Electric Current (Cambridge (CIE) A Level Physics)

Defining electric current

• Electric current is the flow of charge carriers and is measured in units of amperes (A) or amps

• Charge carriers can be either positive or negative

• When a conductor is exposed to a potential difference, charge carriers will flow

Charge flowing between conductors

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

• 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 from the positive terminal of a cell to the negative terminal

  • This is the opposite to the direction of electron flow, as conventional current was described before the discovery of the electron

Conventional current and electron flow

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

• Current is measured using an ammeter

• Ammeters should always be connected in series so that the charged particles flow through the ammeter

An ammeter in a circuit

An ammeter can be used to measure the current around a circuit and always connected in series

Quantisation of charge

• The charge carried by charged particles is quantised

• Charge is carried in discrete amounts or quanta (sometimes referred to as packets)

• Charge is measured in coulombs (C)

  • The charge of an electron is -1.60 × 10^-19 C

  • The charge of a proton by comparison is 1.60 × 10^-19 C

  • This value is known as the elementary charge

• In this way, the quantity of charge is dependent on how many protons or electrons are present; charge always occurs in multiples of the magnitude of the elementary charge