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OCR Physics A (A-level) Unified Physics Unofficial Mark Scheme

Poll

How many marks do you think you got on this paper?

Question 1)

a) A stationary U92238 nucleus decays to form a Th90234 nucleus, write a decay equation for this nuclear reaction. [2 marks]

Spoiler



b) The mass of the U92238 nucleus is 4.0×10-25kg. After the decay the thorium nucleus has a velocity of 2.4×105ms-1, calculate the kinetic energy in MeV of the alpha particle produced in this decay. [4 marks]

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c) Show that when a U92238 nucleus decays to a Pb82206 nucleus 14 particles are emitted. [3 marks]

Spoiler



Question 2)

a) The electric force experienced by current carrying electrons in a hall probe balances the magnetic force. V = p.d. induced across opposite faces of probe, a = separation of faces, B = magnetic flux density, v = mean drift velocity of electrons.
Show that V=Bva [3 marks]

Spoiler



b) Given that for the hall probe
number density = ... m-3
current = ... A
a = ... m
b = ... m
calculate the mean drift velocity. [2 marks]

Spoiler



c) Given that B=0.08T calculate V [2 marks]

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d) A student wants to measure the magnetic flux density between two poles of a horseshoe magnet. It is in the range 0.02-0.04T. Why is the hall probe not suitable for taking this measurement? [1 mark]

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e) Explain how you would carry out a suitable experiment to measure the magnetic flux density using a current carrying wire. [4 marks]

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Question 3)

a) A capacitor circuit so that a capacitor can be charged and then discharged through a resistor. The resistance of the resistor is varied. The p.d. across the capacitor is measured after it has been discharging for 15 seconds. Show that the magnitude of the gradient of a graph of ln(V) against 1/R is 15/C. [2 marks]

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b) Complete a table of results by calculating ln(V) and its uncertainty for V= 3.0 +-0.2 [1 mark]

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c) Complete graph by plotting 2 missing points and error bars [1 mark]

d) Use the graph to calculate the capacitance giving the uncertainty [4 marks]

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Question 4)

a) You are given an ammeter, a 6V supply, a 100Ω resistor and an unlabelled box containing the resistor arrangement below. Describe how you would find out which terminals of the box are A, B, C and D and how you would find the resistances of the two resistors. Include a circuit diagram. [6 marks]



Spoiler



b) You are given a potential divider circuit diagram with an LDR and resistor of unknown resistance, and a graph of LDR resistance against light intensity.
Draw the circuit symbol for an LDR. [1 mark]

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c) An electric switch connected across the LDR will close when the p.d. is ... V and open when it is ... V. It closes when the light intensity is 0.80 Wm-2. Calculate the resistance of the resistor. [3 marks]

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d) Calculate the light intensity when the switch opens. [3 marks]

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Question 5)

a) A speaker emits sound of frequency ... kHz, and amplitude ... m The speed of sound in air is 320ms-1. Draw a graph of displacement of air molecules against distance from speaker and label the axes. Mark a point X on the graph where molecules have maximum speed and a point Y where they have the same speed in the opposite direction. [4 marks]

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b) Calculate the maximum velocity of air molecules. [2 marks]

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c) Calculate crms of oxygen at ... oC given that its molar mass is ... kgmol-1. [3 marks]

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d) The speed of sound can be calculated using two methods a) and b). Only one measurement is taken for each method. Method a) has a microphone and speaker fixed and another speaker moving away. Method b) has two fixed speakers and a microphone moving between them. Explain how the speed of sound can be calculated using each method, and how you would decrease the uncertainty in your calculation. [6 marks]


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Question 6)
[13 marks]

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Please vote to say how many marks you think you got, and reply with corrections for my answers.
(edited 6 years ago)

Scroll to see replies

Hello there,

Thank you for setting this up. It's useful to have a comprehensive post like this. As for amendments, I think that the given light intensity was 0.80Wbm2 0.80 Wb m^{-2} . I may be remembering these figures from elsewhere, but the fixed resistance turned out to be 1500Ω 1500 \Omega .

Question 6 had a string and a bob in two configurations. The first was hanging down at rest, with the extension of the string at 0.050m 0.050 m and the second with it being string around at an acute angle to the downward vertical. The period of the motion was 0.67s 0.67 s , I believe. The first part of the question was to show that the angle the string made with the vertical was 22o 22^o . You would do this by finding the vertical and horizontal components of the tension by considering the bob's weight and the horizontal circular motion respectively (and then applying the inverse tangent).

The second part was to find the extension in the second situation, which you would do by finding the magnitude of the new tension and given that a tension of 1.2N 1.2 N in the first situation came from an extension of 0.050m 0.050 m , you could calculate the extension required for the new tension via Hooke's law. I think the answer was 0.054m 0.054 m

The final part of the question was a projectile motion problem.

I might whip up some diagrams because I've just read through this and it's a little messy.

Smithenator5000
(edited 6 years ago)
Reply 2
I'm pretty sure 1.a) was 2 marks. Question 6 was about circular motion but can't remember the specifics.
Reply 3
The last part of question 6 was two marks on suggesting how the motion would differ if a larger angular speed was used.
Original post by P_Digby
The last part of question 6 was two marks on suggesting how the motion would differ if a larger angular speed was used.


Oh, yes- that's right.
Here is an image I made summarising the information and parts of question 6. The final part isn't included. It asked for a suggestion as to what would happen if the angular speed was increased. Reply with corrections.

diagrams.jpg
For question 2, I believe a a and b b were 5.0mm 5.0 mm and 0.20mm 0.20 mm respectively.
Original post by Smithenator5000
For question 2, I believe a a and b b were 5.0mm 5.0 mm and 0.20mm 0.20 mm respectively.


wasnt there a part where we had to draw another force on fig 6.2? I drew weight as they already gave us tension what did you guys write?
Original post by Samendra
wasnt there a part where we had to draw another force on fig 6.2? I drew weight as they already gave us tension what did you guys write?


There was a part like that. I would imagine they were looking for weight; weight and tension were the two main forces.
Original post by Smithenator5000
There was a part like that. I would imagine they were looking for weight; weight and tension were the two main forces.


yea, some people drew a centripetal force no idea if they would accept that?
Original post by Samendra
yea, some people drew a centripetal force no idea if they would accept that?


I doubt they would, as the centripetal force was merely the horizontal component of the tension in this case.
Original post by Dido Twite
Question 1)

a) A stationary U92238 nucleus decays to form a Th90234 nucleus, write a decay equation for this nuclear reaction. [2 marks]

Spoiler



b) The mass of the U92238 nucleus is 4.0×10-25kg. After the decay the thorium nucleus has a velocity of 2.4×105ms-1, calculate the kinetic energy in MeV of the alpha particle produced in this decay. [4 marks]

Spoiler



c) Show that when a U92238 nucleus decays to a Pb82206 nucleus 14 particles are emitted. [3 marks]

Spoiler



Question 2)

a) The electric force experienced by current carrying electrons in a hall probe balances the magnetic force. V = p.d. induced across opposite faces of probe, a = separation of faces, B = magnetic flux density, v = mean drift velocity of electrons.
Show that V=Bva [3 marks]

Spoiler



b) Given that for the hall probe
number density = ... m-3
current = ... A
a = ... m
b = ... m
calculate the mean drift velocity. [2 marks]

Spoiler



c) Given that B=0.08T calculate V [2 marks]

Spoiler



d) A student wants to measure the magnetic flux density between two poles of a horseshoe magnet. It is in the range 0.02-0.04T. Why is the hall probe not suitable for taking this measurement? [1 mark]

Spoiler



e) Explain how you would carry out a suitable experiment to measure the magnetic flux density using a current carrying wire. [4 marks]

Spoiler



Question 3)

a) A capacitor circuit so that a capacitor can be charged and then discharged through a resistor. The resistance of the resistor is varied. The p.d. across the capacitor is measured after it has been discharging for 15 seconds. Show that the magnitude of the gradient of a graph of ln(V) against 1/R is 15/C. [2 marks]

Spoiler



b) Complete a table of results by calculating ln(V) and its uncertainty for V= 3.0 +-0.2 [1 mark]

Spoiler



c) Complete graph by plotting 2 missing points and error bars [1 mark]

d) Use the graph to calculate the capacitance giving the uncertainty [4 marks]

Spoiler



Question 4)

a) You are given an ammeter, a 6V supply, a 100Ω resistor and an unlabelled box containing the resistor arrangement below. Describe how you would find out which terminals of the box are A, B, C and D and how you would find the resistances of the two resistors. Include a circuit diagram. [6 marks]



Spoiler



b) You are given a potential divider circuit diagram with an LDR and resistor of unknown resistance, and a graph of LDR resistance against light intensity.
Draw the circuit symbol for an LDR. [1 mark]

Spoiler



c) An electric switch connected across the LDR will close when the p.d. is ... V and open when it is ... V. It closes when the light intensity is 0.80 Wm-2. Calculate the resistance of the resistor. [3 marks]

Spoiler




d) Calculate the light intensity when the switch opens. [3 marks]

Spoiler



Question 5)

a) A speaker emits sound of frequency ... kHz, and amplitude ... m The speed of sound in air is 320ms-1. Draw a graph of displacement of air molecules against distance from speaker and label the axes. Mark a point X on the graph where molecules have maximum speed and a point Y where they have the same speed in the opposite direction. [4 marks]

Spoiler



b) Calculate the maximum velocity of air molecules. [2 marks]

Spoiler



c) Calculate crms of oxygen at ... oC given that its molar mass is ... kgmol-1. [3 marks]

Spoiler



d) The speed of sound can be calculated using two methods a) and b). Only one measurement is taken for each method. Method a) has a microphone and speaker fixed and another speaker moving away. Method b) has two fixed speakers and a microphone moving between them. Explain how the speed of sound can be calculated using each method, and how you would decrease the uncertainty in your calculation. [6 marks]


Spoiler



Question 6)
[13 marks]

Spoiler



Please vote to say how many marks you think you got, and reply with corrections for my answers.


Hey, for the question as to why the hall probe was not suitable, I think the reason why was because the two magnets were connected by a steel yoke and therefore the potential difference between the two sides of the semiconductor would be lost because the pd can travel from one side to other.

At least that's what i put, I felt like it had something to do with the steel yoke anyway and that's my best guess.

btw thanks for the mark scheme, I feel a bit more relieved about that paper now!
Original post by Loumstar
Hey, for the question as to why the hall probe was not suitable, I think the reason why was because the two magnets were connected by a steel yoke and therefore the potential difference between the two sides of the semiconductor would be lost because the pd can travel from one side to other.

At least that's what i put, I felt like it had something to do with the steel yoke anyway and that's my best guess.

btw thanks for the mark scheme, I feel a bit more relieved about that paper now!


Hi thank you for the MS by the way for the resistor combination 6 markers. Do you think I can connect voltmeter in parallel between terminal c and D and then ammeter and power supply between a and b so if I get higher voltage on voltmeter it means the one connected to voltmeter is actually the one with higher resistance (potential divider rule)
Original post by Dido Twite
Question 1)

a) A stationary U92238 nucleus decays to form a Th90234 nucleus, write a decay equation for this nuclear reaction. [2 marks]

Spoiler



b) The mass of the U92238 nucleus is 4.0×10-25kg. After the decay the thorium nucleus has a velocity of 2.4×105ms-1, calculate the kinetic energy in MeV of the alpha particle produced in this decay. [4 marks]

Spoiler



c) Show that when a U92238 nucleus decays to a Pb82206 nucleus 14 particles are emitted. [3 marks]

Spoiler



Question 2)

a) The electric force experienced by current carrying electrons in a hall probe balances the magnetic force. V = p.d. induced across opposite faces of probe, a = separation of faces, B = magnetic flux density, v = mean drift velocity of electrons.
Show that V=Bva [3 marks]

Spoiler



b) Given that for the hall probe
number density = ... m-3
current = ... A
a = ... m
b = ... m
calculate the mean drift velocity. [2 marks]

Spoiler



c) Given that B=0.08T calculate V [2 marks]

Spoiler



d) A student wants to measure the magnetic flux density between two poles of a horseshoe magnet. It is in the range 0.02-0.04T. Why is the hall probe not suitable for taking this measurement? [1 mark]

Spoiler



e) Explain how you would carry out a suitable experiment to measure the magnetic flux density using a current carrying wire. [4 marks]

Spoiler



Question 3)

a) A capacitor circuit so that a capacitor can be charged and then discharged through a resistor. The resistance of the resistor is varied. The p.d. across the capacitor is measured after it has been discharging for 15 seconds. Show that the magnitude of the gradient of a graph of ln(V) against 1/R is 15/C. [2 marks]

Spoiler



b) Complete a table of results by calculating ln(V) and its uncertainty for V= 3.0 +-0.2 [1 mark]

Spoiler



c) Complete graph by plotting 2 missing points and error bars [1 mark]

d) Use the graph to calculate the capacitance giving the uncertainty [4 marks]

Spoiler



Question 4)

a) You are given an ammeter, a 6V supply, a 100Ω resistor and an unlabelled box containing the resistor arrangement below. Describe how you would find out which terminals of the box are A, B, C and D and how you would find the resistances of the two resistors. Include a circuit diagram. [6 marks]



Spoiler



b) You are given a potential divider circuit diagram with an LDR and resistor of unknown resistance, and a graph of LDR resistance against light intensity.
Draw the circuit symbol for an LDR. [1 mark]

Spoiler



c) An electric switch connected across the LDR will close when the p.d. is ... V and open when it is ... V. It closes when the light intensity is 0.80 Wm-2. Calculate the resistance of the resistor. [3 marks]

Spoiler




d) Calculate the light intensity when the switch opens. [3 marks]

Spoiler



Question 5)

a) A speaker emits sound of frequency ... kHz, and amplitude ... m The speed of sound in air is 320ms-1. Draw a graph of displacement of air molecules against distance from speaker and label the axes. Mark a point X on the graph where molecules have maximum speed and a point Y where they have the same speed in the opposite direction. [4 marks]

Spoiler



b) Calculate the maximum velocity of air molecules. [2 marks]

Spoiler



c) Calculate crms of oxygen at ... oC given that its molar mass is ... kgmol-1. [3 marks]

Spoiler



d) The speed of sound can be calculated using two methods a) and b). Only one measurement is taken for each method. Method a) has a microphone and speaker fixed and another speaker moving away. Method b) has two fixed speakers and a microphone moving between them. Explain how the speed of sound can be calculated using each method, and how you would decrease the uncertainty in your calculation. [6 marks]


Spoiler



Question 6)
[13 marks]

Spoiler



Please vote to say how many marks you think you got, and reply with corrections for my answers.

FYI , you have extra 1 m for 1b 4d and missed out there was one 2m question for the calculate the resistance of resistor connected to the capacitance.
Reply 14
Original post by Loumstar
Hey, for the question as to why the hall probe was not suitable, I think the reason why was because the two magnets were connected by a steel yoke and therefore the potential difference between the two sides of the semiconductor would be lost because the pd can travel from one side to other.

At least that's what i put, I felt like it had something to do with the steel yoke anyway and that's my best guess.

btw thanks for the mark scheme, I feel a bit more relieved about that paper now!


Looking back on this question, I remember that we were given the dimensions of both the probe and the magnet. I suspect that the probe was probably too large and so stuck out on either side when between the poles of the magnet. This would mean that some parts of it were not in the linear field directly between the poles.
CAn somebody plz link me the past paper for 2017
Reply 16
For Q1b, I used conservation of energy.
So, I used e = mc^2 and 0.5mv^2 for each particle. The energy of the U should equal the energy of the Th and He. So, I get this equation:

(Mass-energy of U) - (kinetic-energy of Th) - (mass-energy of Th) - (mass-energy of He) = (kinetic-energy of He)

At first, I got 12MeV. Then, I figured I'd make it more precise by increasing the number of decimals i had. I then got 43MeV.

According to a separate method involving momentum (as well as an unofficial markscheme), I am supposed to get 4.2MeV.

What am I doing wrong here?
(edited 5 years ago)
Original post by Heirio
For Q1b, I used conservation of energy.
So, I used e = mc^2 and 0.5mv^2 for each particle. The energy of the U should equal the energy of the Th and He. So, I get this equation:

(Mass-energy of U) - (kinetic-energy of Th) - (mass-energy of Th) - (mass-energy of He) = (kinetic-energy of He)

At first, I got 12MeV. Then, I figured I'd make it more precise by increasing the number of decimals i had. I then got 43MeV.

According to a separate method involving momentum (as well as an unofficial markscheme), I am supposed to get 4.2MeV.

What am I doing wrong here?


Hey there,

I think with your method, you end up rearranging for the He \text{He} energy to have a first term of c2(mUmThmHe) c^2 \left(m_\text{U} - m_\text{Th} - m_\text{He}\right) . With the information we have already, we don't know the respective masses to a high enough degree of precision for calculating the mass difference to a good enough accuracy (with the approximation mU=mTh+mHe m_\text{U} = m_\text{Th} + m_\text{He} , the term vanishes) and because of the c2 c^2 factor, it can have a great effect on the final result.

With the method using momentum, you get no such problem. The problem comes to He \text{He} energy E=mTh2vTh22mHe E = \dfrac{m_\text{Th}^2 v_\text{Th}^2}{2 m_\text{He}} . Now, with the mass conservation assumption, we are actually fine because the actual masses won't deviate by a large enough amount to significantly change the fraction mTh2mHe \dfrac{m_\text{Th}^2}{m_\text{He}} and so this ends up being a simpler method.

If you have any questions, don't hesitate. I may have said something a bit nonsensical.

Smithenator5000.
Reply 18
Original post by Smithenator5000
Hey there,

I think with your method, you end up rearranging for the He \text{He} energy to have a first term of c2(mUmThmHe) c^2 \left(m_\text{U} - m_\text{Th} - m_\text{He}\right) . With the information we have already, we don't know the respective masses to a high enough degree of precision for calculating the mass difference to a good enough accuracy (with the approximation mU=mTh+mHe m_\text{U} = m_\text{Th} + m_\text{He} , the term vanishes) and because of the c2 c^2 factor, it can have a great effect on the final result.

With the method using momentum, you get no such problem. The problem comes to He \text{He} energy E=mTh2vTh22mHe E = \dfrac{m_\text{Th}^2 v_\text{Th}^2}{2 m_\text{He}} . Now, with the mass conservation assumption, we are actually fine because the actual masses won't deviate by a large enough amount to significantly change the fraction mTh2mHe \dfrac{m_\text{Th}^2}{m_\text{He}} and so this ends up being a simpler method.

If you have any questions, don't hesitate. I may have said something a bit nonsensical.

Smithenator5000.


I think I get what you mean.
because we don't have the masses to ahigh enough decimal place, using those values to calculate mass difference leads to a high amount of variation?

I've also noticed that for the two particles to have speed, some of the energy fromthe mass of the uranium is turned into energy, whereas my calculations assume strict conservation of mass.

However, the momentum method also makes that mistake, doesn't it? we calculate the mass of the Thorium as being (234/238)*(4 * 10^-25).

I may have wandered, but I'm just trying to make sense of this all in my head.
Original post by Heirio
I think I get what you mean.
because we don't have the masses to ahigh enough decimal place, using those values to calculate mass difference leads to a high amount of variation?

I've also noticed that for the two particles to have speed, some of the energy fromthe mass of the uranium is turned into energy, whereas my calculations assume strict conservation of mass.

However, the momentum method also makes that mistake, doesn't it? we calculate the mass of the Thorium as being (234/238)*(4 * 10^-25).

I may have wandered, but I'm just trying to make sense of this all in my head.


Yeah, it's good to try and get a good sense of this. It'll help you to solve different problems. I'll do a simple but concrete example.

Suppose we have two quantities: A=4±1 A = 4 \pm 1 and B=4±1 B = 4 \pm 1 . If we calculate AB A - B using standard error propagation, we obtain AB=0±1 A - B = 0 \pm 1 . Then if we want the quotient, AB=1±0.4 \dfrac{A}{B} = 1 \pm 0.4 . Note that here, not only is the uncertainty of the difference greater than that of the quotient, but the relative uncertainty of the difference is actually infinite as opposed to the relative uncertainty of 40% 40\% . Of course, this is still bad, but I chose easy numbers here. Generally speaking, the relative uncertainty of a difference is high, when the two terms in the subtraction are similar (i.e. the difference is around zero).

So, it's generally not favourable to use the difference in a numerical calculation when a quotient may be used just as easily. This is especially relevant to the example in the paper due to the fact the the mass difference resides around zero, when this distinction between operations is more noticeable.

This was a university first-year's impression of the issue and even then, I kind of spat this out. So again, please ask if this isn't very clear.

Smithenator5000.

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