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Bar Magnet and Solenoid

The function of a bar magnet and solenoid is to generate the magnetic field. Both the devices have different structures and properties but work the same. Let’s discuss this in detail.

Bar Magnet

A bar magnet is a rectangular piece of metal with permanent magnetic ability. It has two poles, Northand South. It does not require any current to produce the field. It produces its magnetic field. Examples of bar magnets include refrigerator magnets and compass needles. The bar magnet automatically aligns itself along the magnetic field when suspended freely.

The applications of bar magnets include laboratories, radios, televisions, and medical.

The same poles repel each other, while the opposite poles of two magnets attract. The laws are named as the law of attraction and repulsion.

Law of attraction

The magnet’s North Pole will attract towards the South Pole of the other magnet. Similarly, the magnet’s South Pole will attract towards the North Pole of the other magnet.

Law of Repulsion

The North Pole of a magnet will repel the North Pole of the other magnet. Similarly, the South Pole of a magnet will repel the South Pole of the other magnet.

A simple bar magnet is shown below:

Classification of Bar Magnets

There are two types of bar magnets, natural magnets,and artificial magnets.

Neutral magnets are found in nature without any artificial efforts. It has a low magnetic field. The artificial magnets are man-made with different shapes. It has strong magnetic fields. For example, ring magnets, horse magnets, bar magnets, etc. The magnetic strength of the bar magnet is highest at the poles.

Types of bar magnet

There are major two types of bar magnet, which are listed as follows:

1. Cylindrical magnets
2. Rectangular magnets

Cylindrical Magnets

The cylindrical magnets are in the shape of cylinder. It is also known as rod magnets. It has high magnetism properties due to its large thickness. It is used in experiment-related applications.

Rectangular magnet

The rectangular magnet is in the shape of a rectangle. It also has high magnetic strength. A bar magnet is also a type of rectangular magnet. It is used in applications, such as electronic devicesand medical purposes.

Advantages of bar magnets

The advantages of bar magnets are as follows:

• Portable
• No electricity required
• Easily available
• Strong magnetic field
• It has two poles North and South.
• The magnetic field strength is highest at the two poles.
• It is used in various applications, such as speakers, electronic devices, etc.
• It is widely used for experiment purposes.

Solenoid

The solenoid is an electromechanical device that converts electrical energy to mechanical energy. Solenoids are also known as electromagnets because it generates the magnetic field from the electric current flowing in the circuit.

The magnetic property of the solenoid is not permanent. It stays as long as the current flows through the circuit.

The solenoid circuit comprises a coil (wire arranged in the form of a helix), metal core, and the external power supply. The current flows through the coil when the power supply is switched ON. The coil is wounded around the metal core in the form of a helix. It creates a magnetic field, and the metal core behaves like a magnet. We can test it by bringing a metal iron near the electromagnet.

There are various applications of solenoid, such as inductors, antennas, medical machines, engines, etc.

Advantages of the solenoid

The advantages of the solenoid are as follows:

• Inexpensive
• Easy construction
• Simple for beginners
• It is an alternative to the fossil fuels sources used in engines
• Easy to fit
• No pollution
• Easy replacement
• Can operate on both AC and DC voltages
• Wide application in automobiles
• Low power consumption

For detailed information about the solenoids, click on solenoid (https://tutoraspire.com/solenoid).

Bar magnet vs. solenoid

The function of bar magnet and solenoid is to produce magnetic field, but the process of magnetic field generation and their properties are different. The differences between bar magnet and solenoid are listed in the below table:

Category	Bar magnet	Solenoid
Magnetic field	Permanent It depends on its permanent self magnetic field.	Temporary It depends on the current for its magnetic field.
Current dependent	No	Yes It produces magnetic field as long as current flows.
Nature of magnetism	Natural	Artificial
Components	Only a single rectangular shape magnet	Metal core, wire windings, and an external voltage source
Force of attraction	Weak	Strong
Demagnetize	No It cannot be demagnetized because its magnetic property is permanent	Yes It can be demagnetized by removing the external power source or switching off the current.
Increase of field density	No	By increasing the number of turn on the metal core, the field density of the solenoid can be increased

Bar magnet as an equivalent Solenoid

The bar magnet as an equivalent solenoid can be proved using a finite length solenoid, as shown below:

Where,

R is the radius of the solenoid

2d is the total length of the solenoid

N = number of turns on a solenoid

I = current flowing through the solenoid

dx = thickness of a small element

x = distance from the center O to the small element

The magnetic field due to N turns of a solenoid is given by:

dB = μNdxIa²/ 2[(r – x)² + a²]^3/2

Considering the axial field of the solenoid r > > 1 and r > > a, the denominator part can be expressed as:

[(r – x)² + a²]^3/2 = r³

Integrating from -1 to 1,

B = μ NI2a²/ 2r³… (1)

The magnetic moment of a solenoid is given by:

m = N x I × A

Where,

I is the amount of current flowing through the coil

A is the cross-sectional area

N is the number of turns on a solenoid

Substituting the value of magnetic moment in equation (1), we get:

B = μ 2m/ 2r³

Or

B = μ_o 2m/4𝛑 2r³

The magnetic moment of a bar magnet is given by:

m = P × 2d

Where,

P is the pole strength

d is the distance between the two poles (North and South) of a magnet or the total length of the magnet

The magnetic moment of the solenoid is equal to the mar magnet. Thus, we can say that the bar magnet behaves an equivalent solenoid.

Examples

Let’s discuss some examples based on the bar magnet and solenoid, which will be in the form of MCQ’s (Multiple Choice Questions).

Example 1: Solenoids are often known as:

1. Permanent magnets
2. Electromagnets
3. Weak magnets
4. None of the above

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Example 2: Electromagnets are used in applications:

1. Speakers
2. Buzzers
3. Motors
4. All of the above

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Example 3: What is the magnetic moment of a bar magnet when the distance of a pole from center is 3m and the pole strength of 10 Ampere meter?

1. 30 Am²
2. 15 Am²
3. 60 Am²
4. 18 Am²

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Example 4: Unit of the magnetic moment of a magnet is:

1. Am
2. Am²
3. Am^-1
4. Am^-2

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