Which of the following statements about a straight current-carrying wire placed in a uniform magnetic field is correct? The wire experiences ---------------

Analyze the question: We need the true statement about the force (the "motor force") on a current-carrying wire in a uniform magnetic field.

Concept: The force on a straight wire is

F = B I L sin θ

where θ is the angle between the wire (current direction) and the magnetic field. The direction of the force is given by Fleming's Left-Hand Rule, and it is perpendicular to both the current and the field.

Checking each option:

• A. Maximum force if the current reverses direction. Wrong. Reversing the current reverses the direction of the force. It does not make the force maximum.
• B. No motor force if it is parallel to the field. Correct. When the wire is parallel to the field, θ = 0°, so sin 0° = 0 and F = 0. (The "of" in the option is a typo for "if".)
• C. No motor force if it is perpendicular to the field. Wrong. When θ = 90°, sin 90° = 1, so the force is at its maximum, not zero.
• D. Motor force with constant direction if either current or field is reversed. Wrong. Reversing either one reverses the force direction. (Reversing both together would leave the direction unchanged.)

Answer: B.

Tip: Remember "parallel = zero, perpendicular = maximum" for the force on a wire in a magnetic field. The wire must cut across the field lines to feel a force. If it runs along them, nothing happens.