Faraday's Law

Faraday’s law is, together with Ampère’s law, one of the fundamental laws in electromagnetism. It describes how a time varying magnetic field can induce an electromagnetic force.

Ørsted’s experiment shows that a current flowing through a copper wire exerts a force on a magnet. The news about Ørsted’s discovery also reached the Royal Society in London. The physicist Michael Faraday wondered if also the opposite would work: can a moving magnet also exert a force on current or on a current carrying conductor?

Faraday set up an experiment. He moved a magnet over a copper winding and measured a difference in potential between both ends of the winding. In other words: a time varying magnetic field induces a voltage in the copper winding and the winding becomes in fact a voltage source. But how does this work?

The magnetic field lines passing through a certain surface is called the magnetic flux. The word flux originates from the Latin word fluxus that means flow.

The higher the flux (the more magnetic field lines passing through the winding) the higher the induced voltage. The induced voltage is also proportional with the change of the magnetic flux over time.

The free electrons inside the copper experience a force. This force is called the Electro Magnetic Force. This force makes the electrons move to one end of de copper winding and here the potential becomes negative. At the other end of the winding the potential becomes positive because the amount of charge inside the winding has not changed.

When we connect a load between the two ends of the copper winding, for instance a lamp or a resistor, a current starts to flow. This current creates a magnetic field around the winding that opposes the magnetic field of the moving magnet. This phenomenon has been first described by Heinrich Lenz and is since called Lenz’ law.

When we connect more windings in series they form a coil. The induced voltage of each winding adds up, so a coil with 10 windings has a ten times larger voltage at its terminals as a single winding.

We did a similar observation with Ampère’s law. The magnetic flux from one winding is multiplied by putting more windings in series to form a coil.

It is a big thing that happens here. By making a special construction of conducting material, such as a copper coil, we are able to manipulate the magnetic field!

Link to a video about Faraday 's Law