Magnetic Field
Force due to Magnetic Field Biot-Savart Law


Magnetic Field

    Much like an electric field around an electric charge, there is a magnetic field around a magnet. Anything that enters this force (and is magnetically inclined) will experience a force.

   A man named Oerstead once discovered that wires that conduct electric charge also creat a magnetic field. (This is called electromagnetism.) If you want to find the direction of this field, it is much easier than the electric field method. All you need to do is grasp the wire in your left hand, with your thumb in the direction of the flow of charge (negative). Your curved fingers will be pointing in the direction of the field. This is called the Left Hand Rule. Magnetic Fields have a circular direction around a wire. This only deals with straight wires. However, sometimes we make what is called a coil. If you want to make a coil, wrap some wire around and around a pencil, but don't let the wire cross itself. Then remove the pencil. You have a simple coil! There are actually three left hand rules. The second left hand rule states that if your wrap your left hand around a coil of wire, with your fingers in the direction of the flow of charge, your thumb points to the end of the coil that is the north pole. That's right- coils of current conducting wire are also magnets! 

   The presence of a magnetic field is detected by the forces of attraction or repulsion which these exert on similar objects within the range of this field. This magnetic field is characterized by the presence of magnetic lines which seem to be non-intersecting and oriented from one pole of the magnet to the other. A magnetic field is a vector quantity, one which is described by both magnitude and direction. The magnetic field lines are not physical lines but imaginary ones, which help in the understanding of the magnetic field. In short, the orientation and the working range of the field is represented and more easily understood by representing this field in the form of magnetic lines.


Force Due to Magnetic Field

  A current carrying conductor experiences a force when placed in a magnetic field. The direction of force is reversed when the direction of current in the conductor is reversed. The force acting on the current-carrying conductor can be changed by changing the direction of the magnetic field.

Flemings Left Hand Rule

According to this rule, extend the thumb, forefinger, and the middle finger of the left hand in such a way that all the three are mutually perpendicular to each another. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then, the thumb points in the direction of the force exerted on the conductor.


Biot-Savart Law

  Consider a small piece of wire of length ds carrying a current I. This defines a vector ds that points in the direction of the current. The magnetic field dB set up by this piece of current-carrying wire at a point a distance r away is:

Proportional to 1/r^2 .
Proportional to I, the current, and ds, the length of the wire .
In a direction perpendicular to both ds and r, the vector from the wire to the point
Proportional to sinx , where x is the angle between ds and r

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