OCR Physics B G491 20th May 2013

Yeah i got the same! For the question above the last one, did you put stress=Force/cross sectional area so therefore more cross sectionalleads to a smaller stress (i think it was 5 correct me if i am wrong) so therefore if you had 1nm of force it would be 1/1 and 1/5 so therefore 1/5 more?

Sorry if that makes little sense!

Really rough unofficial mark scheme, will be added to when people start commenting.

1. D (Diode) F (Fixed Resistor) L (Filament Lamp) (3)

2a) Difficult to scratch or dent (1)
b) Axe head would become blunt if soft (1)
c) Tough (1) so axe head does not shatter if a crack forms
OR Strong (1) so axe head can withstand large forces (1)

3a) $2 \times 10^{-5} s$
b) $160-20 = 140kHz$
c) $1000$

4a) Current = VG (can't remember values)
b) $15mS$

5a) 5D
b) 4.6 D

6a) 10730J (Is 3sf OK, I settled for 3sf in the end)
b) Energy / 540s = 20W (to 2sf) / 19.9 W (to 3sf)

7. no.of levels = 200/0.5 + 1 = 401
so no. of bits = 9

8a) i) Directly proportional to each other
a)ii) Sensitivity = 1.5
b) 72 bmp
roughly 113 pressure units
c) Heart beats last for same time
Peak pressure heart beats falls then rises then falls...

9a)i) 0.857 Mbyte < 1Mbyte
ii) Estimate and then execution.
b) i) $1.14 \times 10^5 bits^{-1}$
ii) Yes because frequency is higher than the information transfer rate.
iii) Stops deforestation linked to climate change/ etc. (1)
Puts people out of jobs/ too expensive for some people to buy appropriate technology/ etc...

10a) i) Concrete made of stones/ aggregate and cement, which both contribute their properties to the material.
ii) Any four from these and possibly more:
Stones are stronger than cement (1)
Weakest point is at boundary between stones and cement (1)
Under compression, the stress is concentrated on the stone (1)
Under tension, the stress is concentrated on the cement (1)
Brittle fracture occurs sooner in the cement that the stones (1)
b) i) 5
ii) 3
c) i) Stress = F / CSA (1), same force but 5 times CSA leads to 1/5 stress (possibly shown through algebra) (2)
ii) $1.5 \times 10^8 Pa$

Yeah i got the same! For the question above the last one, did you put stress=Force/cross sectional area so therefore more cross sectionalleads to a smaller stress (i think it was 5 correct me if i am wrong) so therefore if you had 1nm of force it would be 1/1 and 1/5 so therefore 1/5 more?

Sorry if that makes little sense!

I did use stress = F/A but talked about the inverse proportionality is. Stress is proportional to 1/A there fore if area is five times larger, stress = k/5A therefore stress x area = k/5 therefore must be a fifth smaller


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Yeah i got the same! For the question above the last one, did you put stress=Force/cross sectional area so therefore more cross sectionalleads to a smaller stress (i think it was 5 correct me if i am wrong) so therefore if you had 1nm of force it would be 1/1 and 1/5 so therefore 1/5 more?

Sorry if that makes little sense!

Yeah I talked about the inverse proportionality as well

does anyone know the marks that go with each question?

Really rough unofficial mark scheme, will be added to when people start commenting.

1. D (Diode) F (Fixed Resistor) L (Filament Lamp) (2)

2a) Difficult to scratch or dent (1)
b) Axe head would become blunt if soft (1)
c) Tough so axe head does not shatter if a crack forms
OR Strong so axe head can withstand large forces (2)

3a) 2 \times 10^{-5} s
b) 160-20 = 140kHz
c) 1000 (1)

4a) Current = VG (can't remember values)
b) 15mS

5a) 5D
b) 4.6 D

6a) 10730J (Is 3sf OK, I settled for 3sf in the end)
b) Energy / 540s = 20W (to 2sf) / 19.9 W (to 3sf)

7. no.of levels = 200/0.5 + 1 = 401
so no. of bits = 9

8a) i) Directly proportional to each other (1)
a)ii) Sensitivity = 1.5
b) 72 bmp (2)
roughly 113 pressure units (2)
c) Heart beats last for same time
Peak pressure heart beats falls then rises then falls...

9a)i) 0.857 Mbyte < 1Mbyte
ii) Estimate and then execution.
b) i) 1.14 \times 10^5 bits^{-1}
ii) Yes because frequency is higher than the information transfer rate. (2)
iii) Stops deforestation linked to climate change/ etc. (1)
Puts people out of jobs/ too expensive for some people to buy appropriate technology/ etc...(1)

10a) i) Concrete made of stones/ aggregate and cement, which both contribute their properties to the material. (2)
ii) Any four from these and possibly more: (4)
Stones are stronger than cement
Weakest point is at boundary between stones and cement
Under compression, the stress is concentrated on the stone
Under tension, the stress is concentrated on the cement
Brittle fracture occurs sooner in the cement that the stones
b) i) 5 (3)
ii) 3 (2)
c) i) Stress = F / CSA (1), same force but 5 times CSA leads to 1/5 stress (possibly shown through algebra) (2)
ii) 1.5 \times 10^8 Pa (1)

Here is metaltron's mark scheme with the marks I am definitely sure about in red:

How did everyone know what concrete was made from? I just knew about ceramics and then talked about their general structure, and how that means that slips are not able to take place? :/

Guys for question 4(b) unfortunately, the answer is not 15mS, it is 30mS. That is because Total conductance across parallel circuits is equal to them being added together. Therefore, 15+15= 30mS. I do hope others got that one right, because many seem to fail to understand why it ISNT 15mS.

Guys for question 4(b) unfortunately, the answer is not 15mS, it is 30mS. That is because Total conductance across parallel circuits is equal to them being added together. Therefore, 15+15= 30mS. I do hope others got that one right, because many seem to fail to understand why it ISNT 15mS.