An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. This is because,

Options

A. the two magnetic forces are equal and opposite, so they produce no net effect
B. the magnetic forces do not work on each particle
C. the magnetic forces do equal and opposite (butnon-zero) work on each particle
D. the magnetic forces are necessarily negligible

Explanation

To solve the question, we need to understand the interaction between the electron and proton and the role of magnetic forces in their motion.

### Step-by-Step Solution:

1. **Understanding the Forces**:
- An electron and a proton are charged particles. When they move, they create electric and magnetic fields. The electric force between them is attractive due to their opposite charges.

2. **Magnetic Forces**:
- As the electron and proton move, they also exert magnetic forces on each other. However, these magnetic forces act perpendicular to the direction of their motion.

3. **Work Done by Forces**:
- Work is defined as the force applied on an object times the displacement of that object in the direction of the force. Mathematically, \( W = F \cdot d \cdot \cos(\theta) \), where \( \theta \) is the angle between the force and the displacement.
- Since the magnetic force is perpendicular to the motion of the particles, the angle \( \theta \) is 90 degrees. Therefore, \( \cos(90^\circ) = 0 \). This means that the magnetic force does no work on the particles.

4. **Conclusion**:
- Because the magnetic forces do not do any work on the electron and proton, they do not contribute to the change in kinetic energy of the system. Thus, when calculating the change in kinetic energy, we can ignore the magnetic forces.

5. **Final Answer**:
- The correct reason for ignoring the magnetic force in the calculation of kinetic energy change is that **magnetic forces do not work on each particle**.