For an isotope with a half-life of one day, at the end of two days the amount that remains is
A) none.
B) one-half.
C) one-quarter.
D) one-eighth.
E) none of the above
Answer: C
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Potential of Point-Charges: Point charges +4.00 ?C and +2.00 ?C are placed at the opposite corners of a rectangle as shown in the figure. What is the potential at point B due to these charges? (k = 1/4??0 = 8.99 × 109 N ? m2/C2) 
A. 8.99 kV B. 11.2 kV C. 89.9 kV D. 899 kV E. 112 kV
An astronomer records the spectrum of light coming from a distant star. Describe what information the astronomer can deduce from this stellar spectrum, and what physical principles she would apply
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In a region of space where the electric field is radial, suppose that Er = C/r3, where C is a positive constant. Is the charge density ? in this region positive, negative, or zero? (Note: ? might depend on r in this region.)
A. Positive B. Negative C. Zero D. This is impossible.
How does a gas cloud become hot?
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