A solenoid is an electromagnet that has a tightly packed rounded coil in the form of a helix. The magnetic field is generated by passing the electricity, which induces magnetism. The bar magnet can be made to act as a solenoid. The rectangular type of magnet is said to be a bar magnet. It is usually made of permanent magnetic material such as iron, steel, etc.

The electromagnets are more beneficial than ordinary permanent magnets as the electric current can be controlled by changing the magnetic field. That is why bar magnets are generally made to act like solenoids. We will learn about the bar magnet as an equivalent solenoid through this article. 

What is a solenoid?

A solenoid is an electromagnet that produces a magnetic field when electricity is passed through it. It is a helical coil with tightly wound turns around a core. The length of a solenoid is greater than its diameter. When the magnetic field is generated, the solenoid starts to act like a magnet, and thus it has north and south poles. 

The magnetic field in the solenoid moves in upward and backward directions. The solenoid does not need to be along a straight line. It can also be bent in the shape of a horseshoe magnet. 

There are various uses of the solenoid. It is widely used in automobiles and doorbells. There are various advantages of using a solenoid instead of a permanent magnet. The most important one is the freedom of changing the electric current and magnetic field. 

Working of Solenoid

The solenoid works on the principle of electromagnetism, i.e., when current is passed through the coil, a magnetic field is generated. A solenoid needs to have three essential components; wire, core, and electricity. Without any of these, the solenoid is of no use. 

When the core is placed in the coil in which electricity is flowing, the magnetic field lines get deposited on the core and increase the induction. Thus, the flux (magnetic field lines per given area) is mainly concentrated on the core. Only a small quantity of flux is found on the outer edge of the core. 

The strength of the solenoid’s magnetic field can easily be increased in two ways. The first method is by increasing the current through the coil, and the next method is by increasing the number of turns of the coil.

Solenoid as a Magnet

When an electric current is passed through the solenoid, it acts as a magnet. Thus solenoid is known to be an electromagnet. The solenoid shows various properties of magnet: 

• The magnetic field line produced in the solenoid is the same as that of a magnet. It moves in the same direction as well

• When the south pole of any magnet is brought near the south pole of the solenoid, it repels it, thus solenoid shows attractive and repulsive properties as well

• The end of the solenoid, connected to the battery’s negative terminal, is taken as the North pole while the other end is taken as the south pole

Bar magnet as an equivalent solenoid

The bar magnet can be represented and function as a solenoid. It can be proven by calculating the axial field lines. Let us prove bar magnet as an equivalent solenoid. 

Suppose we are given a solenoid with the following specifications:

Radius = r

Length = 2l

Number of turns per unit length = n

The current passing through solenoid = I

We take a small thickness of solenoid dx at a thickness of x from the central point O. We will calculate the magnetic field at a point S at a distance of d from the centre. 

Now the magnetic field at the point S through n turns will be:

Thus from the above derivation, it is clear that the magnetic moment of a bar magnet is found to be equal to the solenoid. Therefore the bar magnet is an equivalent solenoid. 

Difference between Solenoid and Bar Magnet 

Let us learn about the primary differences between bar magnet and solenoid:

1. When electricity is passed through a solenoid, it gets magnetised temporarily; that is, the magnetism is lost by removing the current. At the same time, the bar magnet is magnetised permanently. 
2. The solenoid produces a magnetic force of very high strength in comparison to a bar magnet. 
3. By reversing the current direction, the solenoid’s polarity is reversed. But this does not happen with the bar magnet. 
4. The solenoid produces the magnetic field as long as an electric current is passed through it. But the bar magnet produces the field even after the current has been removed. 
5. For the core of solenoid, soft iron or steel is preferred, whereas bar magnets are made up of solid and hard iron and steel. 
6. The quantity of current that flows through a magnet determines the strength of the solenoid. The strength of a bar magnet depends upon the nature of the material used. 

Similarities of Solenoid and Bar Magnet

 Besides the differences mentioned above, solenoid and bar magnets have some similarities. That is why we can take a bar magnet as an equivalent solenoid. The similarities are as follow: 

• The bar magnet and solenoid both have the exact magnetic moment
• Above all, both have the same magnetic field along with the axial point
• The attractive and directive properties are found in both solenoid and bar magnets

Conclusion 

The solenoid has significant usage in various fields as it has more advantages than a permanent magnet. The solenoid has a coil wound around a soft iron or steel core. Each turn acts as a small magnetic dipole that creates a high-strength magnetic field. By increasing the turns, the magnetic field strength can also be increased. We have demonstrated the above bar magnet as an equivalent solenoid. The bar magnet and solenoid both have various similarities.