Solenoid Equivalent to a Bar Magnet
It is important to understand what a bar magnet is and what a solenoid is as well as their similarities and differences in order to truly appreciate how a bar magnet is equivalent to a solenoid. We will learn how a bar magnet is equivalent to a solenoid in this article.
What is a Bar Magnet?
In magnetism, a bar magnet is made up of two poles, north and south, in a rectangular or square object made out of iron or steel.
Natural magnets and artificial magnets are two types of magnets. They are manufactured by humans. They are:
Natural Magnets: These magnets exist naturally and possess weak magnetic fields. Lodestones are an example of natural magnets.
Artificial Magnets: They are designed by humans. Their magnetic fields are stronger. They can be customized in any way. Magnets shaped like bars are called bar magnets.
What is Solenoid?
The solenoid is an electromagnet used to generate controlled magnetic fields through electric current passing through a coil with a length greater than diameter.
How Does It Work?
In the same way as other magnets, solenoids with an activated magnetic field have positive and negative poles. A magnetic solenoid has a negative end that attracts and a positive end that repels. With each forward and backward motion of the piston, an electromagnetic field is produced inside the solenoid.
What is the Use of Solenoid?
A solenoid is used in an automobile's ignition system.
Solenoid relays are used for bringing metals together.
Due to their versatility, solenoids are extremely useful.
For instance, they can be used for:
Activating doorbells.
Activating paintball guns on automatic factory machines.
Activating a doorbell chime.
Bar Magnets and Solenoid Similarities
Bar magnets and solenoids have some similarities:
Attractive and directive properties are shared by both magnets.
These magnets can be aligned with the external magnetic field.
Their axial fields are identical.
Their magnetic moments are also identical.
Solenoid vs. Bar Magnet:
Magnets are permanent while solenoids are electromagnets, which means that they will act as magnets only if an electric current is applied.
The same magnetic properties are shared by both pieces of a split bar magnet, while the magnetic fields of a split solenoid are weaker.
In contrast to a solenoid, the poles of the bar magnet are adjustable.
Unlike a bar magnet, the magnetic field of a solenoid is affected by the electric current which passes through it, while the magnetic field of a bar magnet is unaffected by the current.
What is the Behavior of a Solenoid?
How can a bar magnet be used to determine the north and south poles of a current-carrying solenoid?
Solution:
The solenoid consists of a coil of insulated copper wire wound in circular loops. When current flows through a solenoid, magnetic field lines are created around it. This device produces a magnetic field similar to that produced by a bar magnet.
Bringing the north pole of a bar magnet near the negative terminal of a battery repels the bar magnet as the solenoid is powered.
According to the principle of like poles repelling each other, the end of the solenoid connected to the negative terminal of the battery acts as the North Pole of the device, and the other end acts as the South Pole. In this sense, one end of the solenoid represents the North Pole, while the other end represents the South Pole.
Natural Magnets or Artificial Magnets are Stronger: which is Stronger?
The magnetic field of an artificial magnet is quite stronger than that of a natural magnet. Natural magnets do not have the ability to alter their magnetic field. An artificial magnet, on the other hand, can develop a stronger magnetic field by adding more coils or increasing current flow.
FAQs on Bar Magnet as an Equivalent Solenoid
1. What is the fundamental similarity between a bar magnet and a current-carrying solenoid?
The fundamental similarity is in their magnetic field patterns. When observed from a distance, the magnetic field produced by a finite solenoid carrying current is nearly identical to that of a bar magnet. Both have distinct North and South poles, and their external magnetic field lines form continuous closed loops, appearing to emerge from the North pole and enter the South pole. This equivalence in field geometry is a core concept in magnetism.
2. What are the key differences between a bar magnet and a solenoid?
While they produce similar magnetic fields, a bar magnet and a solenoid differ in several key aspects:
- Nature of Magnetism: A bar magnet is a permanent magnet with a persistent magnetic field due to aligned atomic magnetic moments. A solenoid is an electromagnet, which only produces a magnetic field when an electric current flows through its coils.
- Controllability: The strength of a solenoid's magnetic field can be easily changed by adjusting the current or the number of turns per unit length. The field of a bar magnet is fixed.
- Polarity: The North and South poles of a solenoid can be instantly reversed by reversing the direction of the current. The poles of a bar magnet are permanent and cannot be changed.
- Strength: Solenoids can be designed to be much stronger than typical bar magnets.
3. How does a current-carrying solenoid behave like a bar magnet?
A solenoid consists of many circular loops of wire. When current flows, each loop acts as a small magnetic dipole. The magnetic fields of all these loops add up, creating a strong, uniform magnetic field inside the solenoid and a weaker, non-uniform field outside. The end of the solenoid where the current appears to flow in an anti-clockwise direction acts as the North pole, while the end with a clockwise current acts as the South pole. This arrangement of poles and the resulting field pattern makes it behave exactly like a bar magnet.
4. What is the conceptual basis for the derivation showing a bar magnet is equivalent to a solenoid?
The conceptual basis lies in comparing the mathematical expressions for their magnetic fields. The magnetic field at a distant axial point from the centre of a solenoid is calculated using the Biot-Savart law. The resulting formula for the magnetic field (B) is found to be identical in form to the formula for the magnetic field of a bar magnet (a magnetic dipole) at a similar point. This mathematical analogy proves that, for all external field purposes, a solenoid is equivalent to a bar magnet with a specific magnetic dipole moment.
5. What are some real-world examples of where solenoids are used?
Solenoids are crucial components in many devices due to their ability to convert electrical energy into mechanical motion. Important examples include:
- Automobile Starters: A powerful solenoid engages the starter motor with the engine.
- Electric Door Locks: Used in cars and buildings to move the locking bolt electronically.
- Solenoid Valves: To control the flow of fluids in washing machines, dishwashers, and industrial systems.
- Electromechanical Relays: Act as switches in circuits, allowing a small current to control a much larger one.
6. What would happen if a small bar magnet is placed inside a current-carrying solenoid?
The bar magnet would experience a magnetic torque. The strong, uniform magnetic field inside the solenoid would exert forces on the poles of the bar magnet, causing it to rotate and align its magnetic axis parallel to the solenoid's axis. This principle is used in devices like a moving coil galvanometer, where the deflection of a coil (or magnet) in a magnetic field is measured.
7. If a current-carrying solenoid is cut in half, does it behave like a bar magnet that has been cut in half?
Yes, the analogy holds true. When a bar magnet is cut in half, you get two smaller magnets, each with its own North and South pole. Similarly, if a current-carrying solenoid were cut in half, you would have two shorter solenoids. Each of these new, smaller solenoids would still function as a complete magnet, possessing both a North and a South pole, albeit with a weaker magnetic field than the original.
8. Why is understanding the equivalence between a bar magnet and a solenoid important for the CBSE Class 12 syllabus?
This equivalence is a cornerstone concept for the 2025-26 syllabus because it unifies two different aspects of magnetism. It demonstrates that magnetism, whether from a permanent magnet or an electromagnet, fundamentally originates from moving charges (atomic currents in a magnet and macroscopic current in a solenoid). This deepens the understanding of the origins of magnetic fields and is essential for grasping subsequent topics like Gauss's Law for Magnetism and electromagnetic induction.
