Physics Q

[Guest iblearner]

@kevG

All my physics Q

[kevG]

2. Should be 8% since you add % uncertainties when you divide and divide % uncertainties by 2 when you square root so you get (6+10)/2

3. I would go with C cause you need an exponential graph to show acceleration and they said non zero acceleration and velocity and we know the derivative and double derivative and they both are non zero in C

6. We know speed is 2piR/T = v and they ask us to find the max speed when R is 2V under the same conditions so this includes the time taken. It should be just a question with ratios which then equates to 4piR/T = x and you solve for x which gives 2v. So I would go with that

11. The answer is C, 50 cause gain in energy = loss in energy, so mc(f-i) = mc(i-f), and c gets cancelled cause both the blocks are iron... So you just sub in the i and find f which gives 50

12 isn't in our syllabus anymore, they moved it to an option

I don't think 13 is in our syllabus anymore either

[Guest iblearner]

15 minutes ago, kevG said:

2. Should be 8% since you add % uncertainties when you divide and divide % uncertainties by 2 when you square root so you get (6+10)/2

3. I would go with C cause you need an exponential graph to show acceleration and they said non zero acceleration and velocity and we know the derivative and double derivative and they both are non zero in C

6. We know speed is 2piR/T = v and they ask us to find the max speed when R is 2V under the same conditions so this includes the time taken. It should be just a question with ratios which then equates to 4piR/T = x and you solve for x which gives 2v. So I would go with that

11. The answer is C, 50 cause gain in energy = loss in energy, so mc(f-i) = mc(i-f), and c gets cancelled cause both the blocks are iron... So you just sub in the i and find f which gives 50

12 isn't in our syllabus anymore, they moved it to an option

I don't think 13 is in our syllabus anymore either

12 and 13 is in our options, Engineering Physics.

For Q3, the answer is D, idk why.

The others seem right, thanks. :)) Good luck for your exams tomorrow. 

[kw0573]

When all the questions are from one paper, you can try look at the subject report / examiner's comments as they give some explanations.

6.B@kevG 2 pi R / T is angular velocity or angular frequency. velocity is angular frequency multiplied by R. We can just use circular motion a = v² / r. If r is doubled, v² is doubled. v is only multiplied by a factor of sqrt (2)

Note you won't need to know how to answer multiple choice questions for your options.
12. D. Know that temperature increases with compression along an adiabatic path (hopefully this makes intuitive sense). Also need to consider Q = PΔV + ΔU = 0 for adiabatic, PΔV is negative (smaller final volume) so ΔU is positive. Positive ΔU indicates increase in internal energy. 
13. A Q = PΔV + ΔU = 400000 * (0.004-0.0001) + 1800 = 1200 + 1800 + 3000. Into gas because 3000 is positive. 

14. C This is what the wave looks like at a single moment in time. In one period, the particle at d = 0 moves from 2 to -2 back to 2, a total of 8 cm = 0.08m. The period is given to be = 5ms = 0.005 s. Average speed = total distance traveled / change in time = 0.08 / 0.005 = 16m/s. Note that average velocity is 0 but speed is just the magnitude so it can't be A. If you have to guess, never guess average speed to be 0.

16. B The speakers produce waves in phase. When the difference of distance from each speaker is some integral multiples of wavelength then they are constructive --> max, when they differ by half or 3/2 or 5/2 etc of a wavelength then destructive -- min.

20. D The intensity is always halved for unpolarized light through ONE SINGLE polarizer.

21. B Power dissipation is constant because current, resistances and voltage are constant. 
The most useful form of power equation is P = V²/R. By Kirchhoff's Loop Rule, parallel resistors have same potential difference. In the 20 ohm resistor, it has double the resistance so half the power, so half the energy dissipated in same amount of time.  

22. D Voltmeter works by measuring potential difference by connect to main circuit in parallel, typically around a load. To ensure that only a tiny amount of current goes through the voltmeter, it is made with a really high resistance. What happens by Kirchhoff's loop rule is that the voltmeter, if incorrectly connected in parallel, would draw all the potential difference (leaving 0 to the resistor, which now has such a small R compared to the voltmeter). The current is now determined by the voltmeter. I = V/R, with a huge resistance the current tends to 0. These approximations acceptable by IB.

23. D The particles have same initial velocity, same distance to travel, and same time. They must have also same constant acceleration (s = ut + 1/2 at^2)
a = F / m = electric force / m = E field * charge / m. E field is constant. q/m must be same.

30. C Becquerels is decays / unit time. S basically gives decay/time * time = number of decays. E * S give the total energies by the decays. Note that to get average from integrals you always have to divide by something.

31. A. You are allowed to assume gravitational force is really neglible (it's like saying gravity down a ramp is not affected by mass of ramp). Same electrostatic force, same distance same force. 

32. A kinetic energy = 1/2 * momentum * velocity. By unit analysis only A works. Note in D, numerator is energy, denominator is planck * some R, but in correct answer it's c in denominator and c is planck * frequency not planck * some length. 

Found this online, I don't know what it means though. I think the new nuclear physics doesn't go this in depth. These formula only appear in the old syllabus.


36. A. Albedo measures reflected intensity to incident. So 0.6 * 200 = 120 W m^-2 is reflected. We are told constant temperature so inward intensity = outward intensity. 200 - 120 = 80.

37. D. Energy from black body comes into X, X emits own radiation because its own black body properties. C cannot be right because we need A to be area of blackbody and the black body has different surface area than X. You need to use process of elimination because to actually get the right answer you need to know how the equation is derived. But A, B, C are clearly wrong because either A(rea) is wrong or the black body box is not considered. 
38. D. Power = energy / time = mgh / t = ρ * (V/t) * g * h. g = 10. Power = (1000) (10) (10)(100) = 10⁷ W

Edited May 5, 2016 by kw0573
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