Case Study:

Inside a modern hospital, a patient undergoes a scan in a Magnetic Resonance Imaging (MRI) machine. Unlike X-rays, the MRI produces detailed internal images without using harmful radiation. The patient lies inside a large cylindrical magnet where a strong, uniform magnetic field is applied. Under this field, hydrogen nuclei (protons) in the body begin to move and align in specific ways.

During the scan, these charged particles experience forces due to their motion in the magnetic field. It is observed that the particles follow circular or helical paths depending on their velocity components relative to the magnetic field. The radius of their motion changes when the strength of the magnetic field is adjusted.

To better visualize how these forces dictate direction, you can refer to the Right-hand rule explained with easy diagrams for a clear concept.

Technicians note that increasing the magnetic field strength results in tighter circular motion of the charged particles, improving image resolution. The system precisely controls these particle motions to generate signals that are converted into images of internal organs.

This raises an important question: how does the magnetic field control the motion of charged particles to produce such accurate imaging without any mechanical movement?

CASE-BASED QUESTIONS

MCQ

Q1. The force experienced by a moving charged particle in a magnetic field is maximum when:
A. Velocity is parallel to magnetic field
B. Velocity is perpendicular to magnetic field
C. Velocity is zero
D. Charge is neutral

Q2. In MRI, when magnetic field strength increases, the radius of particle motion:
A. Increases
B. Decreases
C. Remains constant
D. Becomes zero

Assertion - Reason

Q3. Assertion (A): Charged particles move in circular paths inside an MRI machine.
Reason (R): Magnetic force always acts perpendicular to velocity.

A. Both A and R are true, and R is correct explanation
B. Both A and R are true, but R is not correct explanation
C. A is true, R is false
D. A is false, R is true

Application-Based

Q4. Why does increasing magnetic field strength improve MRI image resolution?

Application-Based

Q5. What type of motion will a charged particle exhibit if it has both parallel and perpendicular velocity components in MRI?

Data/Logic-Based

Q6. A proton enters a magnetic field with velocity perpendicular to it. If the velocity is doubled while keeping magnetic field constant, the radius of path becomes:
A. Half
B. Double
C. Same
D. Four times

ANSWER KEY WITH EXPLANATION

A1. B- Magnetic force F=qvBsin⁡θ is maximum when θ=90^∘ , i.e., velocity is perpendicular to magnetic field.

A2.  B-  Radius r=mv/qB​. Increasing B decreases radius, leading to tighter motion.

A3. A- Magnetic force acts perpendicular to velocity, causing circular motion. Hence both are true and correctly related.

A4. Stronger magnetic field reduces radius of particle motion, making paths more controlled and precise, improving signal clarity and image resolution.

MRI is just one example of this technology; explore more about magnetism in daily life: from speakers to MRI machines to see how these principles power everyday gadgets

A5. The particle follows a helical path, combining circular motion (perpendicular component) and linear motion (parallel component).

A6. B-  Since r∝v, doubling velocity doubles radius when magnetic field remains constant.

To master this topic for your boards, test your knowledge with the top 20 important questions from magnetic force on charges

HOTS EXTENSION QUESTIONS

1. If MRI used weaker magnetic fields, how would it affect image quality and why?
2. Can MRI work if charged particles move parallel to the magnetic field? Justify your answer.

Looking for more practice? Download our Class 12 Physics unsolved practice papers to refine your problem-solving skills before the exam

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