1 The Bar Magnet
The Bar Magnet
The Bar Magnet
- AÂ bar magnet has two poles similar to the positive and the negative charges of an electric dipole.
- One pole is designated as North Pole and the other as South Pole.
- When suspended freely, these poles point approximately towards the geographic North and South Poles respectively.
- Like poles repel each other and unlike poles attract each other.
- The poles of a magnet can never be separated.
- Magnetic monopole doesn’t exist.
The Magnetic Field Lines
- Magnetic field line is an imaginary curve, the tangent to which at any point gives us the direction of magnetic field
at that point. - The magnetic field lines of a magnet (or of a solenoid carrying current) form closed continuous loops.
- Outside the body of the magnet, the direction of magnetic field lines is from North Pole to South Pole.
- No two magnetic field lines can intersect each other. This is because at the point of intersection, we can draw two tangents. This would mean two directions of magnetic field at the same point, which is not possible.
Bar Magnet as an Equivalent Solenoid
We know that a current loop acts as a magnetic dipole. The magnetic field lines for a bar magnet and a current carrying solenoid resemble very closely. Therefore, a bar magnet can be thought of as a large number of circulating currents in analogy with a solenoid.
Let
i − Current passing through solenoid
a − Radius of solenoid
2l − Length of solenoid
n − Number of turns per unit length of solenoid
Let ‘P’ be the point at which magnetic field is to be calculated. Consider a small element of thickness dx of the solenoid at a distance ‘x’ from O.
Number of turns in the element =Â ndx
The magnitude of the field at point P due to the circular element is given by,
If P lies at a very large distance from O i.e., r >> a and r >> x, then
Total magnetic field at P due to current carrying solenoid:
If M is the magnetic moment of the solenoid, then
M = Total number of turns × Current × Area of cross-section
This is the expression for magnetic field on the axial line of a short bar magnet.
The magnetic moment of a bar magnet is thus equal to the magnetic moment of an equivalent solenoid that produces the same magnetic field.