Physics, Chemistry and Biology: Post your doubts here!

eureka!

eureka!

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Phoenix Blood said:
eureka!

Q) Explain why energy losses in the transmission cables are lower when the voltage is
high.


Ans) high voltage means) low(er) current (but V is directly prop. to R!)
for given supply power
(low(er) current means) less heat/thermal energy (generated in cables) OR P = I2R (?)
for given resistance (of cables)
cables heated by current

Explain...
Click to expand...
Suppose you wanna send 4 MW of power through a cable of 4 Ω.
P=IV. You've got many options, e.g. sending 10A at 400 000 V, 160A at 25000 V, etc.
Now, what would the p.d. across the wire be if you deploy the 25 000 V solution?
V=IR; 160 x 4 = 640V
Power lost would be 640 x 160 = 102 400 W, just 2.6% of the 4MW being sent.

Therefore, increasing the voltage reduces p.d. across cable, reducing power lost.
 
gary221

gary221

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hard_worker said:
http://papers.xtremepapers.com/CIE/Cambridge IGCSE/Physics (0625)/0625_s10_qp_31.pdf
part (b) Q 11
Click to expand...

Okay, we know that Resistance α length/Area.
Since the coil has a diameter half of that the original one → area of the new coil = ¼ Area of the original one.
Which means, keeping length constant, the new resistance should have been R = length/(Area/4) → R = 4length/Area → R = 4(Resistance of original coil) = 4*4 = 16 ohm.

Whereas, the new resistance is actually V/I → 1.8/0.3 = 6 ohm.
Since R α L → ratio of resistances = ratio of lengths → 6/16 * 100 = 37.5%

Hope you get it.
 
hard_worker

hard_worker

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gary221 said:
Okay, we know that Resistance α length/Area.
Since the coil has a diameter half of that the original one → area of the new coil = ¼ Area of the original one.
Which means, keeping length constant, the new resistance should have been R = length/(Area/4) → R = 4length/Area → R = 4(Resistance of original coil) = 4*4 = 16 ohm.

Whereas, the new resistance is actually V/I → 1.8/0.3 = 6 ohm.
Since R α L → ratio of resistances = ratio of lengths → 6/16 * 100 = 37.5%

Hope you get it.
Click to expand...
THANK YOU
 
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