OCR Physics A - G485: Fields, Particles & Frontiers of Physics - June 2012

Sugar, I wrote PM on my wall chart but it looks like it says AM >.<
so, question 2)b)i) june 11?

Ah ok. SO C-14 decays into C-12? So you can use the formula X=Xo.e^-lambda.t where Xo is the initial ratio and X is the new ratio?

Charge is constant for capacitors in parallel, so work out the charge using CV = Q (1.125x10^-4 x 6 = 6.75x10^-4), then use Q/C = V for the 150uF capacitor (6.75x10^-4/150x10^-6 = 4.5V).

To get the charge for the circuit you need to use the total capacitance; i.e. 1/C1 + 1/C2 = 1/CT

Sort of. The C-14 does decay into C-12, but you can't directly use the ratio of C-12:C-14 to calculate the age. You'd have to work out the activity of the dead sample and the living sample separately. The activity of the dead sample is A, and the activity of the living sample is A0. Therefore, ln(A/A0)/-(lambda) = t.

Ok... i understand comparing Activities completely... but i don't see where the ratio method comes into it :-/ Cheers.

Anyone think MRI is going to come up?
cos' its sort of making me lose hope for the exam

Can someone explain question 2) b) i) in the june 2011 paper please

Anyone think MRI is going to come up?
cos' its sort of making me lose hope for the exam

The strong magnetic field in the MRI machine causes protons in the body to align in the direction of the field. It also causes them to precess, with a frequency called the Larmor frequency. The Larmor frequency depends on the strength of the magnetic field; for this reason, a gradient field is used so that the MRI machine knows the Larmor frequency at every point inside the scanner.

Now, the machine uses an RF coil to send pulsed RF waves into the patient's body. When the frequency of these waves is equal to the Larmor frequency, nuclear resonance occurs and the protons (precessing with the Larmor frequency) "flip" into a high energy state.

After a period of time (the relaxation time), the protons relax back into the low energy state. Due to conservation of energy, this means that they must emit RF waves when they do so. These waves have a frequency equal to the Larmor frequency, and are detected by the RF coil (this is why the waves have to be pulsed; they can't be transmitted and received at the same time!).

The machine knows the position of the proton due to the frequency of RF waves it emits; remember, the gradient field means that the machine knows the Larmor frequency at every point in the field. It can also identify the type of tissue the proton is in based on its relaxation time (since protons have different relaxation times in different tissues). Based on this information, the scanner can produce a 3D image of the body and distinguish between the different types of tissues within it.

Sorry, I amended my response. You could use the ratio.

Basically, if you had C-12:C-14 = 0.5:4 initially, you could take the initial number of C-12 atoms as 0.125.

The ratio would then increase over time, possibly to something like 1:3. So you could take this number of C-12 atoms as 0.333.

Then, use N = N0e^-(lambda)t where N0 is 0.125 and N is 0.333.

Someone correct me if I'm wrong!

Did too much Psychology and left this one behind...Just hoping to get top marks on defintions and long answers. Will have to improvise with the calculator questions.

Pffft psychology is not even a proper subject.

You have put me to shame. I'll get my coat.

The strong magnetic field in the MRI machine causes protons in the body to align in the direction of the field. It also causes them to precess, with a frequency called the Larmor frequency. The Larmor frequency depends on the strength of the magnetic field; for this reason, a gradient field is used so that the MRI machine knows the Larmor frequency at every point inside the scanner.

Now, the machine uses an RF coil to send pulsed RF waves into the patient's body. When the frequency of these waves is equal to the Larmor frequency, nuclear resonance occurs and the protons (precessing with the Larmor frequency) "flip" into a high energy state.

After a period of time (the relaxation time), the protons relax back into the low energy state. Due to conservation of energy, this means that they must emit RF waves when they do so. These waves have a frequency equal to the Larmor frequency, and are detected by the RF coil (this is why the waves have to be pulsed; they can't be transmitted and received at the same time!).

The machine knows the position of the proton due to the frequency of RF waves it emits; remember, the gradient field means that the machine knows the Larmor frequency at every point in the field. It can also identify the type of tissue the proton is in based on its relaxation time (since protons have different relaxation times in different tissues). Based on this information, the scanner can produce a 3D image of the body and distinguish between the different types of tissues within it.

That's not fair.

I seem to make so many calculation errors. I'm going to type each calculation in to my calculator 3 times before I settle with the answer