Electric Current & Electron Flow (OCR A Level Physics): Revision Note

Defining Current

• Electric current is the rate of flow of charge carriers

• This is defined as the movement of

  • Electrons in metals

  • Ions in electrolytes

Electrons in Metals

• In metals, such as copper, mercury and titanium, the charge that flows is electrons

• The metal ions are closely packed and arranged in a crystal lattice structure

• The atoms have many free (delocalised) electrons that are free to move

  • Free electrons make metals good conductors of electricity

• When a conductor is exposed to a potential difference, the free electrons flow

• Free electrons are negatively charged, so they flow toward the positive terminal

• This flow of free electrons is current

Free electrons in metal create current

Ions in Electrolytes

• Ions are atoms that have lost or gained an electron

  • Anions have a negative charge because they have gained an electron

  • Cations have a positive charge because they have lost an electron

• An electrolyte is an electrically conducting solution

  • The charge carriers are not electrons, but cations and anions

• An example of an electrolyte is copper sulfate dissolved in water

Anions are attracted to the anode and cations to the cathode

Current & Electron Flow

• In electrical wires, the current is a flow of electrons

• Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal

• Conventional current is defined as the flow of positive charge

  • It is the direction that positive charge would flow if was positive charge that moved around the circuit (this is what scientists thought was happening before the electron was discovered)

• Conventional current flows from the positive terminal of a cell to the negative terminal

  • This is the opposite to the direction of electron flow

By definition, conventional current always goes from positive to negative (even through electrons go the other way)