OCR B G491, 17th May 2012 Morning.

Original post by When you see it...

You don't get study leave? That is harsh.

I'm 2nd year student all my A2 exams are in june. Got physics, chemistry, biology and maths fml.

Reply 83

Original post by rrfc

Good so far im getting these right...

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

Any value of stress above this value would lead to an elastic strain which is inappropriate for its function. Not sure about that one though, I'll have to check the MS.
Thanks for uploading these BTW. You don't need to upload any of section A because that has already been put up (I think, I'm confused with all the uploads on this thread so far!).

Reply 84

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

Yield stress is when plastic deformation occurs so to keep it well below the yield stress for safety.

Reply 85

Original post by hihihihihi

I'm 2nd year student all my A2 exams are in june. Got physics, chemistry, biology and maths fml.

Oh right. Is this the only AS module you are sitting then?

Reply 86

Original post by When you see it...

Oh right. Is this the only AS module you are sitting then?

Yea I got a D (52/90) in jan 2011, I didnt even revise lol. It was COD black ops fault. I played that instead of revising=/

Reply 87

Original post by ser00

eww electricity :/
and yes materials are soo bad - im gonna learn off the mark schemes cos the answers they want are so specific

Is there anything you don't understand about electricity? I would be happy to explain (unless you want to know 'what is an electron?' lol).

Reply 88

Original post by hihihihihi

Yea I got a D (52/90) in jan 2011, I didnt even revise lol. It was COD black ops fault. I played that instead of revising=/

COD isn't even a good game IMO. I hope you learnt your lesson lol.

Reply 89

rrfc

Ok, I've been starting from section B anyway :L

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

Reply 90

Original post by rrfc

I am typing it word for word

Ok, sorry can you post whole of question 9 in one go?

Nvm you posted it as i typed this lol.

Reply 91

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]
There are two resistors in a circuit so the emf is shared between them.
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]
the resistance of the fixed resistor stays constant. At the thermistor the current increases because resistance is decreasing so more current is flowing around the curcuit and V=IR so the p.d. at the fixed resistor increases

b) all of b is to do with a graph which I can't upload ... Just about sensitivity not that hard

Reply 92

ser00

12

Original post by When you see it...

Is there anything you don't understand about electricity? I would be happy to explain (unless you want to know 'what is an electron?' lol).

that would be amazing!! :biggrin:

ummm...where to start :tongue:

i know total current entering a junction = total current leaving it, but does it just split equally in a parallel circuit? or is it the same for each 'arm' of the circuit?

why does a voltmeter have to be high resistance? wouldnt that just take all the voltage and stop the circuit working?

why does internal resistance increase loads at the end of a cell's life?

ohh and what is a potential divider!?

hahaa im really sorry for all the questions dont feel you haver to answer them if you cba!!

Reply 93

rrfc

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]

Reply 94

i know total current entering a junction = total current leaving it, but does it just split equally in a parallel circuit? or is it the same for each 'arm' of the circuit?

in parallel G = G1 + G2 ... and G = 1/R and R=V/I , so G=I/V
why does a voltmeter have to be high resistance? wouldnt that just take all the voltage and stop the circuit working?
voltmeter don't have high resistance? they only meansure the p.d. across a component

why does internal resistance increase loads at the end of a cell's life?

The amount of chemical energy left is decreasing so the current is decreasing also. and V=IR
so the voltage stays constant

ohh and what is a potential divider!?

It is when two or more reistors are connected in a circuit

Reply 95

Original post by ser00

i know total current entering a junction = total current leaving it, but does it just split equally in a parallel circuit? or is it the same for each 'arm' of the circuit?

I just wrote a fair bit but have come to the conclusion that I can explain it better with a diagram. Bear with me...

Reply 96

ser00

12

Original post by hihihihihi

i know total current entering a junction = total current leaving it, but does it just split equally in a parallel circuit? or is it the same for each 'arm' of the circuit?

in parallel G = G1 + G2 ... and G = 1/R and R=V/I , so G=I/V
why does a voltmeter have to be high resistance? wouldnt that just take all the voltage and stop the circuit working?
voltmeter don't have high resistance? they only meansure the p.d. across a component

why does internal resistance increase loads at the end of a cell's life?

The amount of chemical energy left is decreasing so the current is decreasing also. and V=IR
so the voltage stays constant

ohh and what is a potential divider!?

It is when two or more reistors are connected in a circuit

thank youu xx

Reply 97

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]
m= v/u, so 55x10^-3/67x10^-3 = 0.82

ii) show that the power of the lens in the fixed focus camera is about 18 D [2]
1/v=1/u+1/f so 1/f=1/v-1/u, 1/f= 1/55x10^-3 -(1/-10) = 18.3D

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

i don't know the exact method for this but i did (55x10^-3/10)x66x10^-3 = 3.69x10^-4 = 0.37mm

Reply 98

Original post by ser00

thank youu xx

Btw i mean a series circuit, potential dividers don't work in parallel circuits because current is split

Reply 99