PHY294H - Electricity and magnetism - Lecture 1

Reading for the week: Saslow. Quickly read over the review chapter and Chapter one. You should return to these chapters to gather mathematical and historical background as we go through the course. You should carefully start reading chapter 2. You should also try some problems from the R chapter and chapters 1 and 2.
Also a useful second perspective is: Fishbane, Gasiorowicz and Thornton, pages 585-595
History of EM
  • Ideas about charge and the flow of charge were developed by Gray, Franklin etc in the middle of the 18th century. Coulomb made these ideas concrete by measuring the force between charges and developing a formula to describe these forces (1785). This formula is the basis of electrostatics (stationary charge distributions). However it is often easier to use a reformulation of this law called Gauss' law to solve problems.
  • Ampere and Oersted noticed the fact that a DC current generates a magnetic field. Biot and Savart developed a formula to describe this. This occured in the 1820's. Analysis of the magnetic fields generated by DC currents is called magnetostatics. The Biot-Savart law, or Ampere's law is the basis of magnetostatics.
  • Ampere also noticed that a time varying electric field also induces a magnetic field. Ampere's law describes this phenomenon (1820's).
  • Faraday observed that a time varying magnetic flux leads to an induced emf. Faraday's law describes this phenomenon. This is the basis of electric motors and generators.
  • Maxwell wrote down the equations of electrostatics, magnetostatics, Ampere's law and Faraday's law in a unified form. From this set of equations, he deduced the way in which EM waves propagate. Maxwell's equations are considered a complete theory of classical electricity and magnetism.
  • Often an additional equation is added to this set. This is the Lorentz Force law, which describes the way in which electric and magnetic fields effect a moving charge. This equation was developed by Lorentz (1892/1895).
  • The chronology of these discoveries and other physics discoveries is available at the history of physics from the aps

    Basic concepts about charge

  • There are two types of charge, positive and negative.
  • Charge is conserved.
  • Charge is quantized. e = 1.6 x 10^(-19) C.

    Basic concepts about materials

  • Conductor - If there is a voltage across a conductor, current flows.
  • Insulator - Even if there is a voltage across an insulator, current does not flow.
  • Semiconductor = Insulator at low voltage and temperature.
  • Superconductor = Conductor, for static charges and no applied magnetic field.


    Electrostatics is the study of the forces between static charges. However most of the discussion of electrostatics uses the concepts of the electric field and the electric potential. Nevertheless it is essential to be facile with the analysis of forces before developing the concepts of electric field and electric potential.

    The starting point in the analysis of electrostatics is Coulomb's law. This law describes the force between two charges. However in electrostatics superposition holds, so that the force on one charge due to many other charges is just the sum of the Coulomb force due to each of these charges separately. This sum is a vector sum, so the math can get pretty messy.

    The electrostatic force is very strong compared to the gravitational force. For example the gravitational force between two protons is only 10^(-34) times the electrostatic force between two protons.