A solenoid is an electromagnetic device made out of a coil of wire wound around a cylindrical or elongated core, usually comprised of ferromagnetic material like iron or steel. When an electric current flows through a wire coil, it generates a magnetic field surrounding it, which can exert force on objects in the field or cause mechanical motion. In this article, we will learn in detail about solenoid, its working and application.

What are Electromagnets?

Electromagnets are magnets that use electric current to generate magnetic fields. Unlike permanent magnets, which produce a steady magnetic field, electromagnets may be turned on and off by regulating the flow of electricity through a wire coil. They are commonly utilized in applications that demand changeable and regulated magnetic fields. A coil of wire twisted around a magnetic core, usually made of iron or steel, is what makes up an electromagnet. When electric current travels through a wire coil, it produces a magnetic field surrounding it in accordance with Ampère’s law.

What is a Solenoid ?

A solenoid is a type of electromagnet that consists of a coil of wire wrapped around a ferromagnetic or ferrimagnetic core. When an electric current is passed through the wire, it creates a magnetic field around the core, which results in various applications in industries and everyday devices.

How to Make a Solenoid?

To make a solenoid we need following materials

• Insulated copper wire (preferably enamel-coated)
• Power source (battery or DC power supply)
• Iron or steel rod (optional, as a magnetic core)
• Insulating material (e.g., cardboard tube or plastic pipe)
• Electrical tape
• Wire cutters

The steps to make a solenoid are mentioned below:

• Take a length of insulated copper wire and use wire cutters/strippers to remove any insulation from both ends.
• Make sure the wire is long enough to wrap many layers around the core material you’ve chosen.
• If you’re using a core material (such as an iron or steel rod), place it in the center of the insulating layer.
• Begin winding the wire around the core material, ensuring that each turn is close to the preceding one without overlapping.
• Continue wrapping until there are multiple layers of wire surrounding the center. The magnetic field becomes stronger as the number of turns increases.
• To secure the wire ends to the insulating material, apply electrical tape.
• Turn on the power source to pass current through the wire coil.

When the power is turned on, magnetic field is created which can be tested using iron fillings.

Working of a Solenoid

When an electric current is passed through a circular wire, a magnetic field is generated around it. We know that magnetic field is directional, in simpler terms, if we place another circular wire with electric current passing in the opposite direction to that electric current of the first wire, the magnetic fields of both wires will cancel out each other.

However, if the wires have currents passing in the same direction, the magnetic fields around both wires will sum up. Now, a solenoid is like a collection of multiple circular wires, all of which have current passing in the same direction. Its helix-like structure allows for summing up of all the magnetic fields. As a result, a strong magnetic flux is generated when an electric current is passed through this wire.

Can we control the amount of magnetic force? Yes, we can increase the magnetic field in two ways:

1. Increase the current passing through the coil.
2. Increase the number of turns.

This relationship can be given as:

F = ????₀nl

Where ‘n’ is the number of turns of the wire per unit length, ‘I’ is the current flowing through the wire, and the direction can be determined using the right-hand thumb rule.

The magnetic field inside the solenoid is parallel to the axis of the core. Beyond the solenoid, the magnetic field is very weak.

Magnetic Field in a Current-Carrying Solenoid

The magnetic field inside the solenoid is parallel to the axis of the core. Beyond the solenoid, the magnetic field is very weak. The purpose of solenoid is to generate a magnetic field, so studying it is very important to understand how magnetic field are distributed around a current carrying solenoid.

Inside the current carrying solenoid, magnetic field are parallel to axis of a solenoid. That is another way of saying that magnetic field is same at all the points inside a current carrying solenoid, that is uniform inside a solenoid. Beyond the solenoid, at very large distance magnetic field is nearly zero.

Formula of Magnetic Field inside a current carrying solenoid

The magnetic field inside a current-carrying solenoid can be calculated using the formula:

[Tex]B = \mu \cdot n \cdot I[/Tex]

Where:

• B is the magnetic field strength inside the solenoid (in Tesla, T),
• [Tex]\mu[/Tex] is the permeability of the material inside the solenoid (in Henry per meter, H/m),
• n is the number of turns per unit length of the solenoid (unitless), and
• I is the current flowing through the solenoid (in Amperes, A).

The derivation of this formula involves considering the magnetic field created by each individual turn of the solenoid and summing up the contributions from all the turns. However, I’ll provide a simplified explanation here.

Consider a solenoid with N turns per unit length, carrying a current I. Each turn of the solenoid acts like a circular current loop, producing a magnetic field at the center of the loop. The magnetic field at the center of a single turn of radius R carrying a current I is given by Ampère’s law as:

[Tex]B_{\text{loop}} = \frac{\mu_0 \cdot I \cdot R^2}{2 \cdot (R^2 + x^2)^{3/2}}[/Tex]

Where:

• [Tex]\mu_0[/Tex] is the permeability of free space (constant, [Tex]4\pi \times 10^{-7}[/Tex] H/m),
• x is the distance from the center of the loop along its axis.

For a solenoid with N turns per unit length and length L, the total number of turns in the solenoid is [Tex]N \cdot L[/Tex]. By symmetry, the magnetic field at the center of the solenoid due to all the turns will be the sum of the magnetic fields from each turn, which gives:

[Tex]B_{\text{sol}} = N \cdot B_{\text{loop}} = \frac{\mu_0 \cdot N \cdot I \cdot R^2}{2 \cdot (R^2 + x^2)^{3/2}}[/Tex]

For a long solenoid [Tex]( L \gg R )[/Tex], the magnetic field at the center of the solenoid can be approximated as constant along the length of the solenoid and is given by:

[Tex]B = \mu_0 \cdot n \cdot I[/Tex]

Where [Tex]n = N/L [/Tex] is the number of turns per unit length of the solenoid.

Difference between Solenoid and Bar Magnet

Here are the differences between a solenoid and a bar magnet.