Q10Electromagnetics Waves
Question
Q.10. Write the unit of Poynting vector.
Answer
The Poynting vector S = E×H represents power flow per unit area and has SI unit watt per square metre (W/m²).
The Poynting vector, S = E × H, gives the instantaneous power flow density (magnitude and direction) of an electromagnetic field, i.e., power crossing a unit area perpendicular to the direction of propagation. Since E is measured in volts/metre (V/m) and H in amperes/metre (A/m), their cross product has units of (V/m)(A/m) = VA/m² = W/m² (since one volt-ampere equals one watt for real power). The SI unit of the Poynting vector is therefore the watt per square metre (W/m²), and integrating S over any closed surface gives the total instantaneous power flowing out through that surface — the basis of Poynting's theorem for electromagnetic energy conservation.