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

How do you know the volume remains unchanged? It only says the temperature does.


Posted from TSR Mobile

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

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.

Posted from TSR Mobile

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


Posted from TSR Mobile

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.

Posted from TSR Mobile

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.


Posted from TSR Mobile

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-T_f) rather than (T_f-30)??

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






Posted from TSR Mobile

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.

Posted from TSR Mobile

I would've thought that they provided us with r_0 value?

Here is a similar type of question and what I've done to find the mass of ice

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

Yeah - if you try it the other way around you get an answer like -6.8 degrees which doesn't make sense either!


Posted from TSR Mobile

This is how I've done it

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?

Thanks for the help