AQA Physics PHYA4 - 20th June 2016 [Exam Discussion Thread]

oh right, didn't see the A*AA for camb oops.. You reckon you can do that ey?
Thankfully I only need AAB for Architecture at UCL which means I don't need to stress all too much

Yep, they've been attached

Do we need to know about the experiments for determining the value of the specific heat capacity? It doesn't say we need to on the specification but its been given 3 pages on the nelson thrones book.

Do we need to know about the experiments for determining the value of the specific heat capacity? It doesn't say we need to on the specification but its been given 3 pages on the nelson thrones book.

They're just examples. But AQA can most certainly use one of the this examples and ask you to apply your knowledge

Our teacher usually asks us to 'Describe an experiment to determine....' in his tests, and I suspect that these are past exam questions. Though I haven't yet looked at any past papers so I couldn't say for sure.

I think we could be asked to describe an experiment as a high mark question (4+) in the exam

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I did a unit 5 paper yesterday and the describe the experiment 6 marker was horrible :'(


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What were you made to find out in the experiment?

hey guys, is there a unit 5 thread??
Also is anyone doing Astrophysics as their optional unit???

"4 An electric heater is placed in an insulated container holding 100g of ice at a temperature of -14 degrees C. The heater supplies energy at a rate of 98 joules per second.

4(a) After an interval of 30s, all the ice has reached a temperature of 0 degrees C.Calculate the specific heat capacity of ice."

2100 J per kg per K (from E=mc delta T)"

4(b) Show that the final temperature of the water formed when the heater is left on for a further 500s is about 40 degrees C.
specific heat capacity of water = 4200 J per kg per K
specific latent heat of fusion of water - 3.3 x 10^5 J per kg"

Calculate energy/time to melt ice, then calculate the further change in temperature from the remaining energy/time.

"4(c) The whole procedure is repeated in an uninsulated container in a room at a temperature of 25 degrees C.State and explain whether the final temperature of the water formed would be higher or low that than calculated in part (b)."

Can anyone explain why the answer to 4c) is higher

The water is below 25 degrees for less time than than it is above it. When it is below 25 degrees C the room transfers thermal energy to the water, when above 25 degrees C the water transfers energy to the room/surroundings. Assuming the temperature of the surroundings doesn't change which is a fair assumption given the mass of the surroundings (i.e the air around it) is much more than the mass of water in the beaker.

How can the temperature of the water reach above 40 degrees if it starts transferring thermal energy back to the surroundings, when it reaches above room temp. Does the water just gain thermal energy at a really fast rate so it gains about 15 more degrees before transferring it back?

This question doesn't really have enough information to answer it definitively (you'd need a bit more information). I remember this question when I did my A2s. Because you're supplying thermal energy at a constant rate to the water, but you don't know the rate at which thermal energy is being dissipated by the water into the surroundings. There is Newton's law of cooling which states that the rate of thermal energy dissipation by a body is proportional to the difference in temperatures between the body and its surroundings, but this is only approximately true in convective heat transfer which is approximately true for this question.

To answer the question definitively you'd need to know how fast water dissipates thermal energy to air up to a temperature difference of 15 degrees.

A way to think about it is that since you're supplying energy for a given interval of time, the temperature gain of the water from 0 to 25 should take less time since there is extra thermal energy input by the surroundings. Any temperature after that (above 25), the net thermal energy input to the water is decreased as the surroundings acts as a sink for thermal energy.

So an approximate answer would be since the difference in temperatures between 25 and 40 is less than 0 and 25 and so the net thermal energy input should be increased. => Higher final temperature.