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PHY100 Homework 4

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1. Special Relativity: Consider a firecracker that after being triggered explodes after 1 second. An observer A is standing some distance away from observer B who is traveling with constant velocity towards A with the firecracker. The velocity of B and distance between A and B is such that B will reach A in 1 sec as measured by A. At this instant B triggers the firecracker they are holding. Which of following options is correct.

a. The firecracker explodes exactly when B reaches A

b. The firecracker explodes before reaching A

c. The firecracker explodes after crossing A

d. All the above outcomes are possible

Explain your answer. (20 pts)

2. Educating ancients:

a. How does a glass rod after being rubbed on silk exerts force on another charged object? (7 pts)

b. Describe an experiment that you could use to show that there are two kinds of charges? (13 pts)

3. Fields (electric and magnetic) around a charged object:

a. What field(s) does a stationary (non-magnetic) charged object produce? (7 pts)

b. Does a moving charge particle produce both electric and magnetic field?

Explain your answer. (13 pts)

4. Electromagnetism:

Consider two balls (starting at zero speed) rolling down under the force of gravity (the direction of force by gravity is shown by the arrow) inside conducting copper (non-magnetic material) tubes; the red ball is magnet and blue is not. (This is exactly the situation we discussed in class with a demonstration.)

a. Are current produced in either tube? Explain your answer. (10 pts)

b. What is the direction of electromagnetic force on the two balls, if any? Which ball will reach the end of the tube first? (15 pts)

c. Can the red ball reach the end (bottom) of the tube first? Explain. Hint: Conservation of energy. (15 pts)

d. Can the system be rigged in a way that the magnetic ball is stable with zero velocity and no net forces (without changing the configuration of the set up and the material of the tubes)? Explain your answer. (Bonus 15 pts)