OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread

The ball was possitive. The bottom plate that it was attracted to was the most negative of the two plates.

The second. It had already lost 8 x 10 to power of 6,7 or 8? (can't remember)
so it's only ever going to neutralise it I think.

It was positive? Hmm

I thought the top plate was +9something kv, and the bottom was zero?

It was positive? Hmm

I thought the top plate was +9something kv, and the bottom was zero?

It was, but didn't the question before say that there was an extension in the spring because of the attraction to the bottom plate?

I said that the beta emmiter would neutralise charge and the position of the ball would shift so that kx = Eq, so it moves upwards?

Yep, it was...and the ball was positively charged.

That was another blagged Q on my part as well. I didn't know whether to go for ionizing (and I wouldn't know how the ionizing of air particles would affect things), or beta particles making the ball less positive because of attraction...arp. :P

It was, but didn't the question before say that there was an extension in the spring because of the attraction to the bottom plate?

I said that the beta emmiter would neutralise charge and the position of the ball would shift so that kx = Eq, so it moves upwards?

It didn't mention that it was attracted by the bottom plate per se, it just talked about the spring extending towards it (so common sense would tell you that there's some repulsion/attraction thingymajig happening) - but since the later Q mentions "electrons knocked off the ball", we know that it IS positive

Such a fool (me)

There, there. I have a feeling this is one of those papers where we'll drop a bit here and there, and it'll all add up (despite it being a generally nice paper)...but that would've been the case for most peeps, so grade boundaries might be kinder xD

I refuse to go back a page because I'll see something I dont like, but for the ring cutting one, was it not just the answer from before in webers divided by 0.4?
It goes from a maximum to nothing in this quater turn so I assumed you just use faraday's equation. For my assumption I said that the rotation of the ring is unlikely to be continuous so the rate of flux cutting in the turn won't be constant, therefore it's an average. Probably wrong, I'm crap at the wordy questions.

Just to add to the answers circulating about, I got about 2600kg for the mass of uranium and I got 435 people to get cancer.
Quote me if anyone wants to reply please, I'm not gunna start looking through the full thread, I like to avoid answers.

Also, 3 marks for a E=hc/lambda question? They having a laugh? I must have done this wrong surely in section C was it?

I said that the beta particle ionises the air and ions transfer charge to the ball. If the overall charge is positive, the ball moves downwards, if negative the ball moves upwards due to the electric force. An equalibrium is made where Vq/d = kx. The position is either above the initial position (if the ball is negative) or below (if positive.)

It was a positive ball so i said electrons attracted to the ball, making it less positive so less extension... also wrote maybe they are attracted to the positive plate, making it less positive and so less extension still (but i think thats rubbish)

heard so many different answers for this, reckon the mark scheme will accomodate more than 1 tbh... 1 of my mates was a right smart ass and said it wouldnt reach the ball as beta particles dont travel very far in air, but thinking about it now thats bs because its alpha that doesnt go far lol

Bits in bold exactly what i said mate, seemed thats what they were driving at with the question (i hope!)

I got about 1030 kg of uranium if i remember (1028.something) and am quite confident thats right, other people from my school got that too

i got 4.4 people (so 4) getting cancer, remember it was 3% so multiply by 0.03, not 3...

and the 3 mark E=hf one, there was 3 distinct steps i think it was just pathetically easy, tbh it seemed like they were struggling to give marks on section 3... a 3 marker with about an inch to write your answer! BS!

Did anyone put that the activity would have been higher in the past and thus the calculated value is a minimum?

The mass one. It was 235 X unit mass X how ever many things there was wasn't it?
Can't remember how I worked out how many decays there was though since I can't remember the question.

As for the cancer one, I got 4 at first then changed my answer. It was just 3 times the dose to work out the risk. If its 3% per sievert then its so many % per dose.

Then that% X 25 X number of things per year gave 435 didnt it?