A solenoid is just a coil of wire that forms a cylindrical shape. When an electric current is passed through a solenoid, it tends to adopt the magnetic properties in which electricity is changed to magnetism and then again changed into electricity. The shape of the solenoid reassembles the shape of the bar magnet and has two poles known as the north pole and the south pole respectively. For this reason, solenoids are used for research purposes when there is the presence of a magnetic field. The center of the solenoid marks the uniform magnetic field whereas, at the outer part of the solenoids, the magnetic field lines get far away from each other, making the outside field weaker. The two major types of solenoids – straight and toroidal solenoids have an array of applications and uses.  

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What is a straight solenoid? 

To define a straight solenoid, it is said that the shape is like a long wire that is wrapped around in the form of a cylinder or helix. It is formed with a large number of turns and the gap between the two successive turns is quite small. For this reason, each of the turns can be considered as a closed circular loop. 

When an electric current is passed through it, a magnetic field gets generated and the magnetic field lines start moving from one end to another. In this case, the magnetic field lines that are produced are uniform in nature. The direction of the magnetic field lines in this case is along the direction of the solenoid axis. 

But in comparison to the inside of the solenoid, the outer part has a low magnetic field and magnetic field lines are at larger distances from each other. The magnetic field strength of the straight solenoid depends on the number of turns in the coil and the current that is flowing through it. With the help of the right-hand thumb rule, the direction of the magnetic field and the current can easily be determined. 

 Here is the formula for the magnetic field in a straight solenoid:

B = µ0nI 

where n is the number of turns in the coil and I is the current flowing through it. 

What is a toroidal solenoid? 

The solenoid that is bent into the shape of a circle or like a hollow ring structure is known as a toroidal solenoid. When you see the image of a toroidal solenoid, it seems like continuous loops that are wound nearby without extra spacing. The inside and outside magnetic fields are always zero, meaning there is no magnetic field inside and outside. Low-frequency electric devices use toroidal solenoids. 

According to the right-hand thumb rule, the direction of the magnetic field is always in the clockwise direction. As per the formula of the toroidal solenoids, the magnetic field in the toroidal solenoid is given by B = µ0NI/2πr, where B is the strength of the magnetic field, N is the number of turns in the coil, I is the current that is passed through the coil and r is the radius of the circular toroid. 

Major differences between straight and toroidal solenoids

To define toroidal solenoids and to differentiate them from the straight solenoid, here are a few major differences you need to know: 

• Straight solenoid is a cylinder in shape with wire wrapped around it that forms loops. On the other hand, toroidal solenoids are circular in shape with wire wrapped around them in a circular or ring shape
• In case of a straight solenoid, the magnetic field lines are created outside the solenoid. Whereas in case of toroidal solenoids, the magnetic field lines are created within the device
• You will find a uniform magnetic field present inside of the straight solenoid. But in the case of toroidal solenoids, you won’t find a uniform magnetic field present in the interior of the toroid
• The magnetic field due to solenoids is represented by B = µ0nI. But this isn’t the case with the toroid. In the case of toroids, the magnetic field is zero, regardless of interior or exterior
• Toroids have magnetic fields present within them and the formula of the magnetic field within the toroid is B = µ0NI/2πr

Few similarities between straight and toroidal solenoids

Aside from a handful of differences, there are a few similarities between the straight and toroidal solenoids. These are mentioned as follows:

• Both straight and toroidal solenoids work on the electromagnetism principle
• Both of them have the power to act as electromagnets but there is a necessary condition for it, and that is, an electric current should pass through the devices
• The magnetic field produced by the straight solenoid is similar to the magnetic field present within the toroidal solenoids

Applications of solenoids

Multiple modern-day applications use solenoids and here are a few applications discussed below: 

• The door mechanism that is controlled electronically utilizes solenoids. Until the current flows through the door, it remains closed and safe
• A few medicinal devices such as dialysis machines have solenoids to effectively control the flow of the blood
• An array of industrial devices for pinching, rotating and holding use solenoids

Conclusion 

Even though both straight solenoid and toroidal solenoid work on the same principle yet they have plenty of differences like shape and working. The major difference is the size of solenoid and toroidal as solenoids are cylindrical in shape whereas toroids have a circular shape. Using the right-hand thumb rule formula, the direction of the magnetic field lines is easily found out in case of straight and toroidal solenoids.