A physicist has developed a device that generates a beam of alpha particles (eac

Question

A physicist has developed a device that generates a beam of alpha particles (each of which is a helium nucleus, consisting of two protons, two neutrons, and no electrons). The device ejects a steady stream of 6950 alpha particles per second.

You are testing different shielding materials for a test chamber that will house the alpha particle generator. You prepare a sheet of one potential material in an electrically neutral state, mount it on a perfectly insulating stand, and fire the alpha particle device at it for 12.3 seconds.

Afterward, you measure that the charge of the sheet is 4.19 × 10-15 coulombs. Assume that none of the alpha particles bounced back from the sheet, meaning that the alpha particles either were captured by the shielding or penetrated completely through the shielding. What percentage of the alpha particles penetrated completely through the shielding material?

= _____________ %

Explanation / Answer

We have 6950 alpha particles each with a charge of 2(1.6 X 10-19) C

We fire this many for 12.3 sec

Total charge fired is

(6950)(2)(1.6 X 10-19)(12.3) = 2.74 X 10-14 C

Since the sheet has a charge of 4.19 X 10-15 C, that means that

2.74 X 10-14 - 4.19 X 10-15 = 2.32 X 10-14 C passed through

The percentage that passed through is

2.32 X 10-14/2.74 X 10-14 C = 84.5%

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