CIE A Level Physics复习笔记9.1.1 Electric Current

• Electric current is the flow of charge carriers and is measured in units of amperes (A) or amps
• Charge can be either positive or negative
• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

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 electric current was really understood

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

• There are several examples of electric currents, including in household wiring and electrical appliances
• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

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

• The charge on charge carriers is quantised
• Charge comes in definite bits - e.g. a single proton has a single positive charge, whereas a single electron has a single negative charge
• In this way, the quantity of charge can be quantised dependent on how many protons or electrons are present - positive and negative charge has a definite minimum magnitude and always comes in multiples of that magnitude
• This means that if we say something has a given charge, the charge is always a multiple of the charge of an electron by convention
  • 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 is known as the elementary charge, denoted by e and measured in Coulombs (C) )