Which of the following would a magnetic field not exert a force on?
a stationary current carrying wire aligned at a 90 degree angle to the magnetic field
a rotating current carrying wire in the shape of a rectangle
a charge moving at a 180 degree angle to the magnetic field
an electron beam angled at 45 degrees to the magnetic field

A charge moving at a 180 degree angle to the magnetic field

Explanation:

As we know that magnetic force on a moving charge is given by the formula

here we know that

force on the charge will be zero if the above formula has

i) velocity is zero or charge at rest

ii) magnetic field is zero

iii) charge is moving parallel or opposite to the direction of the magnetic field

So here correct answer would be

A charge moving at a 180 degree angle to the magnetic field