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# Physics watch

1. In an X-ray tube, 10 electrons arrive with a speed of 2*10^5 ms^-1 at a target of mass 0.2kg and shc 500 J kg^-1 K^-1. If the mass of an electron is 9.1*10^-31 kg and assuming 98% of the incident energy is converted into the internal energy of the target, how long will it take for the temperature of the target to increase by 50oC?

I got around 10^15 years. Does anyone else agree?
2. is this non-relativistic mechanics? or do we include relativistic effects?
3. I should hope non-relativistic, it's an extension on our A-level homework.
4. well, if you havent done relativity, then no. (its just that sum sillabi do relativity)

oh, and is it supposed to be "10 electrons a second arrive..."?
5. Yes. What did you get? I got exactly 2.692*10^22 seconds.
6. Power = 0.98 *10 * 0.5 * 9.1 * 10^-31 * 2 * 10^5 * 2 * 10^5
= 1.7836 * 10^-19 W

Energy needed = 0.2 * 500 * 50 = 5000 J

=> time = energy/power = 5000/1.7836*10^-19

= 2.803 * 10^22 s

(we get quite similar answers, mine being out by ~3% to yours, did you remember about the 98%? because that would account for the 3% discrepancy)
7. Ah, pretty similar, I took the percentage from the energy out instead of the energy in. Thanks!

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Updated: April 29, 2004
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