Hi Guys,
Any chance you can help me out with this one – I tried to find my teacher this afternoon, but had no look.
I’ve got some answers, but not sure if they’re right; the working out must be shown.
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The e.m.f of a cell X is measured using a digital voltmeter which gives a reading of 1.53v when connected directly across the terminals of X. When a 10 ohm resistor in connected across the cell terminals, the voltmeter reading falls to 1.48v. Assuming the digital voltmeter takes negligible current determine
a) the cell’s e.m.f (easy)
b) the cell’s internal resistance. (work out current first, then lost volts)
Answer: 0.34
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But how is the question? 😀
Thanks in advance,
Sam
By: PMN - 10th October 2008 at 22:08
Cheers Guys.
Paul, i’d done the other 29 😉
Ask me a question on sound systems, bass guitars or cameras and I’m fine. I’ll leave the clever stuff to you. 😉
Paul
By: B77W - 10th October 2008 at 21:42
Cheers Guys.
Paul, i’d done the other 29 😉
By: PMN - 10th October 2008 at 15:53
Go do your own homework!
And no, I’m not just saying that because I don’t have a clue how to answer the question. 🙂
Paul
By: Creaking Door - 10th October 2008 at 00:21
Current = V/R, so 1.48/10 = 0.148A
Internal Resitance = (Emf – (current x resistance across resistor)) / current
Internal Resitance = (1.53 – (0.148 x 10)) / 0.148
Internal Resitance = (1.53 – 1.48) / 0.148
Internal Resitance = 0.05 / 0.148
Internal Resitance = 0.3378
Internal Resitance = 0.34 Ohms (rounded to two decimal places)
It has been a long day hasn’t it, Sam! 😀
By: B77W - 9th October 2008 at 23:22
Then plug in the numbers to get r=0.338 Ohms
Care to expand on this? It’s been a long day… 😀
By: Arabella-Cox - 9th October 2008 at 18:58
Ah the joys of AS physics.
EDIT: Sorry, my wrong answers are of no use to you anymore. The post’s made while I was writing mine have corrected me. :rolleyes:
By: Flygirl - 9th October 2008 at 18:58
Also you could always visit http://www.physicsforums.com/
By: tomfellows - 9th October 2008 at 18:47
a) is obvious
b) You should know that the voltages in a series circuit always total the EMF.
ie Emf= voltage across resistor + voltage across the internal resistance of cell
From V=IR, we can say:
emf = (current x resistance across resistor) + (current x internal resitance).
Then do some simple algebra to get the Internal Resitance as the subject:
Internal Resitance = Emf – (current x resistance across resistor) / current
Current = V/R, so 1.48/10 = 0.148A.
Then plug in the numbers to get r=0.338 Ohms
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October 9, 2008 at 5:43 pm