### Correct Option: B. The magnetic force increases. ### Detailed Explanation: To understand why the magnetic force acting on a proton increases when the magnetic field strength is increased, we need to refer to the formula for the magnetic force acting on a charged particle moving in a magnetic field. The magnetic force \( F \) can be calculated using the equation: \[ F = qvB \sin(\theta) \] Where: - \( F \) is the magnetic force, - \( q \) is the charge of the particle, - \( v \) is the velocity of the particle, - \( B \) is the magnetic field strength, - \( \theta \) is the angle between the velocity vector and the magnetic field vector. In this scenario: - The proton has a charge \( q \) (approximately \( 1.6 \times 10^{-19} \) coulombs). - The proton is moving with a constant velocity \( v \). - The magnetic field \( B \) is uniform and is perpendicular to the velocity of the proton, which means \( \theta = 90^\circ \). Therefore, \( \sin(90^\circ) = 1 \). Given that the angle is constant at \( 90^\circ \), the formula simplifies to: \[ F = qvB \] Now, let's analyze the situation: 1. **Constant Charge and Velocity**: The charge \( q \) of the proton and its velocity \( v \) remain constant. 2. **Increasing Magnetic Field Strength**: When the magnetic field strength \( B \) is increased, the only variable in the equation that changes is \( B \). Since \( F \) is directly proportional to \( B \), if \( B \) increases, \( F \) must also increase. This means that the magnetic force acting on the proton increases as the magnetic field strength increases. ### Why Other Options Are Incorrect: - **Option A: The magnetic force decreases.** - This option is incorrect because increasing the magnetic field strength \( B \) directly increases the magnetic force \( F \). There is no scenario in this context where an increase in \( B \) would lead to a decrease in \( F \). - **Option C: The magnetic force remains the same.** - This option is also incorrect. The magnetic force cannot remain the same if the magnetic field strength is increased while the other factors (charge and velocity) are held constant. The relationship \( F = qvB \) indicates that any change in \( B \) will affect \( F \). - **Option D: The magnetic force becomes zero.** - This option is incorrect because the magnetic force will not become zero as long as the proton is moving in a magnetic field. The only way for the magnetic force to be zero is if either the velocity \( v \) is zero or the magnetic field \( B \) is zero, neither of which is the case here. ### Summary of Key Points: - The magnetic force on a charged particle is given by \( F = qvB \sin(\theta) \). - In this case, \( \theta = 90^\circ \), simplifying the equation to \( F = qvB \). - Increasing the magnetic field strength \( B \) while keeping charge \( q \) and velocity \( v \) constant results in an increase in the magnetic force \( F \). - Therefore, the correct answer is that the magnetic force increases (Option B). ### Revision Summary: - The magnetic force formula is \( F = qvB \sin(\theta) \). - For a proton moving perpendicular to a magnetic field, \( \theta = 90^\circ \) simplifies the force to \( F = qvB \). - Increasing the magnetic field strength \( B \) increases the magnetic force \( F \). - The correct answer is that the magnetic force increases (Option B).