21.A dipole for which p m is in a uniform field E 7 10⁴ N /C  . What is the external work needed to rotate the dipole from alignment with the field to angle 60° to the field? ( cos60 ^o 0.5;sin 60^o 0.9   )
(A)
(B)  
(C)  
(D)
(E)

17.Consider a parallel-plate capacitor of plate area A and plate separation. A potential difference Vo is applied between the plates. The battery is then disconnected, and a dielectric slab of thickness b and dielectric constant k is placed between the plates. Assuming A 100cm2 , d 1.00cm, Vo 90V, b 0.8cm, k 2.00. What is the capacitance with the slab in place? ( ) (A)14.8pF (B)16.8pF (C)17.9pF (D)20.9pF (E)21.9pF 

18.A 200g top spinning at 20 rev/s makes an angle of 30° to the vertical and precesses at a rate of 1 rev per 10.0s. If its CM (Center of Mass) is 4.0cm from its tip along its symmetry axis, what is the moment of inertia of the top? (A) (B) (C) (D) (E)

19.A phone cord is 4.00m long. The cord has a mass of 0.200kg. If a transverse wave pulse travels from the receiver to the phone box in 0.100s, what is the tension in the cord? (A)50.0N (B)60.0N (C)70.0N (D)80.0N (E)90.0N

20.A thermodynamic system undergoes a process in which its internal energy decreases by 500J. If at the same time, 220J of work is done on the system, what is the thermal energy transferred to or from it? (A)280J (B)280J (C)720J (D)720J (E)1220J

22.A 5A current flows in a circular loop of radius 2cm. The axis of the loop is at 30° to a uniform field 0.06T. What is the magnitude of the torque on the loop? (A) (B) (C) (D) (E)

23.A 15pF parallel-plate capacitor is connected to a 50V battery. The area of each plate is 80cm2 . What is the energy density in the fiele? ( ) (A)  (B) (C)  (D) (E)

24.What minimum frequency of light is needed to eject electrons from a metal whose work function is 4.3 ) (A)  (B) (C) (D)   (E)

25.Through how many volts of potential difference must an electron be accelerated to achieve a wavelength of 0.28nm? ( ́ ) (A)12V (B)16V (C)19V (D)31V (E)34V

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