Physics OCR 2019 A-Level predictions paper 1

Hi how is everyone feeling about the paper on the 20th of May? What do you guys think will likely come up?

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I’m expecting large questions on kinetic theory of gases and a life cycle of stars question. Also any graphical analysis is almost guaranteed to come up so make sure you can find uncertainties in gradients and are confident with logs. There is also the potential for an uncertainty in mean question to come up because its rarely been asked.

Reply 2

Glycerate

Original post by Connor_h77

Line of worst bit, (Steepest gradient) compared to that of the line of best fit etc to work out the uncertainty in the gradient.
Uncertainty in mean? Mean, and the range/2
I'm expecting large questions on Young Modulus (Experiment) or even force constant, the uncertainties tie in nicely here.

Reply 3

ToysAreUs

-To calculate the uncertainty in mean I believe you would add abc uncertainties normally.
-To calculate the percentage uncertainty in mean I think you would add abc uncertainties of each individual value X then, (total abc unc)/(mean value) *100
-say we had (x1 x2 x3)/3=mean %unc=(total abc unc of x1 x2 x3 added)/(mean)*100
Please correct me if I am wrong
What are the potential 6 markers any ideas ?
Latent heat?
SOLID LIQUID GAS?
BIG BANG , CMBR?
star formation / evolution?

If someone could share thoughts / solutions to potential questions that would be great PLEASE and THANKYOU!

(edited 4 years ago)

Reply 4

ToysAreUs

Feel like they could throw in a cheeky kinetic theory 3/4 marker
possibly pressure on walls in terms of molecules eg.
What if volume stayed constant?
WHat if volume changed?
Temperature and volume relationship PV=NRT?

Reply 5

Rakul1479

iI think there's gonna be a 6 marker on SHM or circular motion and latent heat as they have already asked about PV=nRT and the big bang

Reply 6

Glycerate

The was a recent simple harmonic motion experimental question in the 2017 paper; however, there may be a possibility for a circular motion question to appear.
Regarding thermal physics, there is potential for experimental analysis regarding the specific latent of fusion, this question has not been asked before; There is a relatively fair amount of ideas to elaborate on- (Secondary funnel without a heater connection to measure the mass of ice melting from the ambient temperature, using this value to correct the mass melted as a direct result of the heater (Energy supplied = ItV)).
Pressure question regarding volume (Pressure will decrease considering that the frequency of collision between molecules and the walls of the container will reduce if the volume increased etc) It would be nice if there was a pressure question on temperature.

There could be a question on SHM damping, evaluating and analyzing the amplitude frequency graph, whereby forced oscillations at a relatively lower frequency below the natural frequency of the material will oscillate in phase at comparatively large amplitude. Resonance when driving = natural freq (Maximum energy transfer, oscillations are of maximum amplitude, violent, a phase difference of pi/2 radians). Damped oscillations smaller amplitude, out of phase etc.

For materials, there is quite a solid chance for an experimental 6 marker on the Young Modulus experiment. Clamping an end of the wire to a fixed end, placing a metre ruler parallel and beneath the wire (Resolution of 1mm), measure the natural length of the wire from the tip of the ruler to the marker (reference length), to reduce parallax error read at eye level. Measure the diameter of the wire at 3 various points along the wire using a micrometer/vernier callipers, using this information to calculate the cross-sectional area of the wire (Pi R squared), ensure the wire is taut. Placing hanging masses at one end of the wire (100g intervals),(Measure mass using electronic balance), when the wire comes to rest measure extended length and the extension (Final - original length) Repeat for same mass for average. Repeat process for a minimum of 6 different masses. Force = mg.

Two methods: Either work out the stress and strain from each measurement and plot a stress against strain graph gradient = Youngs Modulus. or Since E=FL/Ax, you can plot a force against extension graph. EAx/L =F, the gradient is equal to EA/L , times gradient by L/A to work out E.

Stellar evolution has not been asked before, so there is a good chance this might come up. Nebulae (Collection of dust and gas, consisting of denser regions whereby the gravitational attractive forces result in increased accumulation of dust and gas, this results in gravitational collapse acceleration. The denser regions gain mass, increasing in density, eventually the temperature increases whereby the gravitational potential energy is transferred to thermal energy. Protostar forms, a very hot and dense sphere of dust and gas, for a star to form nuclear fusion must be initiated inside the core. Nuclear fusion can only occur at significantly high pressures and temperatures in order to overcome the electrostatic repulsion of the positively charged hydrogen nuclei. A high temperature is essential to provide sufficient kinetic energy to the extent that the nuclei get close together for the short-range strong nuclear attractive force to take effect for fusion to occur. As temperature increases, volume decreases the gas pressure increases. Radiation pressure exerted by electromagnetic radiation. When the outwards radiation pressure = inwards gravitational compressive force, stable equilibrium is reached. This forms the main sequence star whereby the core undergoes hydrogen fusion.
This depends on the question, since they would not ask the entire evolution of a star, but a segment of it.

They will certainly be questions on Hubble's constant, spectral lines, stellar parallax (p=1/d)- I hope there is not a 6 marker on stellar parallax. The Hubble constant and redshift will go in nicely with CMBR as evidence of the big bang (I have already done a question on this on one of the papers, so not sure if this will come up). It would be good to discuss the anomalies such as dark energy causing acceleration of the universe, or gravitational attraction by other galaxies subverting the trend. Hubble's law only applies to galaxies. There may be a diffraction grating question.

You will have your standard fields questions, gravitational potential or attempting to find the point where the resultant field is zero. Maybe a derviation of either escape velocity, orbital velocity or Kepler's third law (That would be nice).

Reply 7

Glycerate

Original post by ToysAreUs

You get the marks for stating that the molecules are moving in random directions at random speeds, thereby randomly and elastically colliding with each other/walls of the container. Resulting in the distribution of kinetic energies (That would get you 2 marks according to set 2 mark schemes for OCR Paper 3). You could talk about the Maxwell Boltzmann distribution and root mean square speed if they asked about ideal gases and temperature.

Reply 8

Glycerate

"Explain how the kinetic theory of an ideal gas predicts the existence of gas pressure inside the bottle. Go on to explain why this pressure decreases when some of the air is removed from the bottle" (4 Marks)

Reply 9

ToysAreUs

Original post by Glycerate

THANKS

Reply 10

sooky12

what should I put as an answer for explaining why this pressure decreases when some of the air is removed from the bottle?

Original post by Glycerate

Reply 11

conniephillips44

There is a lower number of particles and therefore the frequency of collisions between particles and the bottle will be lower . Leading to the total force extorted by collisions being lower . I think ??

Reply 12

Glycerate

Original post by sooky12

what should I put as an answer for explaining why this pressure decreases when some of the air is removed from the bottle?

There are fewer air molecules within the fixed volume, thereby reducing the frequency and rate of molecular collisions with the walls of the bottle, thereby this reduced the rate of change of momentum, considering force is equal to the rate of change of momentum, a reduction in the rate of change will thereby reduce the force exerted on and by the molecules/wall. Pressure = Total force exerted by all colliding molecules / area of wall If the number of colliding molecules decreases, then the total force decreases.

Reply 13

Glycerate

molecules/particles have momentum (1) momentum change at wall (1) momentum change at wall/collision at wall leads to force (1) [allow impulse arguments] less air so fewer molecules (1) so change in momentum per second/rate of change is less [or per unit per time] (1) pressure is proportional to number of molecules (per unit volume) (1) max 5

Reply 14

ToysAreUs

Original post by sooky12

what should I put as an answer for explaining why this pressure decreases when some of the air is removed from the bottle?

Since P=nRT/V
P is proportional to N
Provided temp and volume of bottle remain unchanged
The amount of molecules of air is less since the air is being removed.
Therefore as moles decreases and temperature remains the same the pressure must decrease.

Similarly ..
an everyday example of this would be opening and closing a oven before and after preheating.
Opening the door after is a lot easier than opening it before because the pressure on the walls is greater.(P proportional to T)
Correct me if I am wrong