Which of the following can be measured with the help of piezo electric crystal?

The correct answer is: All of the above.

A piezoelectric crystal is a material that produces an electric charge when it is subjected to mechanical stress, and conversely, produces a mechanical deformation when an electric voltage is applied to it. This property is called piezoelectricity.

Piezoelectric crystals can be used to measure force, velocity, sound, and pressure.

• Force: When a force is applied to a piezoelectric crystal, it produces an electric charge. The amount of charge produced is proportional to the amount of force applied. This can be used to measure the force of a collision, the weight of an object, or the force of a muscle contraction.
• Velocity: When a piezoelectric crystal is subjected to a velocity, it produces an electric charge. The amount of charge produced is proportional to the square of the velocity. This can be used to measure the speed of a moving object, such as a car or a bullet.
• Sound: When sound waves hit a piezoelectric crystal, they cause it to vibrate. The vibrations of the crystal produce an electric signal. The frequency of the electric signal is proportional to the frequency of the sound waves. This can be used to measure the pitch of a sound, or to detect the presence of sound waves.
• Pressure: When pressure is applied to a piezoelectric crystal, it produces an electric charge. The amount of charge produced is proportional to the amount of pressure applied. This can be used to measure the pressure of a fluid, such as air or water, or to detect the presence of pressure waves.

Piezoelectric crystals are used in a wide variety of applications, including sensors, actuators, and oscillators.