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Solenoid and Toroid are tools which apply the magnetic effect of current to produce magnetic field. Its main advantage over a conventional magnet is that its magnetic intensity can be controlled by regulating the amount of electric current which flows through it. Unlike permanent magnets, they can also be switched on and off, making them ideal for many applications. Solenoid is a coil of insulated wire which is wound on a rod composed of solid iron, solid steel, or powdered iron. While, a toroid looks mostly like a solenoid which is bent into a circular shape in order to close it into a loop-like form.
Read More: Magnetic Field On The Axis Of A Circular Current Loop
| Table of Content |
Key Terms: Solenoid, Toroid, Magnetic Field, Helix coil, Ampere Circuital Law, Current, Electromagnet, Torus, Electrostatic Potential
What is a Solenoid?
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Much like an electromagnet, a solenoid produces a magnetic field using a coil wound to form a tightly packed helix. In case of a long solenoid, the length of the helix is much larger than the radius of the helix. The coil is arranged in such a manner that it produces a uniform magnetic field.
The wire used in building a solenoid is insulated so that the magnetic effect of no two turns can touch one another. As a result, each turn can be considered as a current-carrying loop. The net magnetic field thus produced is the vector sum of individual magnetic fields of each current carrying loop.
| Let us now understand the mathematical formula of the magnetic field produced by a solenoid. Now, by applying Ampere’s Circuital Law to obtain the magnetic field generated by a solenoid carrying a current ‘I’, we can demonstrate the following:
Solenoid carrying Current ‘I’ Let us assume the Amperian loop is “abcd”. Along the path cd, there is no magnetic field. This is because the field outside the ideal solenoid is zero. Similarly, along bc and ad the field is zero. Let the field along ab be B. Thus, the relevant length of the Amperian loop is, L = h. Let n be the number of turns per unit length, then the total number of turns is nh. The enclosed current is, I e = I (n h), where I is the current in the solenoid. From Ampere’s circuital law, BL = µ0Ie ⇒ Bh = µ0I(nh) ⇒ B = µ0In |
Ampere’s Circuital Law Video Explanation
Read More:
| Related Links | ||
|---|---|---|
| Magnetic Spectrum | Torque on Current Loop | Magnetic Properties of Materials |
| Magnetisation and Magnetic Intensity | NCERT Solutions Chapter 4 Moving Charges and Magnetism | Eddy Currents |
What is a Toroid?
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Toroid can be defined as a circular helical structure which has a hole in the middle. Forming itself like a solid body, the axis can further make a revolution passed through a hole. In order to make sure that it does not intersect with the surface, a rectangle often rotates around an axis that is parallel to one of its edges.
The hollow rectangle section ring is produced, so in case the revolving figure ends up like a circle, then the object can be called a Torus.
The outside and inside the magnetic field of a toroid is zero. The direction of the magnetic field inside a toroid always happens to be clockwise. A toroid mostly looks like a solenoid that can be bent into a circular shape in order to close it into a loop structure. The magnetic field due to a toroid can be expressed as,
| \(B = \frac{\mu_0 NI}{2 \pi r}\) |
The toroid carries a hollow circular ring with many turns of a coated wire which can be closely wound with Negligible spacing between any two turns so that the magnetic field inside and outside stands at zero. Further, the term toroid can also be used to define a toroidal polyhedron. A toroid should not necessarily be circular and may possess a number of holes.
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Similarities between Solenoid and Toroid
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There are various similarities between a Solenoid and Toroid. Some of them are:
- They both act according to the principle of Electromagnetism only.
- They can act like electromagnets when the current passes through them.
- It often happens that the magnetic field of the Toroid and Solenoid is the same.
Solenoid and Toroid
Read More: Magnetic Field due to Current Element
Difference between Solenoid and Toroid
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Key differences between Solenoid and Toroid are tabulated below:
| Solenoid | Toroid |
|---|---|
| Solenoid is Cylindrical in shape. | Toroid is often circular. |
| Magnetic field is produced outside in solenoid. | Magnetic field is produced within a Toroid. |
| Uniform magnetic fields can only be seen in Solenoid. | Uniform magnetic fields cannot be seen in Toroid. |
Read More: Magnetic Force
Previous Year Questions
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- A conducting circular loop of radius r carries … [JEE Advanced 2017]
- Two solenoids are given - 1st has 1 turn per unit … [VITEEE 2016]
- The Lenz's law is the consequence of the conservation of … [JKCET 2014]
- A conducting rod of length L is moving in a uniform magnetic field … [JKCET 2012]
- A bar magnet is placed upright on a floor … [JKCET 2012]
- A short solenoid of radius aa, number of turns per unit length … [JKCET 2012]
- A conducting wire of parabolic shape, initially … [JEE Advanced 2019]
- A square loop of side 10 cm and resistance 5 Ohm is placed … [JKCET 2019]
- A square loop of wire, side length … [AMUEEE 2010]
- A particle enters uniform constant magnetic field … [UPSEE 2016]
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Read More: Rare Earth Magnets
Things to Remember
- Solenoid and Toroid are devices which apply the magnetic effect of current to produce magnetic field.
- Solenoid generated a magnetic field using a coil wound to form a tightly packed helix.
- The net magnetic field in a Solenoid thus produced is the vector sum of individual magnetic fields of each current carrying loop.
- Toroid can be defined as circular helical structure that has a hole in the middle.
- The outside and inside magnetic field of a toroid is zero.
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Sample Questions
Ques. (a) In what respect is a toroid different from a solenoid? Draw and compare the pattern of the magnetic field lines in the two cases.
(b) How is the magnetic field inside a given’ solenoid made strong? (All India 2011)(2 marks)
Ans. Differences between toroid and solenoid are as follows:
- A toroid is often much circular, whereas a solenoid is Cylindrical in shape
- Uniform magnetic fields can only be seen in Solenoid and not in toroid.
- The magnetic field is produced outside in solenoid, whereas the magnetic field is produced within a Toroid
The figure on the top is a torroid whereas the figure on the bottom is solenoid.
(b)This can done by either changing the core of the solenoid or increasing the current passing through the solenoid.
Ques. Using Ampere’s circuital law, obtain the expression for the magnetic field due to a long solenoid at a point inside the solenoid on its axis. (All India 2011)(2 marks)
Ans.
Let us assume the amperian loop is abcd. Along the path cd, there is no magnetic field. This is because the field outside the ideal solenoid is zero. Similary, along bc and ad the field is zero.
Let the field along ab be B. Thus, the relevant length of the Amperian loop is, L = h.
Let n be the number of turns per unit length, then the total number of turns is nh. The enclosed current is, I e = I (n h), where I is the current in the solenoid. From Ampere’s circuital law,
Ques. What are electromagnetic waves, and how do they work? (2 marks)
Ans. Electromagnetic waves are waves that propagate as a result of simultaneous periodic changes in electric and magnetic field strength.
Ques. Is it true that there are magnetic fields in space? (2 marks)
Ans. Magnetic fields exist in space, indeed. Based on measurements of a large number of pulsars and the polarisation of their radio signals, the spiral arms of the Milky Way seem to have some very large-scale coordinated magnetic field. Magnetic fields have been discovered in interstellar dust clouds. The fields are intensified as the clouds fall.
Ques. Define the density of magnetic flux. (2 marks)
Ans. The sum of magnetic flux in an area measured perpendicular to the magnetic flux's path is known as magnetic flux density. It is denoted by the letter B and is represented in Tesla units.
Ques: Can one use an electromagnet to generate electricity? (2 marks)
Ans: Magnets' characteristics are employed to generate electricity. Moving magnetic fields attract and repel electrons. Electrons are dispersed around in metals like copper and aluminium. When you move a magnet around a coil of wire or vice versa, the electrons in the wire are pushed out, and an electrical current is created. Kinetic energy (the energy of motion) is converted into electrical energy via electricity generators.
Ques: What occurs when the current flow of an electromagnet is reversed? (2 marks)
Ans: When the current flowing through the wire is reversed, the north and south poles are reversed as well. The north and south poles reverse when the current is reversed again. The north and south poles will swap locations every time the current is reversed.
Ques: How can you tell the difference between an electromagnet and a permanent magnet? (3 marks)
Ans: The fundamental distinction between electromagnets and permanent magnets is that when a current is sent through them, they have a magnetic attraction to other metallic things. It offers several advantages, including the ability to regulate the magnetic attraction's strength by changing the current flow. It is commonly used in research and industry when magnetic attraction is necessary for this purpose.
Electromagnets also offer several benefits over permanent magnets like-
- They can be switched on and off whenever you choose
- The magnetic field's strength can be adjusted, and
- Electromagnets are excellent for gathering scrap steel and iron in scrap yards because of their qualities.
Ques. Draw the magnetic field lines due to a current passing through a long solenoid. Use Ampere’s circuital law, to obtain the expression for the magnetic field due to the current I in a long solenoid having n number of turns per unit length. (CBSE 2014)(3 marks)
Ans. The field lines are given in the diagram below:
Let n be the number of turns per unit length, then the total number of turns is nh. The enclosed current is, I e = I (n h), where I is the current in the solenoid. From Ampere’s circuital law,
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