Case Study:

In early Cathode Ray Tube (CRT) televisions, images were produced without any moving mechanical parts. Instead, a narrow beam of electrons emitted from an electron gun was directed toward a phosphorescent screen. However, engineers observed that by applying varying electric and magnetic fields, the electron beam could be precisely deflected to scan the entire screen and form images line by line.

Inside the CRT, electrons were accelerated through a potential difference of several kilovolts, acquiring high velocities. When these moving charges passed through a region of magnetic field B, they experienced a force perpendicular to both their velocity and the field. This force caused the beam to bend, allowing control over its position on the screen.

In one experiment, an electron beam moving with velocity 2 × 10⁷ m/s enters a uniform magnetic field of 1 × 10^-3 T perpendicular to its motion. The beam is observed to follow a circular path of radius r, enabling precise targeting of pixels.

This phenomenon raises an important question: how do electric and magnetic forces together control the motion of charged particles so accurately that a complete image can be formed on the screen?

2. CASE-BASED QUESTIONS

MCQ

Q1. The force responsible for bending the electron beam inside the CRT is:

A. Gravitational force
B. Electrostatic force only
C. Magnetic Lorentz force
D. Nuclear force

Q2. The direction of force on a moving electron in a magnetic field is:

A. Along velocity
B. Opposite to velocity
C. Perpendicular to both velocity and magnetic field
D. Parallel to magnetic field

Assertion - Reason

Q3. Assertion (A): The electron beam in a CRT follows a curved path in a magnetic field.
Reason (R): A magnetic field exerts a force parallel to the velocity of the charged particle.

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. Explain why increasing the magnetic field strength increases the deflection of the electron beam in the CRT screen.

Application-Based

Q5. If the velocity of electrons is doubled while keeping magnetic field constant, how will the radius of the path change?

Numerical

Q6. Using the relation for circular motion of charged particles:
r = mv / qB,
calculate the radius of the path for an electron:
( m = 9.1 × 10^-31 kg,
  q = 1.6 × 10^-19 C)
 given,   v = 2 × 10⁷ m/s,
              B = 1 × 10^-3 T.

Ready to master this topic for your exams? Check out these Top 20 Important Questions on Magnetic Force to test your knowledge further.

3. ANSWER KEY WITH EXPLANATION

A1. C. Magnetic Lorentz force
Explanation: Moving charges in a magnetic field experience force causing deflection.

A2. C. Perpendicular to both velocity and magnetic field
Explanation: Magnetic force acts perpendicular to motion and field.

To visualize the direction of this force, you can use the Right-Hand Rule to determine exactly how the beam will deflect.

A3. C. A is true, R is false
Explanation: Magnetic force is perpendicular, not parallel.

A4. Since (F = qvB), increasing B increases force, causing greater curvature and hence more deflection.

A5. From r=mv/qB, radius is directly proportional to velocity; doubling velocity doubles radius.

A6. r ≈ 0.114 m

HOTS EXTENSION QUESTIONS

1. How can electric and magnetic fields be adjusted so that the electron beam remains undeflected?

2. Why must the magnetic field vary continuously in a CRT to produce a complete image?

While CRT TVs are a classic example, you can explore other applications of Magnetism in Daily Life, ranging from speakers to high-tech medical imaging.

Related Case Studies

How does an MRI Machine Control Human Body Imaging?
Why does a Compass Needle Deflect near Charging Cables?

Resource Link

Looking for more exam-style practice? Download our Class 12 Physics Practice Papers to perfect your problem-solving skills.