Moving Charges and Magnetism

Moving Charges And Magnetism Class 12th Topics (NCERT Syllabus)

1. Introduction

Electricity and magnetism were known for centuries, but in 1820 Hans Christian Oersted discovered their connection. He observed that a current-carrying wire deflects a nearby magnetic compass needle. The needle aligns tangentially to concentric circles around the wire, showing that moving charges or currents produce a magnetic field in the surrounding space.

2. Magnetic Force

A moving charge in a magnetic field experiences a force:

$
\vec{F}=q(\vec{v} \times \vec{B})
$

It is always perpendicular to both velocity and magnetic field, so it changes direction of motion but does no work.

For a current-carrying conductor:

$
\vec{F}=I(\vec{L} \times \vec{B})
$

Direction is given by the right-hand rule.

• Magnetic Force

3. Motion in a Magnetic Field

When a charged particle enters a magnetic field with velocity perpendicular to the field, it experiences a magnetic force that makes it move in a circular path.

$
r=\frac{m v}{q B}, \quad T=\frac{2 \pi m}{q B}
$

If velocity has both perpendicular and parallel components, the particle follows a helical path.

4. Magnetic Field Due to a Current Element - Biot-Savart Law

The Biot-Savart law gives the magnetic field produced at a point due to a small current element. If a current $I$ flows through a small length element $d \ell$, then the magnetic field at point $P$ is:

$
d \mathbf{B}=\frac{\mu_0}{4 \pi} \frac{I d \ell \sin \theta}{r^2}
$

where $r$ is the distance of the point from the element, and $\theta$ is the angle between $d \ell$ and $r$.

• Biot-Savart Law

5. Magnetic Field on the Axis of a Circular Current Loop

A circular loop of radius $R$ carrying current $I$ produces a magnetic field along its axis. At a point at distance $x$ from the center:

$
B=\frac{\mu_0 I R^2}{2\left(R^2+x^2\right)^{3 / 2}}
$

At the center of the loop ( $x=0$ ):

$
B=\frac{\mu_0 I}{2 R}
$

• Magnetic Field On The Axis Of Circular Current Loop

6. Ampere's Circuital Law

Ampere's circuital law states that the line integral of the magnetic field $\mathbf{B}$ around any closed path is equal to $\mu_0$ times the total current $I$ enclosed by the path:

$
\oint \mathbf{B} \cdot d \mathbf{l}=\mu_0 I_{\mathrm{enclosed}}
$

• Ampere's Circuital Law And Its Applications

7. The Solenoid

A solenoid is a long coil of wire carrying current $I$. It produces a nearly uniform magnetic field inside and a weak field outside.

• Magnetic field inside:$B=\mu_0 n I$

where $n=\frac{N}{L}$ is the number of turns per unit length.

• Solenoid

8. Force Between Two Parallel Currents, The Ampere

Two parallel wires carrying currents $I_1$ and $I_2$, separated by distance $d$, exert a magnetic force on each other:

$
F=\frac{\mu_0 I_1 I_2 L}{2 \pi d}
$

The force is attractive if currents flow in the same direction, and repulsive if they flow in opposite directions.

• Force Between Two Parallel Current Carrying Conductors

9. Torque on a Current Loop, Magnetic Dipole

A current loop in a uniform magnetic field experiences a torque that tends to rotate it:

$
\tau=n I A B \sin \theta
$

$n=$ number of turns, $I=$ current, $A=$ area of the loop, $B=$ magnetic field, $\theta=$ angle between normal to the loop and $B$.
A magnetic dipole is a current loop, and its magnetic moment is:

$
\vec{m}=n I \vec{A}
$

• Torque on a Current Loop, Magnetic Dipole

10. The Moving Coil Galvanometer

A moving coil galvanometer is an instrument used to detect and measure small electric currents.

• It consists of a coil of wire suspended in a uniform magnetic field.
• When current $I$ passes through the coil, it experiences a torque:$\tau=n I A B$

Conversion of Galvanometer into Ammeter and Voltmeter
Ammeter: Connected in series with a low-value shunt resistance to measure large currents.
Voltmeter: Connected in parallel with a high-value series resistance to measure voltage across a component.

• Moving Coil Galvanometer

Related Topics,

NCERT Notes Subject wise link:

• NCERT Notes for class 12 Physics.
• NCERT Notes for class 12 Chemistry.
• NCERT Notes for class 12 Mathematics.
• NCERT Notes for class 12 Biology.

Importance of Moving Charges and Magnetism class 12

For more than a century, magnetism and moving charges, i.e. electricity, have been explored. The phenomenon seen with the alignment of a needle was the forerunner to the connection between the two. It was discovered that its alignment is tangent to an imagined circle with a straight wire in the centre and a plane perpendicular to the wire. When current is passed, however, the needle's orientation shifts. The production of a magnetic field is thought to be caused by the passage of charges. moving charges and magnetism class 12 is an important topic in the board test, as well as a critical component of the joint entrance exam (jee) and the National Eligibility Entrance Test (neet ). Students must be thorough in their studies in order to do well in the category 12 board exam. In order to acquire outstanding scores in class 12, students must prepare properly and practise with ncert solutions for class 12 physics moving charges and magnetism physics.

Exam-wise Weightage of Moving Charges and Magnetism

Approach to Solve Moving Charges and Magnetism Questions

To solve questions in this chapter first understand whether the problem involves a moving charge, a current-carrying wire, a solenoid or a galvanometer. Then choose the right principle, like Lorentz force, Biot-Savart law or Ampere’s law, and write down the relevant formula. Use the right-hand rule to find the direction of force, field or torque. Substitute the given values carefully, check the units, and solve step by step. For special cases, like motion perpendicular or parallel to the field, or multiple currents, apply superposition if needed. This approach makes it easier to solve both numerical and conceptual problems.

Moving Charges and Magnetism Real Life Examples

1. Electric Motors: Convert electrical energy into mechanical motion using current-carrying coils in magnetic fields.
2. Loudspeakers: Use the force on a current-carrying coil to produce sound vibrations.
3. Galvanometers and Ammeters: Detect and measure electric current using the torque on a current loop.

Also Read

• NCERT solutions for class 12 physics chapter moving charges and magnetism
• NCERT exemplar solutions for class 12 physics chapter moving charges and magnetism
• NCERT notes for class 12 physics chapter moving charges and magnetism

NCERT Solutions Subject wise link:

• NCERT solutions for class 12 Physics.
• NCERT solutions for class 12 Chemistry.
• NCERT solutions for class 12 Mathematics.
• NCERT solutions for class 12 Biology.

NCERT Exemplar Solutions Subject wise link:

• NCERT exemplar solutions for class 12 Physics.
• NCERT exemplar solutions for class 12 Chemistry.
• NCERT exemplar solutions for class 12 Mathematics.
• NCERT exemplar solutions for class 12 Biology.