In earlier chapters, we studied electrostatics, where charges were at rest. In this chapter, Moving Charges and Magnetism, we study what happens when electric charges are in motion. When charges move, they produce magnetic effects. This chapter explains the relationship between electric current and magnetism, which forms the basis of many everyday devices like electric motors, generators, MRI machines, loudspeakers, and more.
This chapter is very important for Class 12 Board Exams, JEE Main, and other competitive exams, as it contains conceptual theory + numerical problems.
Moving Charges and Magnetism Class 12 Physics Notes| Important Questions & Numericals | Important Questions & Numericals
1. Magnetic Field and Magnetic Force
Magnetic Field
A magnetic field is the region around a magnet or a current-carrying conductor where magnetic effects can be felt.
- Represented by magnetic field lines
- Direction is taken from North pole to South pole (outside the magnet)
Properties of Magnetic Field Lines
- They form closed loops
- Never intersect each other
- Closer lines indicate stronger magnetic field
2. Lorentz Force
When a charged particle moves in a region having electric field (E) and magnetic field (B), it experiences a force called Lorentz Force.
Formula:
\vec{F} = q(\vec{E} + \vec{v} \times \vec{B})
Where:
- = charge
- = velocity
- = magnetic field
Important Points:
- Magnetic force acts perpendicular to velocity
- Magnetic force cannot change speed, only direction
- No force if particle is at rest
3. Motion of a Charged Particle in a Magnetic Field
(a) Velocity Perpendicular to Magnetic Field
- Particle moves in a circular path
- Magnetic force acts as centripetal force
r = \frac{mv}{qB}
(b) Velocity Parallel to Magnetic Field
- No magnetic force
- Particle moves in a straight line
(c) Velocity at an Angle to Magnetic Field
- Particle moves in a helical path
4. Cyclotron
A cyclotron is a device used to accelerate charged particles to very high energies.
Principle
It works on the principle that:
- Charged particles move in a circular path in a magnetic field
- Frequency is independent of speed
Cyclotron Frequency
f = \frac{qB}{2\pi m}
Uses:
- Nuclear physics experiments
- Cancer treatment
- Research laboratories
Limitations:
- Not suitable for electrons
- Relativistic effects limit speed
5. Magnetic Force on a Current-Carrying Conductor
When a conductor carrying current is placed in a magnetic field, it experiences a force.
Formula:
F = BIL \sin\theta
Where:
- = current
- = length of conductor
Fleming’s Left-Hand Rule
Used to find the direction of force:
- First finger → Magnetic field
- Thumb → Force
- Middle finger → Current
6. Biot–Savart Law
This law gives the magnetic field produced by a small current element.
Statement:
Magnetic field at a point due to a current element is directly proportional to:
- Current
- Length of element
- Sine of angle
And inversely proportional to square of distance.
Formula:
dB = \frac{\mu_0}{4\pi} \frac{I\,dl \sin\theta}{r^2}
7. Magnetic Field Due to Current-Carrying Conductors
(a) Long Straight Conductor
B = \frac{\mu_0 I}{2\pi r}
(b) Circular Current Loop
At centre:
B = \frac{\mu_0 I}{2R}
(c) Solenoid
Inside a long solenoid:
B = \mu_0 n I
Where:
- = number of turns per unit length
Solenoid acts like a bar magnet.
8. Ampere’s Circuital Law
Statement:
The line integral of magnetic field around a closed loop is equal to times the total current enclosed.
\oint \vec{B} \cdot d\vec{l} = \mu_0 I
Applications:
- Infinite straight wire
- Solenoid
- Toroid
9. Force Between Two Parallel Current-Carrying Conductors
- Two parallel currents in same direction attract
- Two parallel currents in opposite direction repel
Force per unit length:
\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi d}
This concept is used to define ampere.
10. Torque on a Current Loop in Magnetic Field
When a current loop is placed in a magnetic field, it experiences a torque.
\tau = N B I A \sin\theta
Where:
- = number of turns
- = area of loop
Moving Coil Galvanometer
Works on this principle.
11. Magnetic Dipole
A magnetic dipole is equivalent to a small current loop.
Magnetic Dipole Moment:
m = I A
Torque on Magnetic Dipole:
\tau = m B \sin\theta
12. Earth’s Magnetism
Earth behaves like a huge bar magnet.
Elements of Earth’s Magnetic Field:
- Magnetic Declination
- Magnetic Inclination (Dip)
- Horizontal Component
13. Important Applications
- Electric motors
- Loudspeakers
- Electric generators
- Magnetic levitation
- MRI scanners
Exam-Oriented Important Points
- Magnetic force does no work
- Right-hand thumb rule for direction of magnetic field
- Fleming’s left-hand rule for force
- Moving charges produce magnetic field
Conclusion-Moving Charges and Magnetism Class 12 Physics Notes
The chapter Moving Charges and Magnetism Class 12 Physics builds a strong foundation for understanding how electricity and magnetism are interconnected. Mastery of this chapter is essential not only for board exams but also for engineering and medical entrance exams. Regular practice of numericals, diagrams, and formulas will help students score high marks.
🔁 Repeated Questions – Moving Charges and Magnetism Class 12 Physics Notes
📘 Very Short Answer Questions (1 Mark)
- What is Lorentz force?
- Does a magnetic field do work on a moving charge? Give reason.
- Write the SI unit of magnetic field.
- What is the nature of path followed by a charged particle moving perpendicular to a uniform magnetic field?
- Name the rule used to find the direction of magnetic force on a current-carrying conductor.
- What is magnetic dipole moment?
- What happens to the magnetic force if the velocity of charge becomes parallel to the magnetic field?
- State one limitation of a cyclotron.
📗 Short Answer Questions (2–3 Marks)
- Define magnetic field and draw magnetic field lines around a straight current-carrying conductor.
- Explain Fleming’s left-hand rule with a neat diagram.
- What is cyclotron frequency? Write its expression.
- State Biot–Savart law. Write its mathematical form.
- What is Ampere’s circuital law? Mention one application.
- Why is a moving charge deflected in a magnetic field but a stationary charge is not?
- Write any two properties of magnetic field lines.
- Why are two parallel current-carrying conductors attracted to each other when currents are in the same direction?
📙 Long Answer Questions (4–5 Marks)
- Derive the expression for the radius of circular path of a charged particle moving in a uniform magnetic field.
- Explain the working principle, construction, and limitations of a cyclotron.
- Derive the expression for magnetic field due to a long straight current-carrying conductor using Biot–Savart law.
- Explain the force between two parallel current-carrying conductors and hence define one ampere.
- Derive the expression for torque acting on a current loop placed in a uniform magnetic field.
- Explain the working principle of a moving coil galvanometer.
- Describe the magnetic field due to a solenoid and state its applications.
🔢 Repeated Numerical Problems
- A charged particle enters a magnetic field at right angles. Find the radius of its circular path.
- Calculate the magnetic field at a distance r from a long straight wire carrying current I.
- Find the force acting on a conductor of length L carrying current I placed in a magnetic field B.
- Two parallel wires carry currents I₁ and I₂. Find the force per unit length between them.
- A proton moves perpendicular to a magnetic field. Calculate its time period of revolution.
📌 Assertion–Reason Questions (Very Common)
- Assertion: Magnetic force does not change the speed of a charged particle.
Reason: Magnetic force always acts perpendicular to velocity. - Assertion: Cyclotron cannot be used to accelerate electrons.
Reason: Relativistic effects become significant for electrons.
📝 Case-Based / Conceptual Questions
- Explain why a charged particle follows a helical path in a magnetic field.
- Compare electric force and magnetic force acting on a charged particle.
- Why does a solenoid behave like a bar magnet?
⭐ Exam Tips (Important)
- Numericals are guaranteed from this chapter
- Diagrams carry extra marks
- Formula-based questions are frequently repeated
- Fleming’s left-hand rule & Lorentz force are must-prepare topics
We are Completed Moving Charges and Magnetism Class 12 Physics Notes.