CIE AS Physics (9702) 2019-2021

Revision Notes

11.1.1 Electric Current

Defining 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

Flow of charge, downloadable AS & A Level Physics revision notes

Charge can flow between two conductors

  • 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

 

Electric current flow, downloadable AS & A Level Physics revision notes

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

Ammeter in series, downloadable AS & A Level Physics revision notes

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

 

Quantisation of Charge

  • 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) )

The Coulomb

  • The coulomb (C) is a measure of electric charge
    • It is equal to Q = It
  • The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. A × s

 

The Coulomb, downloadable AS & A Level Physics revision notes

Definition of the Coulomb

Calculating Electric Charge

  • Current can also be defined as the charge passing through a circuit per unit time
  • Electric charge is measured in units of coulombs (C)
  • Charge, current and time are related by the following equation

 

Charge equation, downloadable AS & A Level Physics revision notes

Charge equation

 

Worked example

Worked example - charge equation, downloadable AS & A Level Physics revision notes

Exam Tip

Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers.

Calculating Current in a Current Carrying Conductor

  • In a conductor, current is due to the movement of charge carriers
  • These charge carriers can be negative or positive, however the current is always taken to be in the same direction
  • In conductors, the charge carrier is usually free electrons
  • In the image below, the current in each conductor is from right to left but the charge carriers move in opposite directions shown by the direction of the drift speed v
    • In diagram A (positive charge carriers), the drift speed is in the same direction as the current
    • In diagram B (negative charge carriers), the drift speed is in the opposite direction to the current

Charge carriers diagram, downloadable AS & A Level Physics revision notes

Conduction in a current-carrying conductor

 

  • The drift speed is the average speed the charge carriers are travelling through the conductor. You will find this value is quite slow. However, since the number density of charge carriers is so large, we still see current flow happen instantaneously

 

  • The current can be expressed in terms of the number density (number of charge carriers per unit volume) n, the cross sectional area A, the drift speed v and the charge of the charge carriers q

Current conductor equation, downloadable AS & A Level Physics revision notes

Current in a conductor equation

 

  • The same equation is used whether the charge carriers are positive or negative

 

Worked example

Worked example - current in a conductor, downloadable AS & A Level Physics revision notes

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now