Preparation for the International Physics Olympiad (IPhO): Electromagnetism

The IPhO syllabus includes:

2.3 Electromagnetic fields

• 2.3.1 Basic concepts: Concepts of charge and current; charge conservation and Kirchhoff's current law. Coulomb force; electrostatic field as a potential field; Kirchhoff's voltage law. Mag­netic B-field; Lorentz force; Ampere's force; Biot-Savart law and B-field on the axis of a circular current loop and for simple symmetric systems like straight wire, circular loop and long solenoid.
• 2.3.2 Integral forms of Maxwell's equations: Gauss' law (for E- and B-fields); Ampere's law; Faraday's law; using these laws for the calculation of fields when the integrand is almost piecewise constant. Boundary conditions for the electric field (or electrostatic potential) at the surface of conductors and at infinity; concept of grounded conductors. Superposition principle for elec­tric and magnetic fields; uniqueness of solution to well- posed problems; method of image charges.
• 2.3.3 Interaction of matter with electric and magnetic fields; Resistivity and conductivity; differential form of Ohm's law. Dielectric and magnetic permeability; relative per­mittivity and permeability of electric and magnetic ma­terials; energy density of electric and magnetic fields; fer­romagnetic materials; hysteresis and dissipation; eddy currents; Lenz's law. Charges in magnetic field: helicoidal motion, cyclotron frequency, drift in crossed E- and B-fields. Energy of a magnetic dipole in a magnetic field; dipole moment of a current loop.
• 2.3.4 Circuits: Linear resistors and Ohm's law; Joule's law; work done by an electromotive force; ideal and non-ideal batter­ies, constant current sources, ammeters, voltmeters and ohmmeters. Nonlinear elements of given V-I charac­teristic. Capacitors and capacitance (also for a single electrode with respect to infinity); self-induction and in­ductance; energy of capacitors and inductors; mutual in­ductance; time constants for RL and RC circuits. AC circuits: complex amplitude; impedance of resistors, in­ductors, capacitors, and combination circuits; phasor di­agrams; current and voltage resonance; active power.

Members of the Spanish National Team can download the course notes here:

• Campo electrostático: 
• Corriente eléctrica
• Campo magnetostático
• Campo electromagnético

Some cognitive conflicts involving Electromagnetism:

• Cognitive Dissonances: Electromagnetism

It is also useful to follow the IPhO's Study Guide by Jaan Kalda

• Study guide: Circuits

Here you can find the solutions to some of the problems:

• Pr 3:  [IPhO1996Theory1] Problem Solution
• Pr 69: [IPhO1994Theory2] Problem Solution
• [IPhO1983Theory2] Problem Solution
• [IPhO2010Theory1] Problem Solution
• [IPhO2001Theory1c] Problem Solution

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Something happens with the light

If we split a collimated light beam by using a half-silvered mirror, then the two resulting beams (A and B) have exactly the same intensity. Since the light is made of photons, that means that half of the photons go through path A, and the other half through path B.

If we now reflect both beams by a mirror and the two beams then pass a second half-silvered mirror and enter two detectors as explained in the picture:

then we expect the A beam to be split into two beams. We will call them A1 and A2. A1 goes to dectector 1, while A2 goes to detector 2. Each one contains 50% of A-photons, that is, 25% of the photons of the original light beam:

On the other hand, we also expect the B beam to be split into two beams. We will call them B1 and B2. B1 goes to dectector 1, while B2 goes to detector 2. Each one contains 50% of B-photons, that is, 25% of the photons of the original light beam:

So the amount of photons that should arrive to detector 1 is 25% + 25% = 50%, and the same for detector 2:

Nevertheless, once we have carried out the experiment, what we found is that 100% of photons arrive to detector 2  and no photon arrives to detector 2!

Moreover, what is even more puzzling, if we obstruct channel A (or B, it does not matter), then we detect the same number of photons in detector 2 as the number detected in detector 1. Are you able to figure it out? Try it!

Please, explain your reasoning. You can post your attempted answers in the comment box below. Please, do not use Facebook or Twitter to give your answers.