Turn on thread page Beta
    • Thread Starter
    Offline

    14
    ReputationRep:
    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?
    Offline

    15
    ReputationRep:
    is this non-relativistic mechanics? or do we include relativistic effects?
    • Thread Starter
    Offline

    14
    ReputationRep:
    I should hope non-relativistic, it's an extension on our A-level homework.
    Offline

    15
    ReputationRep:
    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..."?
    • Thread Starter
    Offline

    14
    ReputationRep:
    Yes. What did you get? I got exactly 2.692*10^22 seconds.
    Offline

    15
    ReputationRep:
    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)
    • Thread Starter
    Offline

    14
    ReputationRep:
    Ah, pretty similar, I took the percentage from the energy out instead of the energy in. Thanks!
 
 
 

University open days

  1. Loughborough University
    General Open Day Undergraduate
    Fri, 21 Sep '18
  2. University of Cambridge
    Churchill College Undergraduate
    Fri, 21 Sep '18
  3. Richmond, The American International University in London
    Undergraduate Open Day Undergraduate
    Fri, 21 Sep '18
Poll
Which accompaniment is best?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.