A blob question and a voltage question from the same physics paper

[aspodkfpo]

Homework Statement

https://www.asi.edu.au/wp-content/uploads/2015/08/NQE_2007_Physics_solutions.pdf

Q13 D) E) F)
Q15 C)

Relevant Equations

Conservation of momentum, conservation of energy

P = v^2/r
P = I^2 R

For Q13 D) E) F), the model I had built was based on elastic collision with the blobs going out of the box, but colliding with air particles and then going back into the box. Thus, momentum would be redirected and it would be higher than what the answer says and kinetic energy would also be conserved.
Is my model wrong and how so?

Q15) C)
Can someone right a clearer, more concise explanation for this?

 

[BvU]

but colliding with air particles and then going back into the box

How likely is that ?

c. Given that the power station is providing a fixed amount of power, should the power station’s voltage output be increased or decreased (or does it not have any effect) to reduce the power losses due to the resistance of the transmission lines? Justify your answer. (Max 6 lines) (4 marks)

Increasing the voltage output of the power station will reduce the power losses through the power lines. The power lost through the power lines is given by I 2 R, and hence increases as I increases. As the power station’s power output is given by P = IV , increasing its voltage will reduce the current it produces, which is the same current flowing through the power lines. (6 lines)

The resolution to the apparent paradox that the power drop, P, across the power lines is both proportional to the square of V, and to the square of I, is that V, the potential drop across the power lines is not the same as the voltage output of the power station, however the current that passes through the power station is the same current that passes through the power lines. As the current output of the power station reduces, the voltage drop across the power lines would also reduce.

Seems pretty clear to me.

The crux is that the voltage drop over the power lines is proportional to $I$, so $P_{\rm\, loss} = I^2 \,R_{\rm power\ lines}$
and $V$ does not appear in there.

 

[haruspex]

Please use separate threads for unrelated questions.

 

[hutchphd]

Whomever made up question 4 should receive a failing grade. Their answer is simply incorrect, and confusingly so. The momentum changes are equal but opposite.

 

[haruspex]

Whomever made up question 4 should receive a failing grade. Their answer is simply incorrect, and confusingly so. The momentum changes are equal but opposite.

Debatable. I read "change" as a magnitude. How much did the temperature change? One degree.
And it is whoever (nominative).

 

[hutchphd]

Yes it is nominative. I am worried because I actually convinced myself otherwise even though it wrong!
But change in a vector is a vector. In uniform circular motion the velocity changes. The question as stated offers no correct answer (if they wanted magnitude it should be so stated). This is an important exam for an unsophisticated cohort.

 

[haruspex]

But change in a vector is a vector.

True.