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Quick Question on AS Physics Unit 1 Paper

Hi, I was wondering if anyone can give me some info about a question on AQA Physics A Specimen Paper(Unit 1).
The question is:
Determine the specific charge of a nucleus of Cs 133(Atomic relative Mass)/55(Proton Number).

So here's what I did.

Specific Charge = Charge / Mass
The electronic charge unit (e) = 1.6 X 10^-19 C
So 55 X e = 8.8 x 10 ^-18 (Which is the charge)
And 133 X Proton mass = 2.2211 X 10 ^-25 (The Mass)

So the out come should be 3962000.81 = 3.962 X 10^7 C Kg-1

My confusion is the AQA Mark scheme says this
specific charge (= charge/mass) = 55 × 1.6 × 10^-19/137 × 1.67 × 10^-27
AQA Answer = 3.85 × 10^7 C kg-1
My Answer = 3.962 X 10^7 C Kg-1

Where Am I wrong? Also I do not understand where AQA have got 137 as the relative mass number for Cs on the periodic table says 133?

Any Help?
(edited 13 years ago)
Reply 1
Avatar for westarmy
westarmy
OP
Anyone?
Perhaps they have used separate values of mass for neutrons and protons? That's all I can think of. The answer is so close it shouldn't be anything else...plus I just got the same as you.

Try calculating the masses off protons + neutrons and you should get the right answer. :smile:
...Naaaah, didn't work, did it? :frown:
Oops, didn't read your post properly :P Definitely something to do with that 137/133 thing. It definitely says 133 in the question then goes on to use 137?
It looks like an error in the mark scheme to me, for some reason they're using Caesium-137, when the question paper clearly states Caesium-133. I've found that the AQA specimen papers quite often have mistakes in their mark schemes, presumably because they're not checked as rigorously as the real papers.
Reply 6
Avatar for westarmy
westarmy
OP
Thanks. I am quite confused on this equation.

From these energies in joules, we can work out the speeds at which the electrons travel using v2 = 2 Ek/m. Mass of an electron = 9.11 × 10-31 kg.
so 100eV equals v = 5.92 × 105 m/s
According to t he mark scheme.
Yet every time i try it. I get a completely different answer.
Reply 7
Avatar for Utd4eva
Utd4eva
Original post by westarmy
Thanks. I am quite confused on this equation.

From these energies in joules, we can work out the speeds at which the electrons travel using v2 = 2 Ek/m. Mass of an electron = 9.11 × 10-31 kg.
so 100eV equals v = 5.92 × 105 m/s
According to t he mark scheme.
Yet every time i try it. I get a completely different answer.


Hi, I was wonderin' do you have an electronic version of this paper? (AQ & MS). If so could you please upload? :smile:...
Hey,
Forces acting on a vertically ascending balloon: Drag, Upthrust and Weight.
Will Drag be upwards or downwards?
Reply 9
Avatar for Andrea03
Andrea03
Original post by westarmy
Thanks. I am quite confused on this equation.

From these energies in joules, we can work out the speeds at which the electrons travel using v2 = 2 Ek/m. Mass of an electron = 9.11 × 10-31 kg.
so 100eV equals v = 5.92 × 105 m/s
According to t he mark scheme.
Yet every time i try it. I get a completely different answer.


I dunno what the poster above is talking about, but ill help, youve made a very simple but common mistake Ek or work is measured in Joules. you need to convert the 100eV to joules.

Multiply 100ev by the charge of one electron (1.6x10^-19) and you will get the Ek

Then put that into the equation and rember to square route the equation.

I got an answer of 5.926x10^6 ms-1

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