(C) The magnetic field $B$ inside a solenoid is given by $B = \mu_0 n I$,where $I$ is the current.The magnetic field energy density $u$ is given by $u

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(C) The magnetic field $B$ inside a solenoid is given by $B = \mu_0 n I$,where $I$ is the current.The magnetic field energy density $u$ is given by $u = \frac{B^2}{2\mu_0} = \frac{(\mu_0 n I)^2}{2\mu_0} = \frac{1}{2} \mu_0 n^2 I^2$.Since the energy density $u$ is proportional to $I^2$,reversing the direction of the current (i.e.,changing $I$ to $-I$) does not change the value of $I^2$.Therefore,the magnetic field energy stored in the solenoid remains constant.Since the Assertion claims the energy decreases,the Assertion is incorrect.The Reason states that energy density is proportional to the square of the current,which is correct.Thus,the Assertion is incorrect and the Reason is correct.

Class 12 Physics Moving Charges and Magnetism Ampere’s circuital law and its application (Solenoid and Toroid)

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Assertion : If the current in a solenoid is reversed in direction while keeping the same magnitude,the magnetic field energy stored in the solenoid decreases.
Reason : Magnetic field energy density is proportional to the square of the current.

AIIMS 2015 All AIIMS

A
If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
B
If both Assertion and Reason are correct,but Reason is not the correct explanation of Assertion.
C
If Assertion is correct but Reason is incorrect.
D
If both the Assertion and Reason are incorrect.

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Moving Charges and Magnetism

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Ampere’s circuital law and its application (Solenoid and Toroid)

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Two toroids with number of turns $400$ and $200$ have average radii $30 \ cm$ and $60 \ cm$ respectively. If they carry the same current,the ratio of magnetic fields in these two toroids is:

EasyAP EAMCET 2024

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$A$ toroid core has an inner radius of $0.24 \ m$ and an outer radius of $0.26 \ m$. $A$ current of $10 \ A$ flows through the wire having $2500$ turns around it. Find the magnetic field inside the core of the toroid.

MediumTS EAMCET 2022

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If the magnetic field inside a solenoid is $B$,then the magnetic energy stored in it per unit volume is (where $c$ is the speed of light in vacuum and $\varepsilon_0$ is the permittivity of free space).

EasyAP EAMCET 2025

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$A$ magnetic field of $100 \;G$ $(1 \;G = 10^{-4} \;T)$ is required which is uniform in a region of linear dimension about $10 \;cm$ and area of cross-section about $10^{-3} \;m^2$. The maximum current-carrying capacity of a given coil of wire is $15 \;A$ and the number of turns per unit length that can be wound round a core is at most $1000 \;turns \;m^{-1}$. Suggest some appropriate design particulars of a solenoid for the required purpose. Assume the core is not ferromagnetic.

Medium

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An infinitely long hollow conducting cylinder with inner radius $R/2$ and outer radius $R$ carries a uniform current density $J$ along its length. The magnitude of the magnetic field,$|\vec{B}|$ as a function of the radial distance $r$ from the axis is best represented by:

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Assertion : If the current in a solenoid is reversed in direction while keeping the same magnitude,the magnetic field energy stored in the solenoid decreases.Reason : Magnetic field energy density is proportional to the square of the current.

Similar Questions

Two toroids with number of turns $400$ and $200$ have average radii $30 \ cm$ and $60 \ cm$ respectively. If they carry the same current,the ratio of magnetic fields in these two toroids is:

$A$ toroid core has an inner radius of $0.24 \ m$ and an outer radius of $0.26 \ m$. $A$ current of $10 \ A$ flows through the wire having $2500$ turns around it. Find the magnetic field inside the core of the toroid.

If the magnetic field inside a solenoid is $B$,then the magnetic energy stored in it per unit volume is (where $c$ is the speed of light in vacuum and $\varepsilon_0$ is the permittivity of free space).

An infinitely long hollow conducting cylinder with inner radius $R/2$ and outer radius $R$ carries a uniform current density $J$ along its length. The magnitude of the magnetic field,$|\vec{B}|$ as a function of the radial distance $r$ from the axis is best represented by:

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Assertion : If the current in a solenoid is reversed in direction while keeping the same magnitude,the magnetic field energy stored in the solenoid decreases.
Reason : Magnetic field energy density is proportional to the square of the current.