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Physics: Post your doubts here!

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Can someone explain this to me please
''For the greatest efficiency the external resistance must be much greater than the internal resistance of the cell. However, for the maximum power to be delivered to the external circuit the internal resistance must be equal to the resistance of the external circuit, although the cell will only be 50% efficient.''
Why should the cell be 50% efficient but not fully efficient please?
 
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20151120_102112 (2).jpg When strain is increased,it implies that length decreases according to strain=extension/length
when length decreases,resistance also decrease according resistance=resistivityXlength/area,right?
Then how come the answer is E?Maybe i can't understand the question itself.Help please!!
Thanks in advance :)
 
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Can someone please give me the answer for this question? The answer is A, right?
 

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Help please! :' I can not understand what happens when cells in a battery are wrongly connected like in the following 2 questions.Thanks in advance guys :)
Question 1 )
Five cells of emf E are connected in series each having internal resistance r. if due to oversight, one cell is connected wrongly, then the equivalent emf and internal resistance are

[a]3E and 5r 2E and 3r

[c] 4E and 5r [d]5E and 3r

question 2)

20151122_120427 (2).jpg 20151122_120450 (2).jpg
 
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Can someone please give me the answer for this question? The answer is A, right?
Yea me too i think the answer is A as constant speed implies that acce=0
therefore 0=driving force i.e F _ opposing force i.e f
therefore f=F.
Can anybody confirm please
 
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Sketch graph with labelled axes to show how current through the wire will vary with p.d. across it if temperature increases as current increases.Can anybody help me out please
 
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