Physics: Discussion, Questions & Potential Solutions

[vox nihili]

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[KiNSKi01]

For 5b i got same answer but I'm not sure if method is alright (cos I don't know anyone else who did it this way and part or working out I repeated in the following question)

Add power loss in transmission lines to power dissipated by globe to get total power (72+48=120). Then divide by 3A to get Voltage output of power supply. (120/3=40V)

[Freddie Hg]

For 5b i got same answer but I'm not sure if method is alright (cos I don't know anyone else who did it this way and part or working out I repeated in the following question)

Add power loss in transmission lines to power dissipated by globe to get total power (72+48=120). Then divide by 3A to get Voltage output of power supply. (120/3=4A)

you mean 40 volts? i did the exact same thing so hopefully its right

[KiNSKi01]

lol yeah thats what i meant 
fixed it now

[Alexmaths]

Did anybody else find that power transmission question the hardest part of the exam? it was the only question I was very lost.

[aussiboi]

Did anybody else find that power transmission question the hardest part of the exam? it was the only question I was very lost.

im with you

[Freddie Hg]

Here are the answers (q) 4 , 5  & 6

4(a) Vrms is the DC equivalent voltage = 4/ SQRT(2) = 2.83 V
4 (b) Doubling in frequency will double the emf and half the period. You should draw double amplitude (+/- 8 V).

(5) (a) Current in the Globe 4 x 3 = 12A, Voltage = 4.0 V, So Power dissipated in the globe = 12x4 = 48W
(b) V output = Vdrop in the line + V Primary coil = 3x8 + 16 = 40 V
(c) P loss = I^2 R = 72 W

(d) Assume Globe still operate at 48 W and the V = 4.0 V.
Current in the Globe = 12A. Therefore the current in transmission line = 12 / 8 = 1.5 A, Ploss = I^2 R = 18 W

(e) - For constant power increasing voltage would reduce current in the transmission line (P = VI). High current contributes to increasing  Fire Risk, thus low current reduces fire risk.

- Also High voltage in the line or lower current in the line contributes to lower lower loss as Ploss = I^2R. So more efficient power transmission can be achieved by High voltage electric power transmission.

for e ive seen so many different answers! i did the ploss one and that an alternative way of minimizing power loss is to use wires with less resistance such as gold, this impractical and expensive increasing voltage is the most efficient way to transmit power

[aussiboi]

I put power loss and more efficient would that give me 2 marks???

[Richard Feynman 101]

for e ive seen so many different answers! i did the ploss one and that an alternative way of minimizing power loss is to use wires with less resistance such as gold, this impractical and expensive increasing voltage is the most efficient way to transmit power

No moving parts and less capital cost.

[Freddie Hg]

No moving parts and less capital cost.

is that what you said? i like it!

[wanigara]

(Q6) Spring
(a) Conservation of Energy mg (2.5) = 1/kx0.5^2, K = 392 N/m
(b) Initially object accelerate at 9.8 ms-2, and speed was increasing. When the ball hit the spring its acceleration begin to decrease as Net force = mg - kx.,, however still speeding up. Once mg = kx, it reaches the maximum speed and acceleration is zero as mg = F (spring force).
SO answer is 0 ms-2
(c) at Maximum speed F= -mg andf F= - kx
So x = mg/ K  = 0.05 m

[wanigara]

Please let me know if you need answers to any particular question (s) first.

[Freddie Hg]

Please let me know if you need answers to any particular question (s) first.

if you put 9.8 ms instead of 9 how many marks do you lose? im guessing both 2 marks

[wanigara]

if you put 9.8 ms instead of 9 how many marks do you lose? im guessing both 2 marks

which  question  ?

[Freddie Hg]

which  question  ?

6 b