which move in opposite directions under the applied field.
• The magnitude of the current depends on the charge on each carrier, the number density of carriers and their mean drift velocity.
• A current of 1 A corresponds to about 6.25 × 10¹⁸ electrons passing a point per second, since 1 ÷ (1.60 × 10⁻¹⁹ C) ≈ 6.25 × 10¹⁸.

Common mistakes and how to put them right

• Thinking that in a metal the positive ions also drift and carry current.

  Put it right: The ions are fixed in the lattice, so only the delocalised electrons move through the metal.

• Believing that electrons travel from the negative terminal to the positive terminal at nearly the speed of light.

  Put it right: The drift velocity is typically a fraction of a millimetre per second, while the electric field signal propagates much faster.

• Assuming an electrolyte conducts because electrons flow through the liquid.

  Put it right: Conduction in an electrolyte is by movement of positive and negative ions, with electrons transferred at the electrode surfaces.

Exam tips

• Read the stem carefully to identify the material; if it is a metal, select the option describing electron flow, and if it is an electrolyte, select the option describing ion movement.
• Check the direction wording: conventional current is from positive to negative, while electron flow in a metal is from negative to positive.
• Use I = ΔQ/Δt with the elementary charge 1.60 × 10⁻¹⁹ C to convert between current and number of carriers per second when a numerical option is offered.