Currents and Magnets

When a current passes through a wire, it produces a magnetic field around the wire. If we take a compass and place it next to the wire, we can see a deflection in the needle if we vary the current or move the compass away from the wire. The strength of the field varies with the distance from the wire. The direction of the field is circular around the wire, and is given by the right hand rule. Wrap your hand around the wire with your thumb pointing in the direction of current. Your fingers will show the direction of the field around the wire. For our example below, the 1st view shows the wire end on with the current coming out of the page. The second view shows the current moving to the left. The field is coming out of the page on the top and moving back into the page on the bottom of the wire. By the way, the symbol for a magnetic field is B and it is a vecotr quantity.  The SI unit for magnetic field is the Tesla (T).

Now, consider a loop of wire passed through a magnetic field of intensity B. Just as passing a current through a wire produces a magnetic field around the wire, if we move a wire through an existing field we create a current in the wire. This is called Electromagnetic Inductance. Electromagnetic Inductance (or Induction) is defined as producing a current and an emf (electro motive force, or voltage) in a circuit by changing a magnetic field around that circuit. In order for this to happen, we need three things:

  • A magnetic field
  • A current-carrying conductor
  • Relative motion between the two

The French scientist Andre Ampere investigated this phenomenon on the 19th century. We can see this by setting up a simple experiment. Attach a coiled copper wire to a galvanometer and drop a bar magnet through the coil. A deflection on the galvanometer shows the current being generated. The direction of the induced current depends upon the direction that the wire is moved through the field and upon the direction of the polarity of the field. The size of the induced emf depends upon:

  • The strength of the magnetic field
  • How fast the wire is moving in the field
  • The length of the wire that is in the field
  • The orientation of the wire to the field.  When the wire is perpendicular to the field, you get the maximum induced emf.