When a charged partical moves at right angles to a mangetic field then which of the following quantities changes-

Options

A. energy
B. momentum
C. speed
D. all of above

Answer

To solve the question, "When a charged particle moves at right angles to a magnetic field, which of the following quantities changes?", we can follow these steps:

### Step 1: Understand the Motion of the Charged Particle
When a charged particle moves in a magnetic field, it experiences a magnetic force that is perpendicular to both its velocity and the magnetic field direction. This force causes the particle to undergo circular motion.

**Hint:** Remember that the force on a charged particle in a magnetic field is given by the Lorentz force equation: \( F = q(\mathbf{v} \times \mathbf{B}) \).

### Step 2: Analyze the Velocity
The velocity of the charged particle is a vector quantity, which means it has both magnitude and direction. While the particle moves in a circular path due to the magnetic force, the direction of the velocity changes continuously. However, the magnitude of the velocity (speed) remains constant because the magnetic field does not do work on the charged particle.

**Hint:** Consider the definition of speed versus velocity. Speed is a scalar quantity, while velocity includes direction.

### Step 3: Consider the Momentum
Momentum (\( p \)) is defined as the product of mass (\( m \)) and velocity (\( v \)), expressed as \( p = mv \). Since the direction of the velocity changes as the particle moves in a circular path, the momentum, which is also a vector quantity, will change as well. The magnitude of the momentum remains constant, but its direction changes.

**Hint:** Remember that momentum is a vector quantity, and its direction is influenced by the direction of velocity.

### Step 4: Evaluate Other Quantities
- **Total Energy:** The total mechanical energy of the charged particle remains constant because the magnetic field does not do work on the particle. The kinetic energy, which depends on the speed, remains constant as the speed does not change.
- **Speed:** Since the speed (magnitude of velocity) remains constant, it does not change.

**Hint:** Think about the conservation of energy in a magnetic field and how it affects the motion of charged particles.

### Conclusion
From the analysis, we conclude that the only quantity that changes when a charged particle moves at right angles to a magnetic field is the momentum. Therefore, the answer is:

**Answer:** Momentum changes.