If a wire is placed parallel to the lines of force in a magnetic field and a current flows in the wire, then
- A.the wire will experience a force in the direction of the magnetic field
- B.the wire will not experience any force at all
- C.the wire will experience a force in the direction opposite to the field
- D.it will experience a force in the direction perpendicular to the lines of force
Correct answer
Explanation
The correct answer is B, the wire will not experience any force at all. The force on a current carrying conductor kept in a magnetic field is given by F equal to B I L sin theta, where theta is the angle between the direction of the current and the direction of the field. When the wire lies along the lines of force that angle is zero, sin theta becomes zero, and the force disappears however strong the field or the current may be. The same formula shows that the force is greatest when the wire is at right angles to the field, because sin theta is then one. Option A is wrong because the force, when there is one, never acts along the field; Fleming's left hand rule places it at right angles to both the field and the current. Option C is wrong for the same reason, as no rule gives a force pointing against the field. Option D is wrong here only because the angle is zero and no force arises at all. Exam tip: parallel means no force, perpendicular means maximum force.
Practice Questions
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