XII PHYSICS NEW SYLLABUS UNIT 3 MCQs

1. The magnetic field at the center O of the following current loop is ---.

Soln. For a semi-circular current loop, B = (µ_o I)/(4πr^2 ) (πr) = (µ_o I)/4r. Ans. (a)

2. An electron moves straight inside a charged parallel plate capacitor of uniform charge density σ. The time taken by the electron to cross the parallel plate capacitor when the plates of the capacitor are kept under constant magnetic field of induction B is ---.
Note (for the Textbook authors & for errata): This qn. should be reworded as follows:
An electron moves straight inside a charged parallel plate capacitor of uniform charge density σ. The time taken by the electron to cross the parallel plate capacitor when the plates of the capacitor are  kept under constant magnetic field of induction B for straight line motion of the electron is ---.

Soln. E = σ / ε_o and for st. line motion, v = E/B = σ / ε_oB 

time t = distance / vel = l / (σ / ε_oB) = ε_olB/σ. Ans. (d)

3. The force experienced by a particle having mass m and charge q accelerated through a potential difference V when it is kept under perpendicular magnetic field B is ---.
Note (for the Textbook authors & for errata): This qn. should be reworded as follows:
A particle having mass m and charge q is accelerated through a potential difference V. The force experienced by it when it is kept under perpendicular magnetic field B is ---.

Soln.