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Help with some SEAI Second Level Math please

  • 07-02-2015 6:55pm
    #1
    Registered Users, Registered Users 2 Posts: 13,151 ✭✭✭✭


    From here
    http://www.seai.ie/Schools/Post_Primary/Subjects/Physics/Unit_5_-_Heat_Quantity/Sample_Problems_-_Specific_Heat_Capacities/

    Comes the following: I am puzzled by the 103 in question 2

    It does not impact the answer as it is in both sides of the heat equation

    Exam tip:
    When solving problems involving specific heat capacities, you only need to know two equations!

    1. Heat = mass × specific heat capacity × temperature change
    Q = mc Dq
    and

    2. Heat lost = Heat gained
    mc Dq = mc Dq

    When reading the question, concentrate on what is losing heat and what is gaining heat - then use the above two equations to solve the problem.

    Worked Examples
    Question 1

    Given that the specific heat capacity of water is 11 times that of copper, calculate the mass of copper at a temperature of 100 °C required to raise the temperature of 200 g of water from 20.0 °C to 24.0 °C, assuming no energy is lost to the surroundings.

    Solution

    Heat lost by copper = heat gained by water
    mcuccuDqcu = mwcwDqw
    mcuccu(100 - 24) = 0.200 × 11ccu(24 – 20)
    76mcu = 8.8
    mcu = 0.116 kg

    Question 2
    Three litres of water at 100 °C are added to 15 litres of water at 40 °C. Calculate the temperature of the mixture. Take the mass of 1 litre of water to be 1 kg and the specific heat capacity of water to be 4.2 × 103 J kg -1 K -1

    Solution

    Let the temperature of the mixture q
    Heat lost = heat gained
    m1c1Dq1 = m2 c2 Dq2
    3 × 4.2 × 103 × (100 – q) = 15 × 4.2 × 103 × (q -40)
    Solving for q gives q = 50°C

    “I can’t pay my staff or mortgage with instagram likes”.



Comments

  • Registered Users, Registered Users 2 Posts: 237 ✭✭Man of Aran


    Likely it's a typo, should be 10 to power3 to get convert 1kg mass to grams since SHC has units [ joule / gram per deg C]. If I understand your query correctly.


  • Registered Users, Registered Users 2 Posts: 13,151 ✭✭✭✭Calahonda52


    Good spot, many thanks indeed.

    “I can’t pay my staff or mortgage with instagram likes”.



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