19. A conductor of radius r, length ℓ and resistivity ρ has resistance R. It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor. The resistance of the new conductor is

(A)\( \frac{1}{16} \)R

(B)\( \frac{1}{4} \)R

(C)R

(D)4R

(E)16R

13. A conductor of radius r, length ℓ , and resistivity ρ has resistance R. It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor. The resistance of the new conductor is . (A) R (B) R (C) R (D) 4 R (E) 16 R

25 A conductor of radius r, length and resistivity has resistance R. It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor. The resistance of the new conductor is (A) (B) (C)R (D)4R (E)16R

17. A conductor of radius r, length and resistivity has resistance R. It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor. The resistance of the new conductor is (A)R (B)R (C)R (D)4R (E)16R

20. A square loop of wire moves with a constant speed v from a field-free region, through a region of uniform B field as shown. Which of the five graphs correctly shows the induced current I in the loop as a function of time t?(A)(B)(C)(D)(E)

21. The coulomb is the same as (A : ampere, s : second, M : mass, T : temperature)(A)A·s(B)A/s(C)T·A/M(D)T/M(E)none of the above

22. A 2-μF capacitor in series with a 2-k resistor is connected to a 60-Hz ac source. Calculate the impedance of the circuit.(A)1500 ohms(B)1800 ohms(C)2100 ohms(D)2400 ohms(E)8600 ohms

23. The figure shows a cross section of a long cylindrical conductor of radius a = 4 cm containing a long cylindrical hole of radius b = 1.5 cm. The central axes of the cylinder and hole are parallel and are distance d = 2 cm apart; current i = 5.25 A is uniformly distributed over the tinted area. The magnitude of the magnetic field at the center of the hole is(A)7.6 μT(B)15.3 μT(C)24.7 μT(D)35.9 μT(E)51.8 μT

24. A proton is accelerated from rest through a potential difference of 2.5 kV and then moves perpendicularly through a uniform 0.60-T magnetic field. What is the radius of the resulting path?(A)15 mm(B)12 mm(C)18 mm(D)24 mm(E)8.5 mm

25. A 30-μF capacitor is charged to 40 V and then connected across an initially uncharged 20-μF capacitor. What is the final potential difference across the 30-μF capacitor?(A)15 V(B)24 V(C)18 V(D)21 V(E)40 V

1. 碳化鎢車刀適宜以碳化矽(SiC)磨粒的磨輪研磨，高速鋼車刀則適以氧化鋁(Al2O3)磨粒的磨輪研磨。 (A)O (B)X

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