Components of loudspeaker :
A loudspeaker consists of permanent magnets, basket, voice cails, flexible supension & rigid speaker cone as shown in figure.

Loudspeaker principle
A light voice coil is mounted so that it can move freely inside the magnetic field of a strong permanent magnet. the paper cone is attached to the voice coil and attached with a flexble mounting to the outer ring of the support .Because there is a definite equilibrium position for the speaker cone and there is elasticity of the mounting structure, there is inevitably a free cone resonant frequency like of a mass on a spring. The frequency can be determined by adjusting the mass and stiffness of the cone and voice coil

Working :
The ratio drives a rapidly changing current through the coil. The current follows that vibrations of speech and the electromagnetic force follows the current changes, pushing the paper cone. Finally the air in front of the loudspeaker is set into vibration following the cone's motion, and sound waves are transmitted to the listener
With reference to the figure of Q.& when the current in the coil is given by `I=I_(c) cos (2000 pi t)` where `t` is in seconds. the coil will experience magnetic force in the positive y-direction in the time intervals of : (Take the initial direction of cirrent shown in figure to be positive)

Options

A. 0 to `5 xx 10^(-4) sec`
B. `2.5 xx 10^(-2)` to `5 xx 10^(-4) sec`
C. 0 to `2.5 xx 10^(-4) sec`
D. `2.5 xx 10^(-4)` to `7.5 xx10^(-4) sec`

Answer

D

Explanation

Force will be in `+y` direction when current is in `-ve` direction
i.e., when `cos (2000 pi t) lt 0`
i.e. when `2.5 xx 10^(-4) lt t lt 7.5 xx 10^(-1)`.