Year 12s: what will be the first way you research your future options? >>

OCR A Level Physics - Unified (paper 3) - Unofficial markscheme

This poll is closed

What mark do you think you got?

65-70 3%

60-64 6%

55-59 10%

50-54 17%

45-49 17%

40-44 17%

35-39 9%

30-34 10%

25-29 4%

20-24 2%

Less than 206%

Total votes: 327

haxhacks2

(Credit to RedGiant who put this together)

Calculate the number of moles of air [2 Marks]: 0.12

Calculate the initial pressure [3 marks]: 1.9x10^5

Show that the force acting upwards on the rocket is 5N [3 Marks]: F=pressure*area. Pressure is the difference between the inside and outside (atmospheric pressure). The answer was 15.4N I believe.

Calculate the initial acceleration [3 Marks]: First work out the total mass, which came out as 0.3kg or something (I think). Then work out the net force, and then the acceleration.
15 - 0.35 * 9.81/ 0.35 =33.047 ≈ 33 ms^-2 (2.sf)

Does the rocket reach a greater or lower height? [4 marks]:
It decreases as the additional water displaces the gas meaning the number of moles decreases. But the volume the gas occupies has decreased. i said that these factors cancel each other out as P is proportional to n and P is inversely proportional to volume. This means that the pressure and upwards force are similar to before. there is more water meaning the weight increases so the inertia of the bottle is larger meaning it will accelerate less than before hence the height decreasing.

Why won't beta emission affect the results? [1 mark]:
Beta radiation cannot penetrate lead, so therefore it won't affect the results.

Complete the nuclear decay equation [2 marks]:

Attachment 826146

Show that ln(n) = (-1/u)d + ln(n0) [1 mark]: Follow through and get in the form of y= mx + c

Calculate the unknown value of ln(N), and it's uncertainty [2 marks]:
Ln of 300 = 5.7
Uncertainty in Ln = 0.13

Graph question [2 marks]:
Draw Line of best fit through middle (approximate) of all of the error bars
Draw Line of worst fit top of first bar to bottom of last bar

Calculate the value of u, and state it's uncertainty [3 marks]:
u = 53.4 +-4.6

6-marker #1 - Efficiency between G.P.E and electrical energy
Measure the GPE for 1 inversion - mgh, so use a balance to get the mass of the magnet and measure the distance the magnet falls (h) using a ruler.
Perform multiple tests, varying the number of inversions. Measure the electrical energy using E = Pt, where P is the power of the LED and t is the time it stays on using a stopwatch. Calculate the total G.P.E for each test by multiplying the G.P.E for 1 inversion by the number of inversions you tested.
Plot a graph of electrical energy against G.P.E - gradient should be efficiency? i.e gradient of 0.1 = 10% efficiency, and so forth.

6-marker #2 - Diffraction grating
dsin(x) = nl (let l be lambda/wave length)
You need d and sin(x) - d is the slit separation, so divide 1mm (in m) by 300. You now need the angle.
Imagine the metre rule, the position of the 1st order and the line connecting them is a triangle. sin = opposite/hypotenuse, so treat the imaginary triangle as a right-angled triangle and use Pythagoras theorem to get the hypotenuse. Calculating the angle gives 11 degrees. You know n = 1, so the wavelength of red light was 667nm.
The uncertainties - the cm ruler is +-0.5mm, and (this I forgot in the exam) the metre rule is +-0.5cm. Use all this to get the percentage uncertainty like normal.

Calculate the energy [2 marks]: 2.16J

Calculate the time that the L.E.D will last for [2 marks]: 43.2 seconds

Calculate the forces F and R [3 marks]: F was 39N, R was 68N

Calculate the centripetal force [2 marks]: 9.0N

Explain why F increases, and R decreases [4 marks]:
There is a new force that has been introduced which acts horizontally to the left. Breaking down the vector of R it is clear to see that it has a horizontal component to the right. The centripetal force acts in a different direction thus decreasing the horizontal vector of R. Overall this decreases R. When breaking F down it is clear to see that it has a horizontal component that acts to the left. This is the same direction as the centripetal force meaning the 2 components are added together thus increasing the horizontal vector of F. Overall this increases F.

Describe the Doppler effect [1 mark]: The wavelength and frequency changes, when there is relative motion between the observer and the wave source.

Show the the speed of the wave was 7.5ms^-1 [2 marks]: First get time period, and do f=(1/T), which was 2 Hz. Then do 2*pi*r to get wavelength. Then use v=f*lamda.

Caluclate the maximum change in frequency [2 marks]:
330/1700 = 0.194...
337.5/0.194... = 1738.6 ≈ 1739Hz
∆F = 39

Show the point x on the path [1 mark]:
left most hand side of the circle
x was 3/4 along the circumference.

Hence, label the graph scale [2 marks]:
1739 marked at peak
1661 marked at trough

Explain the terms accuracy and precision [2 marks]:
Accuracy is how close each measured value is, to the true/actual value.
Precision is how close each measured value is to each other.

(edited 4 years ago)

Scroll to see replies

Reply 1

BiLiRen

I got 40hz ***change in f

怎么不出声啦

Some people got 34 and others got 340, im not sure which is correct.

Original post by BiLiRen

I got 40hz ***change in f

Reply 4

Personal_17

Original post by haxhacks2

Some of the answers: Feel free to add to it.

0.12 moles
1.9x10^5 Pa (?)
44ms^-2
12J (?)
240 seconds (?)
R=8gcos30
F=8gsin30 (or the other way around)
340Hz change in wavelength

Yeah I got similar as well
I forgot some of them but I got miles and F/R right

Reply 5

jessicathompson1

Original post by BiLiRen

I got 40hz ***change in f

same

Think I got 39Hz

I got 170 seconds for the led?

Reply 8

tesconyc

i got 120seconds :frown:

Original post by _zoe

I got 170 seconds for the led?

Reply 9

Levi23317

I got 77hz for the change

Reply 10

Montytheragdoll

Original post by etothepiiplusone

Think I got 39Hz

How did you get this what equation did you use??

Reply 11

laurenhiss

Original post by tesconyc

i got 120seconds :frown:

Same. Did you also get 6J?

Reply 12

Montytheragdoll

Me too

Original post by _zoe

I got 170 seconds for the led?

Reply 13

jessicathompson1

Original post by _zoe

I got 170 seconds for the led?

same

Reply 14

rayan_kotecha

Original post by Levi23317

I got 77hz for the change

same!

Reply 15

ridwan157

I got 77Hz too, I think it depends on which equation you use. I used the 2vcos0 one lol, honestly had no idea which to use

Original post by rayan_kotecha

same!

Reply 16

Desz_king

I got 340Hz

Original post by jessicathompson1

same

Reply 17

Desz_king

I got 43 seconds

Original post by jessicathompson1

same

Reply 18

RedGiant

Calculate the number of moles of air [2 Marks]: 0.12

Calculate the initial pressure [3 marks]: 1.9x10^5

Show that the force acting upwards on the rocket is 5N [3 Marks]: F=pressure*area. Pressure is the difference between the inside and outside (atmospheric pressure). The answer was 15.4N I believe.

Calculate the initial acceleration [3 Marks]: First work out the total mass, which came out as 0.3kg or something (I think). Then work out the net force, and then the acceleration.
15 - 0.35 * 9.81/ 0.35 =33.047 ≈ 33 ms^-2 (2.sf)

Does the rocket reach a greater or lower height? [4 marks]:
It decreases as the additional water displaces the gas meaning the number of moles decreases. But the volume the gas occupies has decreased. i said that these factors cancel each other out as P is proportional to n and P is inversely proportional to volume. This means that the pressure and upwards force are similar to before. there is more water meaning the weight increases so the inertia of the bottle is larger meaning it will accelerate less than before hence the height decreasing.

Why won't beta emission affect the results? [1 mark]:
Beta radiation cannot penetrate lead, so therefore it won't affect the results.

Complete the nuclear decay equation [2 marks]:

Show that ln(n) = (-1/u)d + ln(n0) [1 mark]: Follow through and get in the form of y= mx + c

Calculate the unknown value of ln(N), and it's uncertainty [2 marks]:
Ln of 300 = 5.7
Uncertainty in Ln = 0.13

Graph question [2 marks]:
Draw Line of best fit through middle (approximate) of all of the error bars
Draw Line of worst fit top of first bar to bottom of last bar

Calculate the value of u, and state it's uncertainty [3 marks]:
u = 53.4 +-4.6

6-marker #1 - Efficiency between G.P.E and electrical energy
Measure the GPE for 1 inversion - mgh, so use a balance to get the mass of the magnet and measure the distance the magnet falls (h) using a ruler.
Perform multiple tests, varying the number of inversions. Measure the electrical energy using E = Pt, where P is the power of the LED and t is the time it stays on using a stopwatch. Calculate the total G.P.E for each test by multiplying the G.P.E for 1 inversion by the number of inversions you tested.
Plot a graph of electrical energy against G.P.E - gradient should be efficiency? i.e gradient of 0.1 = 10% efficiency, and so forth.

6-marker #2 - Diffraction grating
dsin(x) = nl (let l be lambda/wave length)
You need d and sin(x) - d is the slit separation, so divide 1mm (in m) by 300. You now need the angle.
Imagine the metre rule, the position of the 1st order and the line connecting them is a triangle. sin = opposite/hypotenuse, so treat the imaginary triangle as a right-angled triangle and use Pythagoras theorem to get the hypotenuse. Calculating the angle gives 11 degrees. You know n = 1, so the wavelength of red light was 667nm.
The uncertainties - the cm ruler is +-0.5mm, and (this I forgot in the exam) the metre rule is +-0.5cm. Use all this to get the percentage uncertainty like normal.

Calculate the energy [2 marks]: 2.16J

Calculate the time that the L.E.D will last for [2 marks]: 43.2 seconds

Calculate the forces F and R [3 marks]: F was 39N, R was 68N

Calculate the centripetal force [2 marks]: 9.0N

Explain why F increases, and R decreases [4 marks]:
There is a new force that has been introduced which acts horizontally to the left. Breaking down the vector of R it is clear to see that it has a horizontal component to the right. The centripetal force acts in a different direction thus decreasing the horizontal vector of R. Overall this decreases R. When breaking F down it is clear to see that it has a horizontal component that acts to the left. This is the same direction as the centripetal force meaning the 2 components are added together thus increasing the horizontal vector of F. Overall this increases F.

Describe the Doppler effect [1 mark]: The wavelength and frequency changes, when there is relative motion between the observer and the wave source.

Show the the speed of the wave was 7.5ms^-1 [2 marks]: First get time period, and do f=(1/T), which was 2 Hz. Then do 2*pi*r to get wavelength. Then use v=f*lamda.

Caluclate the maximum change in frequency [2 marks]:
330/1700 = 0.194...
337.5/0.194... = 1738.6 ≈ 1739Hz
∆F = 39

Show the point x on the path [1 mark]:
left most hand side of the circle
x was 3/4 along the circumference.

Hence, label the graph scale [2 marks]:
1739 marked at peak
1661 marked at trough

Explain the terms accuracy and precision [2 marks]:
Accuracy is how close each measured value is, to the true/actual value.
Precision is how close each measured value is to each other.