# OCR A Level Physics - Exploring (paper 2) - Unofficial MarkschemeWatch

Poll: What mark do you think you got?
95-100 (21)
3.95%
90-94 (21)
3.95%
85-89 (49)
9.23%
80-84 (72)
13.56%
75-79 (70)
13.18%
70-74 (65)
12.24%
65-69 (65)
12.24%
60-64 (45)
8.47%
55-59 (47)
8.85%
Less than 55 (76)
14.31%
#1

MCQ:

C
D
B
C
A
A/B
C
B/C
D
C
B
A
D/A
B
B

(In words):
Name of the law (Kirchoff's 2nd law),
The proportion of ultrasound intensity after 0.5m (0.67),
Current in resistor at t=0 and t=10 (Max at t=0, 37% at t=10),
Which cannot be wavelength, given that path difference was 4 (3cm),
The correct understanding radius for the force between (X+Y+Z),
Mean drift velocity ratios (0.2 m s ^-1^),
Relationship between V and R (curved line, through the origin),
EM waves can be... (plane polarized, diffracted, but not same speed in glass and vacuum/Only 1 and 2),
Nature of wire with four plotting compasses (straight wire),
Correct capacitors equation (0.9=1.5 e^-0.1t^),
Transformers ratio (50 turns primary, 200 turns secondary),
Volume of 235 nucleon nucleus (4.9 x 10 ^-42^).

Label x axis on wave graph (time/ seconds) [1 Mark]

Difference in time taken for two light rays to travel a small distance; show that t = 1.0 x 10^-15^ (Calculate both and subtract one from the other; as required) [3 Marks]

Why is frequency dependent on wavelength, for stationary wave? [3 marks]: The speed is constant; and the frequency has to be an integer multiple of 2f0 from f0 (e.g. f0, 3f0, 5f0, etc...). The wavelength depends on the length of the tube, so the wavelength has to decrease with each harmonic.

EMF question [3 marks]: 8.0 volts (it may actually be 7.0v, need more opinions)

Calculate the number of blue photons emitted per second [3 marks]: First work out energy of photon, using hc/lambda. Then use P = IV to get power of L.E.D. Then do power/energy of photon to get the answer. 5.9x10^16

Will photoelectrons be emitted? [2 marks]: The energy of the photons was 2.6 eV. This was greater than the work function (2.3 eV iirc), so yes, they will be emitted - with some kinetic energy.

Energy released by nuclear fission [4 marks]: 2.2 x 10^12 Joules (2 s.f)

Acceleration of oil droplet [3 marks]: 20 ms^-2

How do you stop a chain reaction? [2 marks]: Use (boron) control rods. This absorbs neutrons, such that only 1 neutron will trigger another nuclear reaction.

Age of Earth [3 marks]: 3.6 billion years (may vary slightly if you rounded early or not).

Graph of stationary wave [2 marks]: You had to draw the anti phase equivalent. There were 4 nodes.

Draw/describe the X-Ray gun [3 marks]: Draw the diagram of what it looks like. There is a hot cathode filament, and the anode is the target metal. A high p.d. is used. A vacuum is needed in the chamber. 99% of energy of electrons is converted to thermal energy; hence the need for water-cooling.

Work out the gradient from the line of best fit [2 marks]: Draw the line of best fit. I got the gradient as 2.3 or something like that.

Why was the student correct about the answer being unchanged by the measuring error? [2 marks]: It would give a systematic error, so the gradient would still be the same. Therefore each measurement varies by a consistent amount, as the ruler was kept at the same initial position.

Prove that gradient = p/EL [2 marks]: Just use standard definitions of variables (R=pl/A), and follow through working.

Calculate B [2 marks]: The answer was 0.23T

Phase difference [1 mark]: 180 degrees.

Show that C is inversely proportional to d [2 marks]: Show that Q*d = constant (Using values from the graph).
Q/V=c=EA/d therefore Q=EAD/d, where E A and D are constant

Explain how a piezoelectric transducer produces ultrasound waves [3 marks]: You apply an alternating P.D across the ends of a piezoelectric crystal, and the shape will rapidly compress/expand (change shape). The frequency must be above 20kHz. It produces (longitudinal) sound waves. Mention resonance.

Explain what physical properties of the issue affect the amount of reflection at the boundary [3 marks]: Talk about the different densities of materials, the different speeds of the ultrasound waves within the media, and z=pc. Mention the absorption coefficient (Ir/I0) = (Z2-Z1/Z2+Z1)^2. The greater the difference in Z value (acoustic impedance), the greater the reflection, and thus the smaller the intensity of the ultrasound waves that pass through. Mention other things like the volume, mass, etc. (I just rambled on).

1st 6 Marker on verifying the relationship, of v=(sqrt(g x d)), where d is the depth [6 marks] (note that there are other ways, which might be better):
Use a 'water ripple generator', connected to a bench power supply and oscilloscope.
Count the squares on the oscilloscope to get the time period, and do time base * period. Then do f = (1/T).
Use a high speed camera to measure the wavelength of the initial waves.
Use v = f * lambda
Measure the depth, d, using a ruler against the side of the tray.
Avoid parallax error; make sure the water is still before measuring.
You might want to repeat the wavelength measurement, for a given frequency, to calculate the mean speed.
Plot a graph of v / (sqrt(d)). This gives a straight line through the origin, with a constant gradient of sqrt(g).
Use a line of best fit.
Hence, v is directly proportional to sqrt(d), and thus v = sqrt(d*g).

2nd 6 Marker on verifying the equation tan∆=qE/w [6 marks] (again, you may have used a different method):
The charge q, distance between the two plates, the position of the camera (when measuring the angle); and w, must all remain constant.
Vary the voltage, as E is directly proportional to V (for a constant distance, d).
Measure the angle for each iteration. Avoid parallax error.
Repeat the angle measurements (for each voltage), to calculate the mean angle.
Use a voltmeter with a high accuracy (many sig figs) to be precise.
Plot a graph of tan(theta) / E (where E=v/d), or alternatively tan(theta) / v, to give a straight line through the origin. Plot a line of best fit (and extrapolate).
The gradient equals q/w (or otherwise, q/wd).
Hence, tan(theta) is directly proportional to E
Last edited by RedGiant; 4 weeks ago
4
4 weeks ago
#2
8V for the potential divider
2.2 * 10^12 J for the fission reaction
20ms^-2 for the acceleration of the oil droplet
3.67 billion years for the age of Earth question

I can't remember what question numbers they were.
1
4 weeks ago
#3
age of universe: 3.6 billion

some people got 7 V and some got 8 V
0
4 weeks ago
#4
I just realised the mistake I made which got me 5.5V lol, and doing it now gets me 8V too
0
4 weeks ago
#5
How did you get acceleration of oil droplet
(Original post by RedGiant)

EMF question [3 marks]: 8.0 volts (it may actually be 7.0v, need more opinions)

Energy released by nuclear fission [4 marks]: 2.2 x 10^12 Joules (2 s.f)

Acceleration of oil droplet [3 marks]: 20 ms^-2

Age of universe [3 marks]: 3.6 billion years (may vary slightly if you rounded early or not).

Graph of stationary wave [2 marks]: You had to draw the anti phase equivalent. There were 4 nodes.

Draw/describe the X-Ray gun [3 marks]: Draw the diagram of what it looks like. There is a hot cathode fillament, and the anode is the target metal. A high p.d. is used. A vacuum is needed in the chamber. 99% of energy of electrons is converted to thermal energy; hence the need for water-cooling.
0
4 weeks ago
#6
Multiple choice
C
D
B
C
A
A
C
B
D
C
B
A
D
B
B
Not sure if they are correct
2
#7
(Original post by limegreen16)
How did you get acceleration of oil droplet
Use the fact that E is directly proportional to V, and therefore F increases by the same factor that V does. So you multiplied the force by 3, minus the weight, and use a = (F / m) to get a.
1
4 weeks ago
#8
Sameee I got 29.4 and wrote 29 but now I keep on seeing people got 20 so I’m confused
(Original post by harry.samuel24)
I got acceleration was 29ms^-1
B = 0.23 T
3.6 billion
8 V for emf
1
4 weeks ago
#9
(Original post by RedGiant)
Use the fact that E is directly proportional to V, and therefore F increases by the same factor that V does. So you multiplied the force by 3, minus the weight, and use a = (F / m) to get a.
0
4 weeks ago
#10
I got B=0.23 T
somehow got 29ms^-2 but everyones saying 20 so idk where I messed up
3.6 Billion years
both 6 markers were a joke
2.2x10^12 J for the fission question

shambolic paper overall
1
#11
(Original post by limegreen16)
Can't remember exactly, but I just did the given weight / 9.81.
0
4 weeks ago
#12
Multiple Choice I got
C
D
B
C
A
B
C
C
D
C
B
A
A
B
B
0
4 weeks ago
#13
That's probably because you used a=F/m , but used 3x the weight instead of 2x the weight. The resultant force on the particle would've been 2x the weight (3.6x10^-15 N) or something, not 5.8x10^-15 N . I did the same thing then realised right at the end of the exam
(Original post by _zoe)
Sameee I got 29.4 and wrote 29 but now I keep on seeing people got 20 so I’m confused
0
4 weeks ago
#14
Yeah 30g isn’t 7.8 moles
(Original post by Malph)
I got 1.3x10^14 for the fission, where did I go wrong?

Mass of uranium = 30g = 7.8 mols = 4.7x10^24 molecules
Energy from 1 reaction = 0.19u * c^2 = 2.8x10^-11joules per reaction

Total energy = number of molecules * energy for a reaction
1
4 weeks ago
#15
Why did some people get 8V for EMF? I worked out current first across 75 ohm resistor then applied v=ir to the 100 ohm resistor
0
4 weeks ago
#16
Ooh I see. Well that’s 3 marks gone
That's probably because you used a=F/m , but used 3x the weight instead of 2x the weight. The resultant force on the particle would've been 2x the weight (3.6x10^-15 N) or something, not 5.8x10^-15 N . I did the same thing then realised right at the end of the exam
0
4 weeks ago
#17
Added the pds of the LED and the 50 ohm resistor together, think it was 3v. Then found the current in the circuit which was 0.05A. Found the pd across the series resistor before the combination and then just added them together to get the total emf of the circuit which was 8v
(Original post by Montytheragdoll)
Why did some people get 8V for EMF? I worked out current first across 75 ohm resistor then applied v=ir to the 100 ohm resistor
0
4 weeks ago
#18
Got slightly different to this. I had the last one as D for sure with the transformer equation. And for the expression of charging the capacitor it needed to be 0.9= not 0.6= because it was charging not discharging so I got B
(Original post by hz__23)
Multiple Choice I got
C
D
B
C
A
B
C
C
D
C
B
A
A
B
B
0
4 weeks ago
#19
It’s fine I also got 29. We will definitely still get 2 marks out of the 3 because we just forgot to change the electric force to the resultant force, every other part of our working was correct
(Original post by _zoe)
Ooh I see. Well that’s 3 marks gone
0
4 weeks ago
#20
Oh I did similar working, actually so I may still get 1 or 2 marks
Added the pds of the LED and the 50 ohm resistor together, think it was 3v. Then found the current in the circuit which was 0.05A. Found the pd across the series resistor before the combination and then just added them together to get the total emf of the circuit which was 8v
0
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