State the definition and SI unit of current

SI unit of current is ampere. 1 ampere is the constant current that will produce an attractive force of 2 × 10−7 Newton per meter of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one meter apart in a vacuum.

Explanation:

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current is a flow of electricity which results from the ordered directional movement of electrically charged particles.

SI unit of current is ampere, or amp

In longitudinal and transverse waves, all the particles in the entire bulk of the medium move in a parallel and a perpendicular direction (respectively) relative to the direction of energy transport. In a surface wave, it is only the particles at the surface of the medium that undergo the circular motion.