Physics Moving Charges & Magnetism Notes Class 12

Stuck with Moving Charges and Magnetism Class 12 notes that feel too heavy and formula-loaded? Relax - this chapter from CBSE Syllabus Class 12 Physics is actually very logical once the basics click. It’s all about what happens when electric charges start moving and how magnetic fields interact with them.

From deflection of a compass needle to electric motors, MRI machines, and particle accelerators - this chapter connects electricity and magnetism in the most practical way possible. So if you want clean, exam-ready moving charges and magnetism notes without confusion, you’re in the right place.

Moving Charges and Magnetism Class 12 Notes

If you’re looking for moving charges and magnetism class 12 notes that don’t sound like a physics textbook, this is it. No over-explanations, no random theory - just clear ideas, sorted formulas, and exactly what you need for boards and numericals.

Whether you’re revising magnetic force, Biot–Savart law, Ampere’s law, or devices like cyclotron and galvanometer, everything is explained step by step below.

What is Moving Charges and Magnetism?

Moving Charges and Magnetism is the branch of physics that studies the relationship between electricity and magnetism. These two are not separate phenomena - they are deeply connected.

You already know that: 

• Charges at rest produce an electric field
• Charges in motion produce a magnetic field

This chapter focuses on what happens when electric charges start moving and how the magnetic fields produced by them interact with other moving charges and current-carrying conductors.

It explains how a magnetic field can change the direction of motion of a charge, how electric current creates magnetic fields around conductors, and why these effects are the foundation of devices like electric motors, generators, and galvanometers.

In short, this chapter connects electric current and magnetism and shows how both work together in real-life applications.

Magnetic Force on a Moving Charge – Class 12 Physics

When a charged particle moves in a magnetic field, it experiences a force called the Lorentz force. This force acts only on moving charges and is responsible for changing the direction of motion of the particle.

• Lorentz Force Law

Magnetic force on a moving charge is given by:

F = q (v × B)

• Magnitude of Magnetic Force: F = q v B sin θ

Where:
q = charge of the particle
v = velocity of the particle
B = magnetic field strength
θ = angle between velocity and magnetic field

Key Points to Remember:

• Magnetic force is maximum when the charge moves perpendicular to the magnetic field
• Magnetic force is zero when the charge moves parallel to the magnetic field
• Force always acts perpendicular to velocity, so it does no work
• Speed of the particle remains constant; only its direction changes

This explains why charged particles move in circular or helical paths inside a magnetic field.

Motion of a Charged Particle in Magnetic Field

When a charged particle enters a magnetic field, it experiences a force that is always perpendicular to its velocity. Because of this, the magnetic field changes the direction of motion, not the speed.

(a) When velocity is perpendicular to the magnetic field

In this case, the magnetic force acts as the centripetal force, making the particle move in a circular path.

Key Observations:

• Radius increases with mass and speed
• Radius decreases with charge and magnetic field strength
• Electrons have a very small radius due to low mass

Time Period of Circular Motion: T = 2πm / qB

Cyclotron Frequency: f = qB / 2πm

• Cyclotron frequency is independent of speed and radius
• This principle is used in the cyclotron

(b) When velocity has both parallel and perpendicular components

The velocity of the particle can be resolved into:

• v⊥ (perpendicular component) → causes circular motion
• v∥ (parallel component) → causes uniform straight-line motion

Because both motions occur simultaneously, the particle follows a helical path.

Pitch of the Helical Path: Pitch = v ∥ T

Magnetic Force on a Current-Carrying Conductor

So, what actually happens when a current flows through a wire kept in a magnetic field? The wire experiences a force. Simple as that.

This happens because the magnetic field produced by the current interacts with the external magnetic field around it.

The force on the conductor is given by: F = I (L × B)

Its magnitude is: F = I L B sin θ

Here,I is the current in the wire
L is the length of the wire
B is the magnetic field
θ is the angle between current and magnetic field

A few things you should always remember:

• The force is strongest when the wire is perpendicular to the magnetic field
• If the wire is parallel, the force becomes zero
• The direction of force can be found using Fleming’s Left-Hand Rule

This simple idea is the reason why devices like electric motors, loudspeakers, and galvanometers actually work.

Force Between Two Parallel Current-Carrying Conductors

When two long wires carrying current are placed close to each other, they exert magnetic forces on one another.

• If the currents flow in the same direction, the wires attract each other
• If the currents flow in opposite directions, the wires repel each other

This behaviour happens because each current-carrying wire produces a magnetic field that affects the other wire.

This concept is important because it is used to define one ampere, the SI unit of electric current.

Magnetic Field Due to Current – Biot–Savart Law

The Biot - Savart law explains how an electric current produces a magnetic field around it.

Using this law, we can find the magnetic field produced by:

• A long straight current-carrying conductor
• A circular current loop
• An arc of a current-carrying wire

This law helps us calculate both the magnitude and direction of the magnetic field at any point due to a current.

Ampere’s Circuital Law – Class 12 Notes

Ampere’s circuital law is basically about one simple idea: current creates magnetic fields.

What the law says is that if you go around a closed loop near a current-carrying conductor, the magnetic field along that loop depends only on the current passing through it. The exact shape of the loop doesn’t matter much - the current inside it does.

This law is most useful when the magnetic field is neat and symmetrical, because in those cases the calculations become straightforward. That’s why we prefer Ampere’s law over Biot - Savart in some problems.

Where Ampere’s Law Is Used

Ampere’s circuital law is mainly applied in cases where the magnetic field pattern is simple, such as:

• Around a long straight wire
• Inside a long solenoid, where the field is almost uniform
• Inside a toroid, where the field stays confined inside the coil

These situations come up a lot in numericals and help explain how things like electromagnets and transformers work.

Motion of a Charged Particle in Combined Electric and Magnetic Fields

Sometimes, charged particles move in a region where both electric (E) and magnetic (B) fields are present. Here’s what you need to know:

Velocity Selector

A velocity selector allows only particles with a certain velocity to pass straight through without being deflected.

• Condition for undeflected motion: Electric force = Magnetic force
• Only particles with v = E / B move straight. Others get deflected.

This is super useful in mass spectrometers to filter particles by velocity.

Cyclotron – Particle Accelerator

A cyclotron is a machine that speeds up charged particles using magnetic and electric fields.

• Uses a uniform magnetic field to bend the particle in a circle
• Uses an alternating electric field to accelerate the particle each time it crosses the gap

Cyclotron Frequency: f = qB / 2πm (independent of speed and radius)

Limitations:

• Cannot accelerate electrons (relativistic effects)
• Cannot accelerate neutral particles
• At very high speeds, synchronization fails

Magnetic Dipole Moment and Torque

A current-carrying loop behaves like a magnetic dipole.

• Magnetic dipole moment: m = N I A
• Torque on the loop in a magnetic field: τ = m B sinθ

Applications:

• Electric motors
• Moving coil galvanometers

Moving Coil Galvanometer

A moving coil galvanometer is used to detect and measure very small currents.

How it works:

• A current-carrying coil in a magnetic field experiences a torque
• This torque rotates the coil, which is opposed by a restoring spring or suspension wire
• At equilibrium: Magnetic torque = Restoring torque

Current sensitivity: θ / I = N A B / k

Why it works well:

• Strong radial magnetic field makes torque proportional to current
• Soft iron core increases magnetic field and sensitivity

Applications of Moving Charges and Magnetism

The ideas of moving charges and magnetic fields are not just theory - they are used in many real-life devices. Here’s a quick overview:

1. Electric Motors: Convert electrical energy into mechanical energy using the torque on a current-carrying coil.

2. Loudspeakers: The coil inside a speaker moves in a magnetic field when current passes through it, producing sound.

3. Galvanometers, Ammeters, Voltmeters: Instruments that measure small currents or voltages using the magnetic force on a coil.

4. Cyclotron & Mass Spectrometers: Accelerate charged particles for research and medical purposes using magnetic and electric fields.

5. Magnetic Storage Devices: Hard disks and tapes store data by aligning magnetic domains.

6. Magnetic Levitation (Maglev) Trains: Trains float above the tracks using magnetic forces, reducing friction and allowing high-speed travel.

These applications show that understanding moving charges and magnetism is not just important for exams - it explains a lot of modern technology around us.

FAQs

Q1. What is a moving charge?

Ans. A moving charge is simply an electric charge that is in motion, like electrons flowing through a wire.

Q2. How does a moving charge create a magnetic field?

Ans. When a charge moves, it produces a magnetic field around it. The faster it moves, the stronger the field.

Q3. What happens to a moving charge in a magnetic field?

Ans. A moving charge in a magnetic field experiences a force called the Lorentz force, which acts perpendicular to both its velocity and the magnetic field.

Q4. How can we find the direction of this force?

Ans. Use the right-hand rule: point your thumb along the direction of the charge’s motion, your fingers along the magnetic field, and your palm shows the direction of the force.

Q5. Can a magnetic field move a stationary charge?

Ans. No. A magnetic field only affects charges that are already moving. Stationary charges remain unaffected.