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

does anyone know where i can find good cosmology notes? thank you in advaance

The Robbie Peck notes in the attachments are pretty good.
If you're looking for something a little more in depth, you could try the CyberPhysics Notes.

Does anyone have model answers (e.g. from a markscheme!) about how x-rays, PET scans, CAT scans, and contrast mediums work. In fact, any of the medical physics stuff - I know the basics but I'm really struggling to write what the mark schemes actually want.

Ionic solids consist of a regular arrangement of positive and negative ions. The figure
below shows two neighbouring ions in a particular ionic solid. The ions A and B may be
considered as two point charges of equal magnitude, 1.6 × 10–19 C, and opposite sign,
with a separation of 2.0 × 10–10 m. The ion A is positive.

(ii) Calculate the magnitude of the electric field strength at the mid point between the
charges.

(ii) Use of E = (1/4πε0)Q/r^2

Sum of two equal terms

E = 2 × 9 × 109 × 1.6 × 10–19 / (2.0 × 10–10)^2

E = 7.2 × 1010 N C–1 or V m–1

I'm having trouble with this question:



I'm using $E={Q\over{4\pi\epsilon_0 r^2}}$

As this is asking for the magnitude between the midpoint, I am using $r = 1\times10^{-10}$. This is not correct in the markscheme. Why does r need to be the whole distance if this is only for 1 particle?

EDIT: Markscheme:

guys i was wondering what is the total ums needed to get and an A? and what is the total need for a B?
like thorughout the whole course?

r is defined as the distance between the charges; so although the question is asking you to find E at the midpoint between the charges, the charges which create E are $2\times10^{-10}$m apart.
The mark scheme has worked out the magnitude of E provided by one of the ions; and then doubles the answer because the other ion provides an equal magnitude of E.

The mark schemes wrong on this question. You should indeed use r=1E=10 not r=2E=10.

guys i was wondering what is the total ums needed to get and an A? and what is the total need for a B?
like thorughout the whole course?

The mark schemes wrong on this question. You should indeed use r=1E=10 not r=2E=10.

i'm confused is r not meant to be the distance between the two charges? thanks

This was posted from The Student Room's Android App on my GT-S5830

From my understanding, if you're using $F={Q_1Q_0\over4\pi\epsilon_0r^2}$ then you're measuring the force between the two point charges. Therefore you need to use the whole distance.

The question before asked for the field at the midpoint. It wants you to find the electric field from both of the particles at that point, then add them up.

When you use $E={Q\over4\pi\epsilon_0r^2}$, you single out one particle and completely ignore the other particle when working out the electric field strength. Therefore the distance between the point charge and the midpoint is half the distance between the two point charges.

After you work out the effect of both of the particles' fields on the midpoint, you add or take away the field strengths to get the total (add if the charges are opposite, subtract if they're the same).

I'd like some clarification about Question 21 b ii (pictured):



The left hand rule should be used as a force is being induced. (Left hand rule for a force, right hand for a current).

The magnetic field is going upwards to the left (to the centre of the imaginary circle that the electron moves on).

From my understanding, the charge would be in the opposite direction to the motion of the electron as it has a negative charge.

Therefore the Force would be going into the page.

The mark scheme seems to assume a conventional current, even though I can't see any references to this in the paper.

Is my reasoning correct? How should I know to use a conventional current if a similar question comes up on the exam?