OCR B G491, 17th May 2012 Morning.

In bits/bytes, do we give a Megabyte as 1 x10^6 bytes, or as 1024^2 bytes? they both exist.

Question 8b in the jan 2012 paper:

When tension in a cable is 5.4x10^4 Newtons the stress is 1.1x10^8 Pa. Calculate the cross sectional area of the cable?

I need to catch up on rrfc's posts!
Thanks for typing out all of those questions BTW.

I just noticed you never uploaded 8 a. Could you type it please?

8a) [2marks] a crane uses steel cables to lift heavy objects. The cables need to be strong. Name one other mechanical property of steel that is important in this application and explain why.
Question 8b in the jan 2012 paper:

When tension in a cable is 5.4x10^4 Newtons the stress is 1.1x10^8 Pa. Calculate the cross sectional area of the cable?

8ci) show that the strain in the cable at a stress of 1.1x10^8 Pa is about 0.05%.

Youngs modulus of steel = 2.1x10^11 Pa

8cii) the total length of a cable on the crane is 650m. Calculate the extension of the cable when the stress is 1.1x10^8 Pa

8d) suggest why the maximum stress is a cable is limited to about 1/3 of its yield stress. [1]

9). A thermistor is to be used as a temperature sensor. It is connected in series with a fixed resistor in a potential divider circuit.

ai) Explain why the circuit can be described as a potential divider [1]

aii) The resistance of the thermistor decreases as the temperature rises. Explain why the p.d measured by the voltmeter across the fixed resistor increases as the temperature of the thermistor rises. [3]

b) all of b is to do with a graph which I can't upload ... Just about sensitivity not that hard
10) some crap about tennis balls and cameras...

a) a tennis ball has a diameter of 67mm. When the ball is 10m from the camera, a sharp image is formed on a CCD 55mm behind the lens.

i) calculate the magnification of the image [1]

ii) show that the power of the lens in the fixed focus camera is about 18 D [2]

iii) show that the diameter of the image of the ball on the CCD is about 0.4mm. Make your method clear [2]

b) there are 70 pixels per mm on the CCD

i) calculate the number of pixels across the image of the 67 mm diameter ball [1]. << I think that's a trick question as it gives the 67 people would think to use that but it asks for the image so I would use the .4mm from the earlier question

ii) calculate the least distance te ball must move sideways for is image to move one pixel [1]

c) the position of the ball on 2 consecutive images can be used to determine the distance it has moved between images. Two such images give a value do the distance moved of 0.080m. State and explain the max possible value for this measurement based on your answer to bii.

11 This question is about electrical light fittings and plugs and the materials used in their
construction.

(a)
(i) A 12 V halogen lamp is rated at 25 W.
Calculate the operating current for the lamp.
current = ................................ ...................... A [1]

(b) Explain in terms of their microscopic structure why metals are good electrical conductors [3]

(ii) Calculate the conductance of the lamp.
conductance = ................................ ...................... S [1]

c)
The pins of a 12 V halogen lamp slot into metal sockets held by ceramic fittings, but plastic
plugs with metal contacts are used to connect appliances to the mains.
(i) Ceramics and plastics are suitable materials for lamp fittings and plugs because of their
good electrical insulation properties.
Suggest a reason why, unlike metals, ceramics and plastics are good insulators.
[1]

(ii) Suggest and explain in terms of their material properties why ceramics are preferred to
plastics for halogen lamp fittings.
[2]

(iii) Suggest a reason, in terms of their material properties, why plastics are preferred to
ceramics for electric plugs.
[1]

8a) [2marks] a crane uses steel cables to lift heavy objects. The cables need to be strong. Name one other mechanical property of steel that is important in this application and explain why.
The cable ends to be stiff so it cannot be stretched under heavy load.

No not ductile because that means it can be drawn into long wires....

Here is the whole Jan 2012 paper for anyone who is interested, I think the mark scheme was posted earlier in this thread, but let me know if not and I can upload that too.

Thanks for uploading. Where on earth did you find it?

Here is the whole Jan 2012 paper for anyone who is interested, I think the mark scheme was posted earlier in this thread, but let me know if not and I can upload that too.

Omg ty so much, I'm going to do the this paper again because I had to imagine everything in section B because no picture

Ok I'll simply just ask you to kindly answer question 10 b of may 2010:

'Explain why metals are usually ductile, but glasses are brittle.
In your explanation, you should make clear how the differences in arrangement of atoms account for the large-scale behavoir or the materials'

Metals - Giant metallic structure. Positive ions in a sea of delocalised electrons. Metallic bonding is non directional so ions can slip past one another.

Glass - Giant amorphous structure. Covalently bonded atoms. Covalent bonding is directional therefore atoms cannot slip past one another.