20. Consider a purely resistive heater connected to an AC power source (rms voltage 120 V, frequency 60 Hz) with an average heating power of 1 kW. If an inductor is connected in series with the heater, causing the average heating power to drop to 1/4 of its original value, what is the phase angle between the voltage and the current?
(A) 15°
(B) 30°
(C) 45°
(D) 60°
(E) 90°

21. Consider an ideal parallel plate capacitor that is charged. If the capacitor plates are slightly pulled apart, which of the following statements is correct during the process? (A) The electric field strength inside the capacitor decreases. (B) The energy density inside the capacitor increases. (C) The potential difference between the capacitor plates remains unchanged. (D) The energy stored in the capacitor increases. (E) The capacitance of the capacitor increases.

22. For a hydrogen atom, the number of states in a shell with principal quantum number n is (A) n (B) 2n (C) 2n+1 (D) n² (E) 2n²

23. If the average intensity (energy per unit time per unit area) of an electromagnetic wave at some point in space is, what is the peak electric field at that point? ε₀ and μ₀ are the vacuum permittivity and permeability, respectively. (A) (B) (C) (D)(E)

24. Consider a soap film of thickness t and index of refraction n (＞1) suspending in air (index of refraction ~1). Light of wavelength λ is incident on the soap normally. If the intensity of the reflection is 0 (destructive completely), what is the maximum of 2t? (A) nt (B) 2t (C) 2nt (D) 4t (E) 4nt

25. In an experiment of photoelectric effect, when the light intensity increases, (A) the photoelectric current decreases (B) the stopping potential decreases (C) the photoelectric current does not change (D) the stopping potential does not change (E) None of the above

1. The number of significant figures of (0.00150 m×3.2 cm) is: (A) 2 (B) 3 (C) 4 (D) 5 (E) 6.

2. A 32-N force, parallel to the incline, is required to push a certain crate at constant velocity up a frictionless incline that is 30° above the horizontal. The mass of the crate is: (A) 3.3 kg (B) 3.8 kg (C) 5.7 kg (D) 6.5 kg (E) 16 kg.

3. When a certain rubber band is stretched a distance x, it exerts a restoring force F = ax + bx², where a and b are constants. The work done in stretching this rubber band from x = 0 to x = L is: (A) aL2/2 + bL³/3 (B) aL+bL2/2 (C) a + 2bL (D) aL + 2bL² (E) aL² + bLx³.

4. The potential energy of a 2.0-kg particle moving along the x axis is given by U(x)=5.0J/m²)x4 J.When the particle is at x = 1.0 m it is traveling in the positive x direction with a speed of 5.0 m/s. It next stops momentarily to turn around at about x = (A) 0 (B) 1.56 m (C) -1.56 m (D) 3.02 m (E) -3.02 m.

5. A sled loaded with sand, slides along a horizontal frictionless track. As the sled moves, sand trickles out at a constant mass-reduced rate through a hole in the back. The speed of the sled will: (A) decrease at a constant rate (B) increase at a constant rate (C) remain the same (D) decrease at a variable rate (E) increase at a variable rate.

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