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Dodgy answers in Investigating Physics?

  • 01-03-2016 08:28PM
    #1
    TSMGUY
    Closed Accounts Posts: 855 ✭✭✭


    Started electricity (finally) and it's no way near as bad as I thought. My only qualm with it is the LONG calculations you have to do. So I was doing the questions on Capacitance (ex. 16.2) and I noticed some of the answers looked dodgy. The figures were right but they were to the wrong power of 10

    Like on question 5i) my answer was 2.492x10^-9 but in the back of the book it was 2.492x10^-12. I've plugged in the values several times and the formula is relatively simple so am I wrong or is the book? I'm a bit neurotic about these things so I'd appreciate it if anyone could clear this up. I hate nothing more than wrong answers to exercises, studying it is hard enough as it is without second guessing whether the answers at the back of the book are reliable or not

    Thanks in advance


Comments

  • #2
    lolo97
    Registered Users, Registered Users 2 Posts: 28 ✭lolo97


    Is this exercise in a book? If so which one ?


  • #3
    aibreann_
    Registered Users, Registered Users 2 Posts: 22 ✭aibreann_


    The same type of thing happens to me in the heat calculations, really annoying.


  • #4
    emersyn
    Registered Users, Registered Users 2 Posts: 430 ✭✭emersyn


    I don't do physics but I know for maths the answers in the back of my textbook (Active Maths) tend to be quite unreliable. Now this could be because they were in a rush to publish them after the project maths curriculum came out and therefore didn't proofread properly, but with anything involving long calculations there's bound to be some human error somewhere, so if you're confident you have the right method and you've gotten the right answers to other questions of the same type I wouldn't worry


  • #5
    rainbowtrout
    Registered Users, Registered Users 2 Posts: 15,404 ✭✭✭✭rainbowtrout


    TSMGUY wrote: »
    Started electricity (finally) and it's no way near as bad as I thought. My only qualm with it is the LONG calculations you have to do. So I was doing the questions on Capacitance (ex. 16.2) and I noticed some of the answers looked dodgy. The figures were right but they were to the wrong power of 10

    Like on question 5i) my answer was 2.492x10^-9 but in the back of the book it was 2.492x10^-12. I've plugged in the values several times and the formula is relatively simple so am I wrong or is the book? I'm a bit neurotic about these things so I'd appreciate it if anyone could clear this up. I hate nothing more than wrong answers to exercises, studying it is hard enough as it is without second guessing whether the answers at the back of the book are reliable or not

    Thanks in advance

    Hard to say without seeing the question, but the difference between your answer and the answer in the back of the book is a factor of 3, so it's possible that you have used the wrong unites e.g. kilojoules instead of joules.

    What values and formula are you using?


  • #6
    Fionn111
    Registered Users, Registered Users 2 Posts: 2 ✭Fionn111


    Sorry for the late reply, only saw this now. The answer in the book is correct. I'm assuming you went wrong converting mm^2 to m^2. Anyway to get the answer:
    A=4mm^2 =4x10^-6m^2
    d=0.1mm =10^-4m
    e=6.23x10^-11
    Plug them in to the formula C=eA/d
    to get C=2.492x10^-12F
    Hope this helped


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  • #7
    TSMGUY
    Closed Accounts Posts: 855 ✭✭✭TSMGUY


    emersyn wrote: »
    I don't do physics but I know for maths the answers in the back of my textbook (Active Maths) tend to be quite unreliable. Now this could be because they were in a rush to publish them after the project maths curriculum came out and therefore didn't proofread properly, but with anything involving long calculations there's bound to be some human error somewhere, so if you're confident you have the right method and you've gotten the right answers to other questions of the same type I wouldn't worry
    active maths has to be the biggest pile of crap I've ever encountered. About 20% of their answers make no sense. You should get a different book, even if your teacher uses active maths. It's really disgraceful how badly written it is.
    Fionn111 wrote: »
    Sorry for the late reply, only saw this now. The answer in the book is correct. I'm assuming you went wrong converting mm^2 to m^2. Anyway to get the answer:
    A=4mm^2 =4x10^-6m^2
    d=0.1mm =10^-4m
    e=6.23x10^-11
    Plug them in to the formula C=eA/d
    to get C=2.492x10^-12F
    Hope this helped

    Thanks so much! Was bugging me a lot.

    No worries then rainbowtrout. I stated the question number and exercise number in the OP if you wanna take a look just for the sake of it.


  • #8
    rainbowtrout
    Registered Users, Registered Users 2 Posts: 15,404 ✭✭✭✭rainbowtrout


    TSMGUY wrote: »
    active maths has to be the biggest pile of crap I've ever encountered. About 20% of their answers make no sense. You should get a different book, even if your teacher uses active maths. It's really disgraceful how badly written it is.


    Thanks so much! Was bugging me a lot.

    No worries then rainbowtrout. I stated the question number and exercise number in the OP if you wanna take a look just for the sake of it.

    My class don't have that book. I teach from Physics Plus from Edco


  • #9
    randylonghorn
    Registered Users, Registered Users 2 Posts: 29,509 ✭✭✭✭randylonghorn


    rainbowtrout wrote: »
    My class don't have that book. I teach from Physics Plus from Edco
    You mean you haven't written your own one yet? :pac:


  • #10
    rainbowtrout
    Registered Users, Registered Users 2 Posts: 15,404 ✭✭✭✭rainbowtrout


    randylonghorn wrote: »
    You mean you haven't written your own one yet? :pac:


    Gotta give someone else a chance


  • #11
    TSMGUY
    Closed Accounts Posts: 855 ✭✭✭TSMGUY


    rainbowtrout wrote: »
    My class don't have that book. I teach from Physics Plus from Edco

    Oh you're a teacher! That explains your burning curiosity. I was thinking it was strange that another student would be so eager to take a look at a random electricity question.


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