The work done in moving a unit positive charge between two points in an electric field is called: MCQ with Answer and Explanation

Explanation:
Beat frequency = |260 - 256| = 4 Hz ⇒ 4 beats/second. Interference of close frequencies causes amplitude modulation. Memory aid: 'Beats = frequency difference; used in tuning instruments'. Wave interference calculation frequently tested in competitive exams.

Explanation:
Mirror formula: 1/f = 1/u + 1/v. Sign convention: f negative for concave mirror? Wait, standard Cartesian: f = -20 cm (concave), u = -30 cm (object real). Thus 1/(-20) = 1/(-30) + 1/v ⇒ -1/20 = -1/30 + 1/v ⇒ 1/v = -1/20 + 1/30 = (-3+2)/60 = -1/60 ⇒ v = -60 cm. Magnitude 60 cm, negative sign indicates real image in front of mirror. Many exams use magnitude-only options; here 60 cm is correct. Memory tip: 'Concave mirror: object beyond C (2f) ⇒ image between F and C, real, inverted'. This numerical tests mirror formula application with sign conventions, crucial for optics in competitive exams.

Explanation:
Maxwell's equations predicted EM waves, light is EM.