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AQA Physics PHYA5 - Thursday 18th June 2015 [Exam Discussion Thread] Watch

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    (Original post by Sbarron)
    Thanks I just worked it out another way that took ten times as long lol I started finding the ratio of moles I didn't think I could do it that way because the volume of the tank stays the same
    How do you know the volume remains unchanged? It only says the temperature does.


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    surely V1 is 8 not 10 if they say the container has volume 8?
    Then V2 is 10
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    (Original post by betbi3etwerrd)
    surely V1 is 8 not 10 if they say the container has volume 8?
    Then V2 is 10
    The air initially occupies both the pump and container. After one pump it now occupies just the container.

    Also, if you think of it logically, the bike pump compresses the air into the container so the tyres can be pumped up. The volume must therefore reduce in order for the pressure within the tyre to increase. Assuming the temperature is constant.


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    Do we assume r_0 = 1.4 \times 10^{-15}? I've seen several accepted values stated.


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    How do I know when to use I1/i2 = (X1/x2 )^2

    And when to use i1/i2=(x2/X1) ^2

    I'm so confused

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    (Original post by CD223)
    Do we assume r_0 = 1.4 \times 10^{-15}? I've seen several accepted values stated.


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    I would've thought that they provided us with r_0 value?
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    When they say that it has 0.375 times as many carbon-14 atoms as an equal mass of living wood do they mean its comparing the content of carbon-14 in modern day wood to old wood from the boat??? Not sure what that statement means in the picture attached.

    Also, the question the final temperature of the cola drink. In the mark scheme it subtracts 30 from the final temeprature for the change in temperature of the beaker which is initially at 30 degrees celcius but shouldn't be final temperature - initial temperature???

    Thanks in advance for any help.

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    (Original post by CD223)
    How do you know the volume remains unchanged? It only says the temperature does.


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    I was mistakenly considering the tank and the pump seperately and the volume of the tank doesn't change so that's where I got confused
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    (Original post by Sbarron)
    I was mistakenly considering the tank and the pump seperately and the volume of the tank doesn't change so that's where I got confused
    Oh I see! Yeah when they mean volume in the equations they're referring to the volume of the gas itself, so when you're given multiple volumes just think of what volume the gas will occupy and use that in the formulas


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    (Original post by MSB47)
    When they say that it has 0.375 times as many carbon-14 atoms as an equal mass of living wood do they mean its comparing the content of carbon-14 in modern day wood to old wood from the boat??? Not sure what that statement means in the picture attached.

    Also, the question the final temperature of the cola drink. In the mark scheme it subtracts 30 from the final temeprature for the change in temperature of the beaker which is initially at 30 degrees celcius but shouldn't be final temperature - initial temperature???

    Thanks in advance for any help.

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    The way I do those temperature ones and decide wether to take initial from final or visa versa is that if there is energy lost you want your temperature value to be negative... Works for me every time
    With the boat one, if I remember correctly they are comparing the number of c14 in the boat (a dead tree) to the C14 in a living tree
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    (Original post by CD223)
    Oh I see! Yeah when they mean volume in the equations they're referring to the volume of the gas itself, so when you're given multiple volumes just think of what volume the gas will occupy and use that in the formulas


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    Yep ok my memory of that was getting a bit rusty so thanks for freshening it up! LolThe whole valve part didn't help
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    (Original post by MSB47)
    When they say that it has 0.375 times as many carbon-14 atoms as an equal mass of living wood do they mean its comparing the content of carbon-14 in modern day wood to old wood from the boat??? Not sure what that statement means in the picture attached.

    Also, the question the final temperature of the cola drink. In the mark scheme it subtracts 30 from the final temeprature for the change in temperature of the beaker which is initially at 30 degrees celcius but shouldn't be final temperature - initial temperature???

    Thanks in advance for any help.

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    The Carbon 14 content of once living organisms gradually decays over time. As the ratio is less than one, it may help to think of it as "the ancient piece of wood has 0.375 times the content of the piece of living wood" which makes sense - the living wood should have more as its alive and photosynthesising. Whereas the ancient piece of wood is dead and therefore has a small fraction of the Carbon 14 content of the living piece of wood.

    The other question refers to when the cola and glass are in "thermal equilibrium". In other words they are the same temperature so there is no net transfer of thermal energy between them.

    The cola is initially at 3 degrees and the glass is at 30 degrees.

    Logically, the cola will warm up due to the heating effect from the glass so there will be a net transfer of thermal energy to the cola from the glass until it reaches thermal equilibrium. Equally, the glass will cool as it transfers energy to the cola.

    This means the glass cools from 30 degrees to Tf, and the cola warms up from 3 degrees to Tf.

    If you place the Tf terms so it reads (final temperature - initial temperature) for both then the thermal energy of the glass will be negative, which leads to the incorrect answer.

    The reason for this is because the mark scheme equates the loss of energy from the glass to the gain in energy of the cola. This makes both quantities positive and allows the final temperature to be calculated.


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    (Original post by CD223)
    The Carbon 14 content of once living organisms gradually decays over time. As the ratio is less than one, it may help to think of it as "the ancient piece of wood has 0.375 times the content of the piece of living wood" which makes sense - the living wood should have more as its alive and photosynthesising. Whereas the ancient piece of wood is dead and therefore has a small fraction of the Carbon 14 content of the living piece of wood.

    The other question refers to when the cola and glass are in "thermal equilibrium". In other words they are the same temperature so there is no net transfer of thermal energy between them.

    The cola is initially at 3 degrees and the glass is at 30 degrees.

    Logically, the cola will warm up due to the heating effect from the glass so there will be a net transfer of thermal energy to the cola from the glass until it reaches thermal equilibrium. Equally, the glass will cool as it transfers energy to the cola.

    This means the glass cools from 30 degrees to Tf, and the cola warms up from 3 degrees to Tf.

    If you place the Tf terms so it reads (final temperature - initial temperature) for both then the thermal energy of the glass will be negative, which leads to the incorrect answer.

    The reason for this is because the mark scheme equates the loss of energy from the glass to the gain in energy of the cola. This makes both quantities positive and allows the final temperature to be calculated.


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    Ahh ok I get the first part now, thanks.

    For the second part because one is losing thermal energy and one is gaining thermal energy can you equate the two equations like this...

    mc-(dT)=mc(dT)

    where dT= change in temperature and because the glass is cooling it temperature drops hence a negative change of temperature and then that can deduce the (30-Tf) rather than (Tf-30)??
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    (Original post by MSB47)
    Ahh ok I get the first part now, thanks.

    For the second part because one is losing thermal energy and one is gaining thermal energy can you equate the two equations like this...

    mc-(dT)=mc(dT)

    where dT= change in temperature and because the glass is cooling it temperature drops hence a negative change of temperature and then that can deduce the (30-Tf) rather than (Tf-30)??
    Yeah - if you try it the other way around you get an answer like -6.8 degrees which doesn't make sense either!


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    (Original post by gcsestuff)
    How do I know when to use I1/i2 = (X1/x2 )^2

    And when to use i1/i2=(x2/X1) ^2

    I'm so confused

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    This is how I've done it
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    (Original post by MSB47)
    When they say that it has 0.375 times as many carbon-14 atoms as an equal mass of living wood do they mean its comparing the content of carbon-14 in modern day wood to old wood from the boat??? Not sure what that statement means in the picture attached.

    Also, the question the final temperature of the cola drink. In the mark scheme it subtracts 30 from the final temeprature for the change in temperature of the beaker which is initially at 30 degrees celcius but shouldn't be final temperature - initial temperature???

    Thanks in advance for any help.

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    Here is a similar type of question and what I've done to find the mass of ice
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    (Original post by betbi3etwerrd)
    I would've thought that they provided us with r_0 value?
    I had one question where I was expected to work it out first and then put it in my further calculations and it defiantly was not 1.4*10^15 ... That gave the wrong answer
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    Does anyone know where the 1.22 comes from in this formula for working out the radius with diffraction? And do we need to remember the formula for the exam?
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    For turning points I think it would be a good idea to compile a list of "significance of results".

    E.g. The significant of Einstein's photon model of light is that light can be shows wave-particle duality: Light's particle behavior is photoelectric effect and wave behavior is diffraction/interference.

    Please correct my one if it's wrong and post similar ones for things like Hertz, Millikan, Thomson, Michelson-Morley, Maxwell, etc.
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    (Original post by Sbarron)
    Here is a similar type of question and what I've done to find the mass of ice
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    Thanks that question seems pretty decent!
 
 
 
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