## Saturday, April 09, 2011

### All India Pre-Medical/Pre-Dental Entrance Examination (AIPMT) 2011 Questions on Magnetic Force

“I have no special talents. I am only passionately curious.”

– Albert Einstein

Two questions involving magnetic force on current carrying conductors and moving charges were included in the AIPMT 2011 (Preliminary) question paper. Here are those questions with solution: (1) A current carrying closed loop in the form of a right angled isosceles triangle ABC is placed in a uniform magnetic field acting along AB. If the magnetic force on the arm BC is F, the force on the arm AC is

(1) √2 F

(2) √2 F

(3) F

(4) F

Since the current loop is placed in a uniform magnetic field, the net force acting on the loop is zero.

[Note that a current loop in a uniform magnetic field will experience a torque but no net force. If the magnetic field is non-uniform there will be a torque as well as a net force].

Since the arm AB of the loop is parallel to the magnetic field, the magnetic force on AB is zero. Since the magnetic force on the arm BC is F (as given in the question), the magnetic force on the arm Ac must be F (so as to make the net force on the loop zero).

(2) A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of the fields, then the electron

(1) will turn towards left of the direction of motion

(2) will turn towards right of the direction of motion

(3) speed will decrease

(4) speed will increase

The magnetic force will be zero since the electron is projected parallel to the field. The electric field will exert a force opposite to the direction of motion of the electron and hence its speed will decrease [Option (3)].