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# OCR Physics G484 - June 2013 Unit 4 (OFFICIAL RETAKE THREAD) Watch

1. (Original post by Kreayshawn)
do we ever actually have to use
x = A sin(2pft)
I haven't seen any question on the past papers so far that requires you to use sin. But if the displacement starts at midpoint (graph starts at 0), then you have to use sin. If the displacement starts at max (graph starts at 1), it's cos. Also, the most important thing for both of them: USE RADIANS!

Anyways, here are some things I think are likely to come up tomorrow:
• Resonance - when it's useful i.e. microwaves, radios; and problems i.e. bridges, vehicles etc.
• Proving Kepler's Law
• Assumptions of kinetic theory of gases
• Circular motion - there wasn't much circular motion so I'm thinking maybe the 'why's there a force required to keep an object in circular motion' question might turn up.
• SMH - graphs maybe. As long as you get the displacement one, the others are fairly easy. Velocity is a 1/4 ahead of displacement, and acceleration is anti-phase of displacement.

Overall, I think there might be easy calculation questions, and more of the wordy questions. I hope it's the other way around though haha.

Well, good luck y'all!
2. Any ideas on what might come up tomorrow? anyone notice anything that hasn't come up at all?
3. (Original post by Jullith)
Am I right in saying, that on a temperature-time graph for ice melting, when in a solid state the specific heat capacity is less so the gradient of that point in the graph is steeper, but when it becomes a liquid the specific heat capacity is more so the gradient of that point on the graph is less steep?
yes that sounds correct to me

What are the benefits of a high/low specific heat capacity value? (is higher better?)
5. (Original post by Jullith)
Am I right in saying, that on a temperature-time graph for ice melting, when in a solid state the specific heat capacity is less so the gradient of that point in the graph is steeper, but when it becomes a liquid the specific heat capacity is more so the gradient of that point on the graph is less steep?
the way i work this out is though

P = W/t = mcT/t

so T = (P/mc) t with P/mc the gradient.

6. (Original post by Kreayshawn)

What are the benefits of a high/low specific heat capacity value? (is higher better?)
A low heat capacity means that less energy has to be supplied per unit mass to raise its temperature by 1 K. So if the specific heat capacity is low, the substance will heat up faster.
the way i work this out is though

P = W/t = mcT/t

so T = (P/mc) t with P/mc the gradient.

Ohhhh, that's great. Thanks! I've never seen that equation before. I'll have to keep it in mind.
8. Can anybody write out their Brownian Motion experiment summary please??
9. I'm quite looking forward to this exam. I was shocked that I dropped so many marks on it in January (39/60 - they'd missed a few marks but even so it was a lot lower than what I thought I'd get) so idk how they'll mark this paper. I'm expecting a few awkward questions, hopefully not so much emphasis on geostationary satellites. Either way, after how badly I think I did on FPFP last week, my hopes for an A* have been shattered, and I'm doubtful that I'll get an A >.< Good luck everyone, hopefully the exam will go well for us all
10. (Original post by absism)
I haven't seen any question on the past papers so far that requires you to use sin. But if the displacement starts at midpoint (graph starts at 0), then you have to use sin. If the displacement starts at max (graph starts at 1), it's cos. Also, the most important thing for both of them: USE RADIANS!

Anyways, here are some things I think are likely to come up tomorrow:
• Resonance - when it's useful i.e. microwaves, radios; and problems i.e. bridges, vehicles etc.
• Proving Kepler's Law
• Assumptions of kinetic theory of gases
• Circular motion - there wasn't much circular motion so I'm thinking maybe the 'why's there a force required to keep an object in circular motion' question might turn up.
• SMH - graphs maybe. As long as you get the displacement one, the others are fairly easy. Velocity is a 1/4 ahead of displacement, and acceleration is anti-phase of displacement.

Overall, I think there might be easy calculation questions, and more of the wordy questions. I hope it's the other way around though haha.

Well, good luck y'all!
What do you mean by the last point about the acceleration and velocity being ahead of displacement? Would you have to find the gradient if the curve to find the velocity?

Also can someone tell me whether to use centigrade or kelvin when working out e=mct? I normally use centigrade but in some past papers where they've asked us to work out the temperature change, they've assumed the answer was in kelvin and that we didn't have to add 273 to convert the answer from the equation from centigrade into kelvin (like you'd normally do)
11. (Original post by imogen--)
Can anybody write out their Brownian Motion experiment summary please??
First introduce smoke grains into a glass cell. Then illuminate the glass cell using a 12v filament lamp and finally place glass cell onto a microscope. Also use glass rod to enhance viewing of the smoke grains.
12. (Original post by imogen--)
Can anybody write out their Brownian Motion experiment summary please??
- Draw diagram (if asked) of a smoke cell encasing smoke, the cell covered by a cover slip and a microscope on top. From one side draw an arrow labelled "light" coming towards the smoke cell.
- Smoke grains show up as tiny specks of reflected light but are too small for details to be distinguished.
- Each grain follows a jerky, erratic path as a consequence of moving, and also have haphazard motion.

Conclusions:
- Movement of smoke particles caused by randomly moving air molecules.
- Smoke particles are continuously moving because air molecules are continuously moving.
- Smoke particles are visible but air molecules are not, so air molecules must be very small (smaller than smoke particles).
- Small movement of smoke particles is due to the large numbers of air molecules hitting from all sides.

I've probably missed some stuff out but hopefully this has helped
13. (Original post by Myocardium)

Also can someone tell me whether to use centigrade or kelvin when working out e=mct? I normally use centigrade but in some past papers where they've asked us to work out the temperature change, they've assumed the answer was in kelvin and that we didn't have to add 273 to convert the answer from the equation from centigrade into kelvin (like you'd normally do)
Temperature change in E=mc(change in) T will be the same whether Kelvin or Degrees Celsius as they have the same divisions/scale.
An increase of 5'C is the same as an increase of 5K

You need to use Kelvin for pV=nRT and E=3/2kT.
14. Are the plateus in a temperature against time graph indicate the latent heat of fusion and latent heat of vaporisation?
15. What is the difference between acceleration and mean acceleration? 'A plane accelerated from rest to v=55ms-1 in 2.2secs' Calculate mean acceleration.
Do I need to halve the velocity valuee and use a=(v-u)/t?? It gives mass as well.
Thanks.
16. (Original post by abu125)
Are the plateus in a temperature against time graph indicate the latent heat of fusion and latent heat of vaporisation?
When it plateaus, energy used to break bonds, not to heat the substance. So I assume it must be as the definitions are 'a substance at a constant temperature'?
17. is there a definition for period and phase difference?
+ how do we calculate time period from number of oscillations? time/number of oscillations?
so frequency would be n/t (as it's 1/[t/n])

thanks jullith!
18. Besides these experiments (see list below) are there any others they could ask about?

Oscillation of a loudspeaker cone
Simple harmonic motion of a spring-trolley system
Damping experiment with hacksaw blade/stringy metal strip
Barton's pendulums
Brownian motion experiment
Specific heat capacity experiments
Boyle's law experiment
Charles' law experiment
19. Is this the right experiment for finding the specific heat capacity of a metal??? Or are we not allowed to presume we know the specific heat capacity of water???

-Measure the mass of an empty beaker, then fill the beaker with water at room temperature and measure the water's mass (in kg) by subtracting the weight of the beaker

-Heat a metal in a hot bath or with a heated rod and record its temperature.

-Record the temperature of the water at room temperature and immediately place the heated metal in the water, submerged.

-Observe the increasing temperature of the water and record the stabilised temperature.

-Remove the heated metal, drying it, and weigh it to find its mass in kg.

Now by knowing the specific heat capacity of water...

(water) mcT=(metal) mcT
20. Do all these questions, and you will be unstoppable!
Attached Images
21. A2 G484 The Newtonian World Definitions.pdf (250.6 KB, 103 views)
22. Circular Motion and SHM Questions.pdf (262.1 KB, 415 views)
23. Thermal Physics Questions.pdf (150.0 KB, 420 views)
24. Attached Files
25. Circular Motion and SHM Answers.rtf (324.4 KB, 63 views)
26. Thermal Physics Answers.rtf (148.7 KB, 70 views)

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