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# Edexcel Physics Unit 2 "Physics at work" June 2013 Watch

• View Poll Results: The last question - Does resistance increase or decrease?
It increases ( using V=IR or some other method)
70.73%
It decreases using the 'lattice vibrations' theory
29.27%

1. The experiment says:

set the voltage so that the motor can lift a mass of 100g a height of 1m in the time t.
Calculate the power in using P=IV
and the time taken for the mass to be raised at the corresponding voltage.
Repeat at different voltages
calculate efficiency using Power out/Power in
2. Power out is calculated using mgh/t
3. (Original post by ALevel96)
Power out is calculated using mgh/t
Yup :P
4. (Original post by hamzeh h)
Yup :P
Can't believe I was so stupid on the drift velocity question haha.
The answer was indeed V/2!

How did you do in unit 1?
5. (Original post by hamzeh h)
i did it in january :P
Oh I see what did you get?
6. (Original post by GCSE-help)
No because it was being operated at below it's normal rating! I think people are missing a massive key point. The question asked for the effect of low current ON the resistance. So it's not asking us to consider WHY the current was low, just accept that it was! And then we had to find the effect of the low current on the resistance!
But I based my entire argument on the "fact" that they are in parallel to the mains power supply so the sum of the voltage should be still equal to 230!
Your answer makes more sense but the question didn't mention if they were taken out of the mains or not?!

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7. (Original post by sarah.102)
Negative. At least I hope so.
I made a thread here on the physics forums so hopefully some genius can help us its driving me insane. http://www.physicsforums.com/showthr...=1#post4408037
8. (Original post by Mo_maths)
I made a thread here on the physics forums so hopefully some genius can help us its driving me insane. http://www.physicsforums.com/showthr...=1#post4408037
This came a LOT in previous papers.
Its obvious its negative!

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9. (Original post by StUdEnTIGCSE)
This came a LOT in previous papers.
Its obvious its negative!

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Some guy before said it was positive because if you look were the voltmeter is its not over the load resistance, not sure if he's right or wrong its just making me worry
10. (Original post by Mo_maths)
Some guy before said it was positive because if you look were the voltmeter is its not over the load resistance, not sure if he's right or wrong its just making me worry
The voltmeter was across the internal resistance.
V=E-rI

So why worry?

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11. (Original post by StUdEnTIGCSE)
The voltmeter was across the internal resistance.
V=E-rI

So why worry?

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Its usually above the load resistance so the resistor if you look at the pics online such as
http://www.s-cool.co.uk/a-level/asse...resistance.gif
12. I read online this

"For an external load resistor, you connect a voltmeter's (say) negative terminal to the terminal of the battery, and measure the voltage across the resistor. You get Ohm's Law: V=IR
For an internal source resistance, you connect a voltmeter's negative terminal to the negative terminal of the battery, and the positive terminal of the voltmeter to the positive terminal of the battery. Increasing the current drawn from the battery causes the voltage of the battery to decrease. In this case, the voltage seems change like -IR, the opposite of Ohm's Law above." so its negative right????
13. (Original post by Mo_maths)
Its usually above the load resistance so the resistor if you look at the pics online such as
http://www.s-cool.co.uk/a-level/asse...resistance.gif

Well both the circuits are one and the same.
Voltmeter has very high resistance therefore no current flows through it.
In this and the circuit in the exam the voltmeter is measuring the potential difference across the cell AND the load resistance (including that of the ammeter) because in parallel the voltage is the same.

So no worry!

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14. (Original post by StUdEnTIGCSE)
Well both the circuits are one and the same.
Voltmeter has very high resistance therefore no current flows through it.
In this and the circuit in the exam the voltmeter is measuring the potential difference across the cell AND the load resistance (including that of the ammeter) because in parallel the voltage is the same.

So no worry!

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AHHH YEAHHH!!!!!! Thanks man, feel much better now
15. On the last question. Wasnt it a series circuit. Cause of it was in series the. Of 1 bulb Is switched off the other one dont light up as it is an incomplete Circuit. :O

Am i wrong?

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16. (Original post by Freddy-Francis)
On the last question. Wasnt it a series circuit. Cause of it was in series the. Of 1 bulb Is switched off the other one dont light up as it is an incomplete Circuit. :O

Am i wrong?

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yeah it was series
17. whats the explanation for the bulbs in series and not the other way?
18. On the E.M.F question did people have a straight or curved gradient? Mine was curved and I think that's wrong how many marks would I get for doing it negative and stating how to work them out correctly?
19. (Original post by StephenNaulls)
On the E.M.F question did people have a straight or curved gradient? Mine was curved and I think that's wrong how many marks would I get for doing it negative and stating how to work them out correctly?
Mine was a straight line with a negative gradient
the y intercept is the e.m.f
the negative of the gradient is the internal resistance
i guess there were 3 marks for the graph and 2 for explaning how to find e.m.f and r so i think you'll get some credit
20. Guys i added a poll for you to vote
i just want to see what the majority went with ...

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