# Weird electricity question?

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#1
Hi,

There is a physics past paper question that is causing quite a bit of confusion with me and my peers. It is OCR June 2009, Unit G482 - Electrons, Waves, and Photons, question 2) (b) (i). (See image attached)

First bit - easy peasy. Total resistance = 0.8Ω. No problem.

The second bit is weird. According to Kirchoff's second law, the sum of the e.m.f.'s should be equal to the sum of the p.d.'s - therefore Σe.m.f.'s should = 14V + 7.6V = 21.6V. Yet for some reason, the mark scheme says the answer is 6.4V (which you would get by doing 14V - 7.6V). We can't seem to figure out why you have to subtract the p.d.'s, though I suspect it has something to do with the fact that the positive terminal of the battery is connected to the positive terminal of the charger.

Any help would be much appreciated. Cheers!
0
4 years ago
#2
If you connect positive terminals in sequence, i.e. + to +, as above, you subtract the potential differences. Likewise, if you connect two cells in series with positive to negative, you will add together the potential differences. Makes sense if you think of it in the sense of each cell is trying to push a current, and if they push in to each other, you take them away, if they are in sequence, you add them up.

Hope I helped!
1
4 years ago
#3
(Original post by cakeboi)
Hi,

There is a physics past paper question that is causing quite a bit of confusion with me and my peers. It is OCR June 2009, Unit G482 - Electrons, Waves, and Photons, question 2) (b) (i). (See image attached)

First bit - easy peasy. Total resistance = 0.8Ω. No problem.

The second bit is weird. According to Kirchoff's second law, the sum of the e.m.f.'s should be equal to the sum of the p.d.'s - therefore Σe.m.f.'s should = 14V + 7.6V = 21.6V. Yet for some reason, the mark scheme says the answer is 6.4V (which you would get by doing 14V - 7.6V). We can't seem to figure out why you have to subtract the p.d.'s, though I suspect it has something to do with the fact that the positive terminal of the battery is connected to the positive terminal of the charger.

Any help would be much appreciated. Cheers!
I hope the example shows in the following link explain the ideas behind it.

There is a sub-section title called Signs and directions. It would have what you need.
1
4 years ago
#4
I done physics last year, wasn't a strong point so I can't help sorry! Was just wondering though, what level is it? (I'm scottish, so our highers are like A-levels?)
1
#5
(Original post by fergijane88)
I done physics last year, wasn't a strong point so I can't help sorry! Was just wondering though, what level is it? (I'm scottish, so our highers are like A-levels?)
Hey, this is from an A-Level paper.
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