Recent content by erok81

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    Change of Variables - Finding New Limits

    Ooooh yes. I see now. That is very straightforward. :) Now I have: 2 ≤ v ≤ 3 0 ≤ u ≤ 1 I am guessing you choose which for v and which for u based on plotting the graph and seeing which corresponds to which axis? I tried swapping them and it wasn't working so well (I got results with x...
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    Change of Variables - Finding New Limits

    Okay, this should be right. I have x=(3u-v)/5 and y=(2v-u)/5 But I think I am doing something wrong as this is just getting messier and messier the further I go. Using the first boundary y=-(x/3)+1 and my two equations there I end up with v=(5-2u)/3. Which I am pretty sure is wrong because...
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    Change of Variables - Finding New Limits

    Homework Statement Integrate the following over the set E. \int_E \frac{2x+y}{x+3y} dA Bounded by the lines: y = −x/3+1 y = −x/3+2/3 y = −2x y = −2x + 1 Homework Equations None. The Attempt at a Solution I can up to the same point everytime, but always get stuck on finding the new...
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    Finding Electric Field of Two Charges

    I am assuming I can't take the inverse of that thing like I did? Would the more appropriate solution to cross multiply everything to get r alone?
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    Finding Electric Field of Two Charges

    Homework Statement There is a charge Q=1 μC located at x=0 and a charge -Q/2 located at x=4 m. What is the E-field. At what value of x is the E-field zero? Express your answer in meters and do not approximate decimals digits. Homework Equations E=\frac{q_1 q_2 k_c}{r^2} Removing...
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    How Hot Must the Air in a Balloon Be to Lift a Given Mass?

    Hmm...I'm not sure that it depends on temp. Rspec=R/M where R is gas constant and M is molar mass. With that said, I don't see how it can vary with temp.
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    How Hot Must the Air in a Balloon Be to Lift a Given Mass?

    Homework Statement I have a balloon with a volume of 500m3 Outside air temp of 300K Mass to lift of 300kg Molar mass of air is 28 g/mol (I didn't end up using this) I am to find the temperature inside the balloon to barely lift the given mass. I have apparently forgotten everything...
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    Entropy and Black Hole Temperature

    Homework Statement In a previous problem I had to find the entropy of a black hole where I ended with this: S_{BH}=\frac{8 \pi^2 G M^2 k}{h c} Now I am to find the temp, given the energy of a black hole is mc2. Homework Equations T=(\frac{\partial S}{\partial u})^{-1} The...
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    Calculating Amount of Ice to Cool Tea to 65°C

    Ooooh. I see where you are headed now! It starts at -15C. So I need to figure how much heat it's taking from -15 to zero, the latent heat part from freezing to non freezing, then 0C up to 65C it's final temperature. And look at that; I just happen to have three stages just like you...
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    Calculating Amount of Ice to Cool Tea to 65°C

    If I am understanding correctly (which I don't think I am)... For melting ice 80 cal/g For boiling water 540 cal/g I don't have work for those as they are given in the text. Which is just the latent heat formula L=Q/m I'm not sure on the three stages you mentioned. The ice will only go from...
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    Calculating Amount of Ice to Cool Tea to 65°C

    The ice ends up at the same final temp as the tea, so 65. And it will go from solid to a liquid. I tried throwing in the latent heat (H) for melting ice at 80 cal/g but it didn't work out correctly.
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    Calculating Amount of Ice to Cool Tea to 65°C

    Homework Statement Your 200-g cup of tea is boiling-hot. About how much ice should your add to bring it down to a comfortable sipping temperature of65°C . Assume that the ice is initially at−15°C . The specific heat capacity of ice is 0.5cal g⋅°C , for water is 1 cal g⋅°C. The latent...
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    Finding Final Temperature of Boiling Water and Potatoes

    So in this case ΔT isn't necessarily Tf-Ti like most Δ's. It's more for interpretation...if that makes sense. I get what you are saying. For this problems the final and initial are "swapped" since the water has to lose heat at the end. If I do final - initial, the ΔT is negative and...
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    Finding Final Temperature of Boiling Water and Potatoes

    However if I swap the temp values like an example I saw... cwmw(Twi-Tf)=cpmp(Tf-Tpi) After the algebra I end up with... T_f = \frac{(c_p m_p T_{pi}) + (c_w m_w T_{wi})}{(c_w m_w) + (c_p m_p)} I get 85.125C. Which is a lot more reasonable. If that's the case, why do the temps...
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