AQA PHYA4 electric fields multiple choice question Watch

Mar1805
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Two identical positive point charges, P and Q, are separated by a distance of 4.0 m. The resultant electric potential at point M, which is mid-way between the charges,is 25.0 V. What would be the resultant electrical potential at a point 1.0 m closer to P?

A 8.3 V
B 12.5 V
C 33.3 V
D 37.5 V

(The answers C)
I know it cant be the top two cause the answer has to be bigger than 25.0V. But i don't know how to get 33.3V

(http://filestore.aqa.org.uk/subjects...1-QP-JUN14.PDF Q15)
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AshQueen
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The resultant potential on the middle particle is sum of the potentials from each side charge. The resultant potential is 25.0V. As the charges are idential you can divide 25.0 by 2 and you get each potential is 12.5V. Now you can calculate each charge (Q). To find the effect after the replacing it you can calculate each potential again and add them up so:

V=KQ/r

12.5=9.0x10^9 x Q/2

Q=2.77x10^-9

V=9.0x10^9 x 2.77x 10^-9/1 = 24.93V

V=9.0x10^9x2.77x10^-9/3=8.31

24.93+8.21=33.3V
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Ramy12343
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(Original post by AshQueen)
The resultant potential on the middle particle is sum of the potentials from each side charge. The resultant potential is 25.0V. As the charges are idential you can divide 25.0 by 2 and you get each potential is 12.5V. Now you can calculate each charge (Q). To find the effect after the replacing it you can calculate each potential again and add them up so:

V=KQ/r

12.5=9.0x10^9 x Q/2

Q=2.77x10^-9

V=9.0x10^9 x 2.77x 10^-9/1 = 24.93V

V=9.0x10^9x2.77x10^-9/3=8.31

24.93+8.21=33.3V
why do you add the V for each one not subtract
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Eimmanuel
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(Original post by Ramy12343)
why do you add the V for each one not subtract
Why would you think you "should" subtract?

The potential due to positive charge at a point is positive.
If there are several charges around the point, the potential at the point is the scalar sum of the potential due to the all the charges.
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