Unit IV
Electromagnetism

Revision date:   July 20        Time 2:45 PM

Electromagnetism was discovered by Hans Oersted when he observed that a current carrying conductor causes the needle of a magnetic compass to change direction.

Oersted's Principle:
  Charge moving through a conductor produces a circular magnetic field around the conductor.

Any current carrying conductor will have surrounding it a magnetic field sometimes called a B field, which was mapped origanally by Andre Ampere.
The magnetic field lines surrounding a wire carrying a current I will form concentric circles around the wire.
The direction of the magnetic field lines is perpendicular to the wire but are they clockwise or counterclockwise?
To determine the direction of the magnetic field lines (clockwise or counterclockwise) wrap the fingers of your right hand around the wire so that your thumb points in the direction of the current flow. Your fingers will be in the direction of the magnetic field lines; as indicated in the diagram.
This procedure is called the Right-Hand Rule for a Straight Conductor

If a magnetic compass is held near the wire in the above diagrams, the needle will align itself tangent to the circle passing crosswise through the shaft. The north pole will point in the direction of the arrows.

Current Conventions

There are two two conventions used to indicate the direction of current flow or should I say electron flow. And this is where the difference lies.
In a battery or electrochemical cell, electrons are produced at on electrode called the anode (where oxidation occurs, the negative terminal) and are passed throught the external circuit to the second electrode called the cathode (where reduction occurs, the positive terminal). This convention used in electrochemistry is called the electron flow convention.

Here's a picture of an electrochemical cell showing electron flow. Similiar to the one in class exept this diagram has a salt bridge.

Originally nobody knew about electron or how a battery produced electrons so Benjamin Franklin decide that the current flowed from the positive terminal to the negative terminal. This was an arbitrary decision and is called the current flow convention.
I suppose you could say in physics use current flow convention and in chemistry use electron flow convention.
Since we will be using the current flow convention right hand rules will be used. Note: if electron flow convention is used the just switch hands and left hand rules will be just as valid.

Electromagnetic Effect

Is based on the idea that field lines (H) can be concentrated into a small area and reinforce each other.

In a helix the H lines as shown in green below, are concentrated into the central core of the helix and produce strong magnetic flux field effects especially at the ends of the helix.

Diagrams are to be found in your text book on these pages. Make sure you study and can explain & understand these diagrams.
  1. Page 458 Fig 12.7
  2. Page 523 Fig 14.12 & 14.13
  3. Page 527 Fig 14.20 & 14.21 & 14.22
  4. Page 528 Fig 14.23

Factors that effect the strength of an electromagnet are listed in the chart below

Two of these effects are shown in this diagram

Electromagnets in action is found in section 14.4 in text book. Four examples of electromagnets are listed in the text. Here's a chart listing three such uses.



A photo of our class bell can be seen

Students at working ringing the bell;

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